Nano Res

1

Low haze transparent electrodes and highly conducting

air dried films with ultra-long silver nanowires

synthesized by one-step polyol method Teppei Araki () Jinting Jiu Masaya Nogi Hirotaka Koga Shijo Nagao Tohru Sugahara Katsuaki Suganuma Nano Res Just Accepted Manuscript bull DOI 101007s12274-013-0391-x

httpwwwthenanoresearchcom on November 13 2013

copy Tsinghua University Press 2013

Just Accepted

This is a ldquoJust Acceptedrdquo manuscript which has been examined by the peer‐review process and has been

accepted for publication A ldquoJust Acceptedrdquo manuscript is published online shortly after its acceptance

which is prior to technical editing and formatting and author proofing Tsinghua University Press (TUP)

provides ldquoJust Acceptedrdquo as an optional and free service which allows authors to make their results available

to the research community as soon as possible after acceptance After a manuscript has been technically

edited and formatted it will be removed from the ldquoJust Acceptedrdquo Web site and published as an ASAP

article Please note that technical editing may introduce minor changes to the manuscript text andor

graphics which may affect the content and all legal disclaimers that apply to the journal pertain In no event

shall TUP be held responsible for errors or consequences arising from the use of any information contained

in these ldquoJust Acceptedrdquo manuscripts To cite this manuscript please use its Digital Object Identifier (DOIreg)

which is identical for all formats of publication

Nano Research DOI 101007s12274‐013‐0391‐x

1

Table of Contents

Ultra-long silver nanowires (u-long AgNWs) with lengths up to 230 μm have been

successfully prepared by a one-step polyol process at a low reaction temperature and a

low stirring speed of 110 degC and 60 rpm respectively Compared to conventional

AgNWs with lengths of 10 μm and indium tin oxide u-long AgNW electrodes achieved

a low haze of 22 with low sheet resistance of 49 Ω at an optical transmittance of

96 and the electrodes also showed a low sheet resistance without post-treatment at

room temperature and pressure

2

Low haze transparent electrodes and highly conducting air dried films with ultra-long

silver nanowires synthesized by one-step polyol method

Teppei Araki1 Jinting Jiu2 Masaya Nogi2 Hirotaka Koga2 Shijo Nagao2 Tohru

Sugahara2 Katsuaki Suganuma2

1Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan

2Institute of Scientific and Industrial Research Osaka University Ibaraki Osaka

567-0047 Japan

Corresponding author

Teppei Araki

teppeiecosankenosaka-uacjp

Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan

3

Abstract

Transparent electrodes made of silver nanowires (AgNWs) exhibit higher flexibility

when compared to those made of tin doped indium oxide (ITO) and are expected to be

applied in plastic electronics However these transparent electrodes composed of

AgNWs show high haze because the wires cause strong light scattering in the visible

range Reduction of the wire diameter has been proposed to weaken light scattering

although there have seldom been any studies focusing on the haze because of the

difficulty involved in controlling the wire diameter In this report we show that the haze

can be easily reduced by increasing the length of AgNWs with a large diameter

Ultra-long (u-long) AgNWs with lengths in the range of 20ndash100 μm and a maximum

length of 230 μm have been successfully synthesized by adjusting the reaction

temperature and the stirring speed of a one-step polyol process Compared to typical

AgNWs (with diameter and length of 70 nm and 10 μm respectively) and ITO a

transparent electrode consisting of u-long AgNWs of 91 nm in diameter demonstrated a

low haze of 34ndash16 and a low sheet resistance of 24ndash109 Ω at a transmittance of

94ndash97 Even when fabricated at room temperature without any post-treatment the

electrodes comprising of u-long AgNWs achieved a sheet resistance of 19 Ω at a

transmittance of 80 which was six orders of magnitude lower than that of typical

4

AgNWs

Keywords

Ultra-long silver nanowires One-step synthesis Transparent electrodes Haze

Introduction

Fabrication of flexible and stretchable transparent electrodes has attracted much

attention in the view of the increasing demand for plastic electronics in displays touch

screens and solar cells Attempts have been made to replace the widely used albeit stiff

and rarely available tin doped indium oxide (ITO) with new materials such as carbon

nanotubes (CNTs) [1ndash3] graphene [4ndash6] metal grids or metal wires [7ndash12] Among

the many candidates silver nanowire (AgNW) films show excellent electrical

conductivities because silver has the lowest resistivity of 16 10ndash6 Ωcm among metals

and exhibits high stability and flexibility AgNW electrodes with a transparency of 80

and sheet resistances of around 10 Ω have been fabricated and applied to the above

mentioned devices at the laboratory scale [11ndash18]

5

However one of the disadvantages of using AgNW films is the high haze derived from

light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low

haze leads to less obscuration of transparent materials which is immensely required in

display-based applications Conventional ITO films exhibit a haze of about 1ndash3

with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used

in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum

(which is disadvantageous for high speed production) or high temperature (over 300 degC

which limits the use of low-heat-resistant substrates) On the other hand CNT graphene

and AgNW electrodes can be fabricated by solution processing for high speed

production by roll-to-roll processing with treatments that preclude damage to the

polymer substrate With new nanomaterials like CNTs or graphene haze values below

1 have been achieved [22 23] while the sheet resistances are generally far above 100

Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can

achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag

[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the

application of AgNW electrodes in display devices [10 20 25 26] If the haze can be

decreased to the level of ITO films in combination with the high conductivity AgNW

6

electrodes will open new frontiers in terms of applications in devices

Kim et al reported an inverse relationship between haze and transmittance in AgNW

electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance

When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3

[20] which is similar to the values shown by ITO films Unfortunately the AgNW

electrodes of such low haze showed poor conductivity or were even non-conductive

because the junctions between the wires collapsed in the film comprising wires with

typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the

haze while maintaining the high conductivity it is necessary to extend the length of the

wires to make network junctions between the AgNWs Moreover smaller diameter

AgNWs directly decrease the haze in the visible wavelength range because of the

decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining

the conductivity as well as transparency has been a great challenge In general

decreasing the diameter and extending the length of wires have been expected to be a

promising strategy to achieve low haze with high conductivity and high transparency

Synthesis of AgNWs by the polyol process is now widely known as a scalable and a

7

simpler method when compared to other techniques [27 28] Recently AgNWs with

lengths typically below 20 μm were elongated to several hundred micrometers by a

multistep repetitive process involving the polyol method [14 29] Transparent

electrodes consisting of long AgNWs has been shown to possess superior electrical

properties at high transmittances ranging to up to 95 when compared to carbon

nanomaterials and other nanometals However AgNWs with diameters above 100 nm

are considered to lead to an increase in haze [25 26] In this report low-haze AgNW

electrodes with high conductivity were obtained when the length of the AgNWs was

increased while maintaining their large diameter A modified one-step polyol method

was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to

100 μm (the maximum length achieved was 230 μm) by adjusting the reaction

temperature and stirring speed AgNW electrodes based on these u-long NWs showed

lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high

optical transmittance of 94ndash97 The sheet resistance was comparable to or even

better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]

Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a

transmittance of 80 just after drying at room temperature without any post-treatments

which was six orders of magnitude lower than the values shown by typical AgNWs

8

1 Experiment

11 Synthesis of AgNWs and fabrication of the electrodes

The AgNWs were synthesized by the reduction of silver nitrate in the presence of

polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from

Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)

was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600

μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene

glycol) were added rapidly into the PVP solution within a minute The mixture was

subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm

for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm

After the reaction the solutions were washed in ethanol thrice centrifuged and the

product was dispersed in ethanol for further use To break the ultralong AgNWs their

suspension was ultrasonicated at 195 KHz and 300 W for 10 min

A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the

suspension was streamed along the longitudinal and transverse directions on the

substrate surface followed by air drying Then the AgNW electrode was prepared with

9

and without heating at 200 degC for 10 min The sheet resistance of the transparent

electrode fabricated by this method showed no large deviation at different locations of

the film

12 Characterization of the AgNW electrodes

The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was

measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co

Ltd) with the substrate as the reference The haze and Tp were measured under D65

illumination haze meter with a strong visible light source (HZ-V3 Suga Test

Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter

light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference

substrate was 92 and 01 respectively under D65 illumination Many researchers

consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis

as the generally accepted data Here we compared the Tp measured with the UV-Vis

spectra at 550 nm and the haze meter The results were identical Hence the measuring

the Tp with the haze meter is also an acceptable method to determine the transmittance

of the AgNW electrode (Figure S-1)

The sheet resistance of the electrode of 25 mm times 30 mm was measured using the

10

four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The

AgNWs on the substrate were observed using an optical microscope (VH-Z500

VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)

(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working

distance of 8 mm

2 Results and Discussion

Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)

exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)

The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It

has been known that lower reaction temperatures used in the polyol method results in

longer wires and the length was about 20 μm at 130 degC [30] In order to extend the

length we attempted to further decrease the reaction temperature to 110 degC When the

temperature was decreased to 110 degC at the same stirring speed the AgNWs

(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative

frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of

74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of

typical AgNWs However the low-temperature sample included some nanoparticles too

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

1

Table of Contents

Ultra-long silver nanowires (u-long AgNWs) with lengths up to 230 μm have been

successfully prepared by a one-step polyol process at a low reaction temperature and a

low stirring speed of 110 degC and 60 rpm respectively Compared to conventional

AgNWs with lengths of 10 μm and indium tin oxide u-long AgNW electrodes achieved

a low haze of 22 with low sheet resistance of 49 Ω at an optical transmittance of

96 and the electrodes also showed a low sheet resistance without post-treatment at

room temperature and pressure

2

Low haze transparent electrodes and highly conducting air dried films with ultra-long

silver nanowires synthesized by one-step polyol method

Teppei Araki1 Jinting Jiu2 Masaya Nogi2 Hirotaka Koga2 Shijo Nagao2 Tohru

Sugahara2 Katsuaki Suganuma2

1Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan

2Institute of Scientific and Industrial Research Osaka University Ibaraki Osaka

567-0047 Japan

Corresponding author

Teppei Araki

teppeiecosankenosaka-uacjp

Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan

3

Abstract

Transparent electrodes made of silver nanowires (AgNWs) exhibit higher flexibility

when compared to those made of tin doped indium oxide (ITO) and are expected to be

applied in plastic electronics However these transparent electrodes composed of

AgNWs show high haze because the wires cause strong light scattering in the visible

range Reduction of the wire diameter has been proposed to weaken light scattering

although there have seldom been any studies focusing on the haze because of the

difficulty involved in controlling the wire diameter In this report we show that the haze

can be easily reduced by increasing the length of AgNWs with a large diameter

Ultra-long (u-long) AgNWs with lengths in the range of 20ndash100 μm and a maximum

length of 230 μm have been successfully synthesized by adjusting the reaction

temperature and the stirring speed of a one-step polyol process Compared to typical

AgNWs (with diameter and length of 70 nm and 10 μm respectively) and ITO a

transparent electrode consisting of u-long AgNWs of 91 nm in diameter demonstrated a

low haze of 34ndash16 and a low sheet resistance of 24ndash109 Ω at a transmittance of

94ndash97 Even when fabricated at room temperature without any post-treatment the

electrodes comprising of u-long AgNWs achieved a sheet resistance of 19 Ω at a

transmittance of 80 which was six orders of magnitude lower than that of typical

4

AgNWs

Keywords

Ultra-long silver nanowires One-step synthesis Transparent electrodes Haze

Introduction

Fabrication of flexible and stretchable transparent electrodes has attracted much

attention in the view of the increasing demand for plastic electronics in displays touch

screens and solar cells Attempts have been made to replace the widely used albeit stiff

and rarely available tin doped indium oxide (ITO) with new materials such as carbon

nanotubes (CNTs) [1ndash3] graphene [4ndash6] metal grids or metal wires [7ndash12] Among

the many candidates silver nanowire (AgNW) films show excellent electrical

conductivities because silver has the lowest resistivity of 16 10ndash6 Ωcm among metals

and exhibits high stability and flexibility AgNW electrodes with a transparency of 80

and sheet resistances of around 10 Ω have been fabricated and applied to the above

mentioned devices at the laboratory scale [11ndash18]

5

However one of the disadvantages of using AgNW films is the high haze derived from

light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low

haze leads to less obscuration of transparent materials which is immensely required in

display-based applications Conventional ITO films exhibit a haze of about 1ndash3

with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used

in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum

(which is disadvantageous for high speed production) or high temperature (over 300 degC

which limits the use of low-heat-resistant substrates) On the other hand CNT graphene

and AgNW electrodes can be fabricated by solution processing for high speed

production by roll-to-roll processing with treatments that preclude damage to the

polymer substrate With new nanomaterials like CNTs or graphene haze values below

1 have been achieved [22 23] while the sheet resistances are generally far above 100

Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can

achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag

[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the

application of AgNW electrodes in display devices [10 20 25 26] If the haze can be

decreased to the level of ITO films in combination with the high conductivity AgNW

6

electrodes will open new frontiers in terms of applications in devices

Kim et al reported an inverse relationship between haze and transmittance in AgNW

electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance

When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3

[20] which is similar to the values shown by ITO films Unfortunately the AgNW

electrodes of such low haze showed poor conductivity or were even non-conductive

because the junctions between the wires collapsed in the film comprising wires with

typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the

haze while maintaining the high conductivity it is necessary to extend the length of the

wires to make network junctions between the AgNWs Moreover smaller diameter

AgNWs directly decrease the haze in the visible wavelength range because of the

decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining

the conductivity as well as transparency has been a great challenge In general

decreasing the diameter and extending the length of wires have been expected to be a

promising strategy to achieve low haze with high conductivity and high transparency

Synthesis of AgNWs by the polyol process is now widely known as a scalable and a

7

simpler method when compared to other techniques [27 28] Recently AgNWs with

lengths typically below 20 μm were elongated to several hundred micrometers by a

multistep repetitive process involving the polyol method [14 29] Transparent

electrodes consisting of long AgNWs has been shown to possess superior electrical

properties at high transmittances ranging to up to 95 when compared to carbon

nanomaterials and other nanometals However AgNWs with diameters above 100 nm

are considered to lead to an increase in haze [25 26] In this report low-haze AgNW

electrodes with high conductivity were obtained when the length of the AgNWs was

increased while maintaining their large diameter A modified one-step polyol method

was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to

100 μm (the maximum length achieved was 230 μm) by adjusting the reaction

temperature and stirring speed AgNW electrodes based on these u-long NWs showed

lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high

optical transmittance of 94ndash97 The sheet resistance was comparable to or even

better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]

Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a

transmittance of 80 just after drying at room temperature without any post-treatments

which was six orders of magnitude lower than the values shown by typical AgNWs

8

1 Experiment

11 Synthesis of AgNWs and fabrication of the electrodes

The AgNWs were synthesized by the reduction of silver nitrate in the presence of

polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from

Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)

was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600

μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene

glycol) were added rapidly into the PVP solution within a minute The mixture was

subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm

for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm

After the reaction the solutions were washed in ethanol thrice centrifuged and the

product was dispersed in ethanol for further use To break the ultralong AgNWs their

suspension was ultrasonicated at 195 KHz and 300 W for 10 min

A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the

suspension was streamed along the longitudinal and transverse directions on the

substrate surface followed by air drying Then the AgNW electrode was prepared with

9

and without heating at 200 degC for 10 min The sheet resistance of the transparent

electrode fabricated by this method showed no large deviation at different locations of

the film

12 Characterization of the AgNW electrodes

The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was

measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co

Ltd) with the substrate as the reference The haze and Tp were measured under D65

illumination haze meter with a strong visible light source (HZ-V3 Suga Test

Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter

light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference

substrate was 92 and 01 respectively under D65 illumination Many researchers

consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis

as the generally accepted data Here we compared the Tp measured with the UV-Vis

spectra at 550 nm and the haze meter The results were identical Hence the measuring

the Tp with the haze meter is also an acceptable method to determine the transmittance

of the AgNW electrode (Figure S-1)

The sheet resistance of the electrode of 25 mm times 30 mm was measured using the

10

four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The

AgNWs on the substrate were observed using an optical microscope (VH-Z500

VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)

(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working

distance of 8 mm

2 Results and Discussion

Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)

exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)

The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It

has been known that lower reaction temperatures used in the polyol method results in

longer wires and the length was about 20 μm at 130 degC [30] In order to extend the

length we attempted to further decrease the reaction temperature to 110 degC When the

temperature was decreased to 110 degC at the same stirring speed the AgNWs

(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative

frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of

74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of

typical AgNWs However the low-temperature sample included some nanoparticles too

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

2

Low haze transparent electrodes and highly conducting air dried films with ultra-long

silver nanowires synthesized by one-step polyol method

Teppei Araki1 Jinting Jiu2 Masaya Nogi2 Hirotaka Koga2 Shijo Nagao2 Tohru

Sugahara2 Katsuaki Suganuma2

1Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan

2Institute of Scientific and Industrial Research Osaka University Ibaraki Osaka

567-0047 Japan

Corresponding author

Teppei Araki

teppeiecosankenosaka-uacjp

Graduate School of Engineering Osaka University Ibaraki Osaka 567-0047 Japan

3

Abstract

Transparent electrodes made of silver nanowires (AgNWs) exhibit higher flexibility

when compared to those made of tin doped indium oxide (ITO) and are expected to be

applied in plastic electronics However these transparent electrodes composed of

AgNWs show high haze because the wires cause strong light scattering in the visible

range Reduction of the wire diameter has been proposed to weaken light scattering

although there have seldom been any studies focusing on the haze because of the

difficulty involved in controlling the wire diameter In this report we show that the haze

can be easily reduced by increasing the length of AgNWs with a large diameter

Ultra-long (u-long) AgNWs with lengths in the range of 20ndash100 μm and a maximum

length of 230 μm have been successfully synthesized by adjusting the reaction

temperature and the stirring speed of a one-step polyol process Compared to typical

AgNWs (with diameter and length of 70 nm and 10 μm respectively) and ITO a

transparent electrode consisting of u-long AgNWs of 91 nm in diameter demonstrated a

low haze of 34ndash16 and a low sheet resistance of 24ndash109 Ω at a transmittance of

94ndash97 Even when fabricated at room temperature without any post-treatment the

electrodes comprising of u-long AgNWs achieved a sheet resistance of 19 Ω at a

transmittance of 80 which was six orders of magnitude lower than that of typical

4

AgNWs

Keywords

Ultra-long silver nanowires One-step synthesis Transparent electrodes Haze

Introduction

Fabrication of flexible and stretchable transparent electrodes has attracted much

attention in the view of the increasing demand for plastic electronics in displays touch

screens and solar cells Attempts have been made to replace the widely used albeit stiff

and rarely available tin doped indium oxide (ITO) with new materials such as carbon

nanotubes (CNTs) [1ndash3] graphene [4ndash6] metal grids or metal wires [7ndash12] Among

the many candidates silver nanowire (AgNW) films show excellent electrical

conductivities because silver has the lowest resistivity of 16 10ndash6 Ωcm among metals

and exhibits high stability and flexibility AgNW electrodes with a transparency of 80

and sheet resistances of around 10 Ω have been fabricated and applied to the above

mentioned devices at the laboratory scale [11ndash18]

5

However one of the disadvantages of using AgNW films is the high haze derived from

light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low

haze leads to less obscuration of transparent materials which is immensely required in

display-based applications Conventional ITO films exhibit a haze of about 1ndash3

with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used

in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum

(which is disadvantageous for high speed production) or high temperature (over 300 degC

which limits the use of low-heat-resistant substrates) On the other hand CNT graphene

and AgNW electrodes can be fabricated by solution processing for high speed

production by roll-to-roll processing with treatments that preclude damage to the

polymer substrate With new nanomaterials like CNTs or graphene haze values below

1 have been achieved [22 23] while the sheet resistances are generally far above 100

Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can

achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag

[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the

application of AgNW electrodes in display devices [10 20 25 26] If the haze can be

decreased to the level of ITO films in combination with the high conductivity AgNW

6

electrodes will open new frontiers in terms of applications in devices

Kim et al reported an inverse relationship between haze and transmittance in AgNW

electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance

When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3

[20] which is similar to the values shown by ITO films Unfortunately the AgNW

electrodes of such low haze showed poor conductivity or were even non-conductive

because the junctions between the wires collapsed in the film comprising wires with

typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the

haze while maintaining the high conductivity it is necessary to extend the length of the

wires to make network junctions between the AgNWs Moreover smaller diameter

AgNWs directly decrease the haze in the visible wavelength range because of the

decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining

the conductivity as well as transparency has been a great challenge In general

decreasing the diameter and extending the length of wires have been expected to be a

promising strategy to achieve low haze with high conductivity and high transparency

Synthesis of AgNWs by the polyol process is now widely known as a scalable and a

7

simpler method when compared to other techniques [27 28] Recently AgNWs with

lengths typically below 20 μm were elongated to several hundred micrometers by a

multistep repetitive process involving the polyol method [14 29] Transparent

electrodes consisting of long AgNWs has been shown to possess superior electrical

properties at high transmittances ranging to up to 95 when compared to carbon

nanomaterials and other nanometals However AgNWs with diameters above 100 nm

are considered to lead to an increase in haze [25 26] In this report low-haze AgNW

electrodes with high conductivity were obtained when the length of the AgNWs was

increased while maintaining their large diameter A modified one-step polyol method

was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to

100 μm (the maximum length achieved was 230 μm) by adjusting the reaction

temperature and stirring speed AgNW electrodes based on these u-long NWs showed

lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high

optical transmittance of 94ndash97 The sheet resistance was comparable to or even

better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]

Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a

transmittance of 80 just after drying at room temperature without any post-treatments

which was six orders of magnitude lower than the values shown by typical AgNWs

8

1 Experiment

11 Synthesis of AgNWs and fabrication of the electrodes

The AgNWs were synthesized by the reduction of silver nitrate in the presence of

polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from

Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)

was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600

μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene

glycol) were added rapidly into the PVP solution within a minute The mixture was

subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm

for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm

After the reaction the solutions were washed in ethanol thrice centrifuged and the

product was dispersed in ethanol for further use To break the ultralong AgNWs their

suspension was ultrasonicated at 195 KHz and 300 W for 10 min

A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the

suspension was streamed along the longitudinal and transverse directions on the

substrate surface followed by air drying Then the AgNW electrode was prepared with

9

and without heating at 200 degC for 10 min The sheet resistance of the transparent

electrode fabricated by this method showed no large deviation at different locations of

the film

12 Characterization of the AgNW electrodes

The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was

measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co

Ltd) with the substrate as the reference The haze and Tp were measured under D65

illumination haze meter with a strong visible light source (HZ-V3 Suga Test

Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter

light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference

substrate was 92 and 01 respectively under D65 illumination Many researchers

consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis

as the generally accepted data Here we compared the Tp measured with the UV-Vis

spectra at 550 nm and the haze meter The results were identical Hence the measuring

the Tp with the haze meter is also an acceptable method to determine the transmittance

of the AgNW electrode (Figure S-1)

The sheet resistance of the electrode of 25 mm times 30 mm was measured using the

10

four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The

AgNWs on the substrate were observed using an optical microscope (VH-Z500

VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)

(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working

distance of 8 mm

2 Results and Discussion

Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)

exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)

The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It

has been known that lower reaction temperatures used in the polyol method results in

longer wires and the length was about 20 μm at 130 degC [30] In order to extend the

length we attempted to further decrease the reaction temperature to 110 degC When the

temperature was decreased to 110 degC at the same stirring speed the AgNWs

(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative

frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of

74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of

typical AgNWs However the low-temperature sample included some nanoparticles too

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

3

Abstract

Transparent electrodes made of silver nanowires (AgNWs) exhibit higher flexibility

when compared to those made of tin doped indium oxide (ITO) and are expected to be

applied in plastic electronics However these transparent electrodes composed of

AgNWs show high haze because the wires cause strong light scattering in the visible

range Reduction of the wire diameter has been proposed to weaken light scattering

although there have seldom been any studies focusing on the haze because of the

difficulty involved in controlling the wire diameter In this report we show that the haze

can be easily reduced by increasing the length of AgNWs with a large diameter

Ultra-long (u-long) AgNWs with lengths in the range of 20ndash100 μm and a maximum

length of 230 μm have been successfully synthesized by adjusting the reaction

temperature and the stirring speed of a one-step polyol process Compared to typical

AgNWs (with diameter and length of 70 nm and 10 μm respectively) and ITO a

transparent electrode consisting of u-long AgNWs of 91 nm in diameter demonstrated a

low haze of 34ndash16 and a low sheet resistance of 24ndash109 Ω at a transmittance of

94ndash97 Even when fabricated at room temperature without any post-treatment the

electrodes comprising of u-long AgNWs achieved a sheet resistance of 19 Ω at a

transmittance of 80 which was six orders of magnitude lower than that of typical

4

AgNWs

Keywords

Ultra-long silver nanowires One-step synthesis Transparent electrodes Haze

Introduction

Fabrication of flexible and stretchable transparent electrodes has attracted much

attention in the view of the increasing demand for plastic electronics in displays touch

screens and solar cells Attempts have been made to replace the widely used albeit stiff

and rarely available tin doped indium oxide (ITO) with new materials such as carbon

nanotubes (CNTs) [1ndash3] graphene [4ndash6] metal grids or metal wires [7ndash12] Among

the many candidates silver nanowire (AgNW) films show excellent electrical

conductivities because silver has the lowest resistivity of 16 10ndash6 Ωcm among metals

and exhibits high stability and flexibility AgNW electrodes with a transparency of 80

and sheet resistances of around 10 Ω have been fabricated and applied to the above

mentioned devices at the laboratory scale [11ndash18]

5

However one of the disadvantages of using AgNW films is the high haze derived from

light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low

haze leads to less obscuration of transparent materials which is immensely required in

display-based applications Conventional ITO films exhibit a haze of about 1ndash3

with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used

in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum

(which is disadvantageous for high speed production) or high temperature (over 300 degC

which limits the use of low-heat-resistant substrates) On the other hand CNT graphene

and AgNW electrodes can be fabricated by solution processing for high speed

production by roll-to-roll processing with treatments that preclude damage to the

polymer substrate With new nanomaterials like CNTs or graphene haze values below

1 have been achieved [22 23] while the sheet resistances are generally far above 100

Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can

achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag

[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the

application of AgNW electrodes in display devices [10 20 25 26] If the haze can be

decreased to the level of ITO films in combination with the high conductivity AgNW

6

electrodes will open new frontiers in terms of applications in devices

Kim et al reported an inverse relationship between haze and transmittance in AgNW

electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance

When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3

[20] which is similar to the values shown by ITO films Unfortunately the AgNW

electrodes of such low haze showed poor conductivity or were even non-conductive

because the junctions between the wires collapsed in the film comprising wires with

typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the

haze while maintaining the high conductivity it is necessary to extend the length of the

wires to make network junctions between the AgNWs Moreover smaller diameter

AgNWs directly decrease the haze in the visible wavelength range because of the

decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining

the conductivity as well as transparency has been a great challenge In general

decreasing the diameter and extending the length of wires have been expected to be a

promising strategy to achieve low haze with high conductivity and high transparency

Synthesis of AgNWs by the polyol process is now widely known as a scalable and a

7

simpler method when compared to other techniques [27 28] Recently AgNWs with

lengths typically below 20 μm were elongated to several hundred micrometers by a

multistep repetitive process involving the polyol method [14 29] Transparent

electrodes consisting of long AgNWs has been shown to possess superior electrical

properties at high transmittances ranging to up to 95 when compared to carbon

nanomaterials and other nanometals However AgNWs with diameters above 100 nm

are considered to lead to an increase in haze [25 26] In this report low-haze AgNW

electrodes with high conductivity were obtained when the length of the AgNWs was

increased while maintaining their large diameter A modified one-step polyol method

was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to

100 μm (the maximum length achieved was 230 μm) by adjusting the reaction

temperature and stirring speed AgNW electrodes based on these u-long NWs showed

lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high

optical transmittance of 94ndash97 The sheet resistance was comparable to or even

better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]

Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a

transmittance of 80 just after drying at room temperature without any post-treatments

which was six orders of magnitude lower than the values shown by typical AgNWs

8

1 Experiment

11 Synthesis of AgNWs and fabrication of the electrodes

The AgNWs were synthesized by the reduction of silver nitrate in the presence of

polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from

Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)

was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600

μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene

glycol) were added rapidly into the PVP solution within a minute The mixture was

subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm

for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm

After the reaction the solutions were washed in ethanol thrice centrifuged and the

product was dispersed in ethanol for further use To break the ultralong AgNWs their

suspension was ultrasonicated at 195 KHz and 300 W for 10 min

A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the

suspension was streamed along the longitudinal and transverse directions on the

substrate surface followed by air drying Then the AgNW electrode was prepared with

9

and without heating at 200 degC for 10 min The sheet resistance of the transparent

electrode fabricated by this method showed no large deviation at different locations of

the film

12 Characterization of the AgNW electrodes

The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was

measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co

Ltd) with the substrate as the reference The haze and Tp were measured under D65

illumination haze meter with a strong visible light source (HZ-V3 Suga Test

Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter

light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference

substrate was 92 and 01 respectively under D65 illumination Many researchers

consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis

as the generally accepted data Here we compared the Tp measured with the UV-Vis

spectra at 550 nm and the haze meter The results were identical Hence the measuring

the Tp with the haze meter is also an acceptable method to determine the transmittance

of the AgNW electrode (Figure S-1)

The sheet resistance of the electrode of 25 mm times 30 mm was measured using the

10

four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The

AgNWs on the substrate were observed using an optical microscope (VH-Z500

VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)

(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working

distance of 8 mm

2 Results and Discussion

Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)

exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)

The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It

has been known that lower reaction temperatures used in the polyol method results in

longer wires and the length was about 20 μm at 130 degC [30] In order to extend the

length we attempted to further decrease the reaction temperature to 110 degC When the

temperature was decreased to 110 degC at the same stirring speed the AgNWs

(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative

frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of

74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of

typical AgNWs However the low-temperature sample included some nanoparticles too

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

4

AgNWs

Keywords

Ultra-long silver nanowires One-step synthesis Transparent electrodes Haze

Introduction

Fabrication of flexible and stretchable transparent electrodes has attracted much

attention in the view of the increasing demand for plastic electronics in displays touch

screens and solar cells Attempts have been made to replace the widely used albeit stiff

and rarely available tin doped indium oxide (ITO) with new materials such as carbon

nanotubes (CNTs) [1ndash3] graphene [4ndash6] metal grids or metal wires [7ndash12] Among

the many candidates silver nanowire (AgNW) films show excellent electrical

conductivities because silver has the lowest resistivity of 16 10ndash6 Ωcm among metals

and exhibits high stability and flexibility AgNW electrodes with a transparency of 80

and sheet resistances of around 10 Ω have been fabricated and applied to the above

mentioned devices at the laboratory scale [11ndash18]

5

However one of the disadvantages of using AgNW films is the high haze derived from

light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low

haze leads to less obscuration of transparent materials which is immensely required in

display-based applications Conventional ITO films exhibit a haze of about 1ndash3

with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used

in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum

(which is disadvantageous for high speed production) or high temperature (over 300 degC

which limits the use of low-heat-resistant substrates) On the other hand CNT graphene

and AgNW electrodes can be fabricated by solution processing for high speed

production by roll-to-roll processing with treatments that preclude damage to the

polymer substrate With new nanomaterials like CNTs or graphene haze values below

1 have been achieved [22 23] while the sheet resistances are generally far above 100

Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can

achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag

[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the

application of AgNW electrodes in display devices [10 20 25 26] If the haze can be

decreased to the level of ITO films in combination with the high conductivity AgNW

6

electrodes will open new frontiers in terms of applications in devices

Kim et al reported an inverse relationship between haze and transmittance in AgNW

electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance

When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3

[20] which is similar to the values shown by ITO films Unfortunately the AgNW

electrodes of such low haze showed poor conductivity or were even non-conductive

because the junctions between the wires collapsed in the film comprising wires with

typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the

haze while maintaining the high conductivity it is necessary to extend the length of the

wires to make network junctions between the AgNWs Moreover smaller diameter

AgNWs directly decrease the haze in the visible wavelength range because of the

decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining

the conductivity as well as transparency has been a great challenge In general

decreasing the diameter and extending the length of wires have been expected to be a

promising strategy to achieve low haze with high conductivity and high transparency

Synthesis of AgNWs by the polyol process is now widely known as a scalable and a

7

simpler method when compared to other techniques [27 28] Recently AgNWs with

lengths typically below 20 μm were elongated to several hundred micrometers by a

multistep repetitive process involving the polyol method [14 29] Transparent

electrodes consisting of long AgNWs has been shown to possess superior electrical

properties at high transmittances ranging to up to 95 when compared to carbon

nanomaterials and other nanometals However AgNWs with diameters above 100 nm

are considered to lead to an increase in haze [25 26] In this report low-haze AgNW

electrodes with high conductivity were obtained when the length of the AgNWs was

increased while maintaining their large diameter A modified one-step polyol method

was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to

100 μm (the maximum length achieved was 230 μm) by adjusting the reaction

temperature and stirring speed AgNW electrodes based on these u-long NWs showed

lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high

optical transmittance of 94ndash97 The sheet resistance was comparable to or even

better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]

Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a

transmittance of 80 just after drying at room temperature without any post-treatments

which was six orders of magnitude lower than the values shown by typical AgNWs

8

1 Experiment

11 Synthesis of AgNWs and fabrication of the electrodes

The AgNWs were synthesized by the reduction of silver nitrate in the presence of

polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from

Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)

was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600

μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene

glycol) were added rapidly into the PVP solution within a minute The mixture was

subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm

for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm

After the reaction the solutions were washed in ethanol thrice centrifuged and the

product was dispersed in ethanol for further use To break the ultralong AgNWs their

suspension was ultrasonicated at 195 KHz and 300 W for 10 min

A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the

suspension was streamed along the longitudinal and transverse directions on the

substrate surface followed by air drying Then the AgNW electrode was prepared with

9

and without heating at 200 degC for 10 min The sheet resistance of the transparent

electrode fabricated by this method showed no large deviation at different locations of

the film

12 Characterization of the AgNW electrodes

The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was

measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co

Ltd) with the substrate as the reference The haze and Tp were measured under D65

illumination haze meter with a strong visible light source (HZ-V3 Suga Test

Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter

light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference

substrate was 92 and 01 respectively under D65 illumination Many researchers

consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis

as the generally accepted data Here we compared the Tp measured with the UV-Vis

spectra at 550 nm and the haze meter The results were identical Hence the measuring

the Tp with the haze meter is also an acceptable method to determine the transmittance

of the AgNW electrode (Figure S-1)

The sheet resistance of the electrode of 25 mm times 30 mm was measured using the

10

four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The

AgNWs on the substrate were observed using an optical microscope (VH-Z500

VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)

(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working

distance of 8 mm

2 Results and Discussion

Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)

exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)

The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It

has been known that lower reaction temperatures used in the polyol method results in

longer wires and the length was about 20 μm at 130 degC [30] In order to extend the

length we attempted to further decrease the reaction temperature to 110 degC When the

temperature was decreased to 110 degC at the same stirring speed the AgNWs

(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative

frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of

74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of

typical AgNWs However the low-temperature sample included some nanoparticles too

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

5

However one of the disadvantages of using AgNW films is the high haze derived from

light scattering which occurs to a lesser extent in ITO and very thin CNTs [10] Low

haze leads to less obscuration of transparent materials which is immensely required in

display-based applications Conventional ITO films exhibit a haze of about 1ndash3

with a sheet resistance exceeding 50 Ω at a transparency of 85 and have been used

in display devices [10 19ndash21] although the ITO layers are fabricated under vacuum

(which is disadvantageous for high speed production) or high temperature (over 300 degC

which limits the use of low-heat-resistant substrates) On the other hand CNT graphene

and AgNW electrodes can be fabricated by solution processing for high speed

production by roll-to-roll processing with treatments that preclude damage to the

polymer substrate With new nanomaterials like CNTs or graphene haze values below

1 have been achieved [22 23] while the sheet resistances are generally far above 100

Ω at a transmittance of 90 [3 12 24] AgNW electrodes at 90 transmission can

achieve still low sheet resistances of 20ndash100 Ω because of the high conductivity of Ag

[10ndash13 20] However the high haze of 5ndash15 is a major disadvantage for the

application of AgNW electrodes in display devices [10 20 25 26] If the haze can be

decreased to the level of ITO films in combination with the high conductivity AgNW

6

electrodes will open new frontiers in terms of applications in devices

Kim et al reported an inverse relationship between haze and transmittance in AgNW

electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance

When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3

[20] which is similar to the values shown by ITO films Unfortunately the AgNW

electrodes of such low haze showed poor conductivity or were even non-conductive

because the junctions between the wires collapsed in the film comprising wires with

typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the

haze while maintaining the high conductivity it is necessary to extend the length of the

wires to make network junctions between the AgNWs Moreover smaller diameter

AgNWs directly decrease the haze in the visible wavelength range because of the

decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining

the conductivity as well as transparency has been a great challenge In general

decreasing the diameter and extending the length of wires have been expected to be a

promising strategy to achieve low haze with high conductivity and high transparency

Synthesis of AgNWs by the polyol process is now widely known as a scalable and a

7

simpler method when compared to other techniques [27 28] Recently AgNWs with

lengths typically below 20 μm were elongated to several hundred micrometers by a

multistep repetitive process involving the polyol method [14 29] Transparent

electrodes consisting of long AgNWs has been shown to possess superior electrical

properties at high transmittances ranging to up to 95 when compared to carbon

nanomaterials and other nanometals However AgNWs with diameters above 100 nm

are considered to lead to an increase in haze [25 26] In this report low-haze AgNW

electrodes with high conductivity were obtained when the length of the AgNWs was

increased while maintaining their large diameter A modified one-step polyol method

was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to

100 μm (the maximum length achieved was 230 μm) by adjusting the reaction

temperature and stirring speed AgNW electrodes based on these u-long NWs showed

lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high

optical transmittance of 94ndash97 The sheet resistance was comparable to or even

better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]

Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a

transmittance of 80 just after drying at room temperature without any post-treatments

which was six orders of magnitude lower than the values shown by typical AgNWs

8

1 Experiment

11 Synthesis of AgNWs and fabrication of the electrodes

The AgNWs were synthesized by the reduction of silver nitrate in the presence of

polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from

Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)

was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600

μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene

glycol) were added rapidly into the PVP solution within a minute The mixture was

subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm

for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm

After the reaction the solutions were washed in ethanol thrice centrifuged and the

product was dispersed in ethanol for further use To break the ultralong AgNWs their

suspension was ultrasonicated at 195 KHz and 300 W for 10 min

A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the

suspension was streamed along the longitudinal and transverse directions on the

substrate surface followed by air drying Then the AgNW electrode was prepared with

9

and without heating at 200 degC for 10 min The sheet resistance of the transparent

electrode fabricated by this method showed no large deviation at different locations of

the film

12 Characterization of the AgNW electrodes

The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was

measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co

Ltd) with the substrate as the reference The haze and Tp were measured under D65

illumination haze meter with a strong visible light source (HZ-V3 Suga Test

Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter

light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference

substrate was 92 and 01 respectively under D65 illumination Many researchers

consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis

as the generally accepted data Here we compared the Tp measured with the UV-Vis

spectra at 550 nm and the haze meter The results were identical Hence the measuring

the Tp with the haze meter is also an acceptable method to determine the transmittance

of the AgNW electrode (Figure S-1)

The sheet resistance of the electrode of 25 mm times 30 mm was measured using the

10

four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The

AgNWs on the substrate were observed using an optical microscope (VH-Z500

VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)

(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working

distance of 8 mm

2 Results and Discussion

Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)

exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)

The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It

has been known that lower reaction temperatures used in the polyol method results in

longer wires and the length was about 20 μm at 130 degC [30] In order to extend the

length we attempted to further decrease the reaction temperature to 110 degC When the

temperature was decreased to 110 degC at the same stirring speed the AgNWs

(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative

frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of

74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of

typical AgNWs However the low-temperature sample included some nanoparticles too

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

6

electrodes will open new frontiers in terms of applications in devices

Kim et al reported an inverse relationship between haze and transmittance in AgNW

electrodes [20] The haze of AgNW electrodes decreased with increase in transmittance

When the transmittance exceeded 95 the AgNW electrodes showed a haze below 3

[20] which is similar to the values shown by ITO films Unfortunately the AgNW

electrodes of such low haze showed poor conductivity or were even non-conductive

because the junctions between the wires collapsed in the film comprising wires with

typical lengths of around 10 μm and diameters of around 70 nm In order to decrease the

haze while maintaining the high conductivity it is necessary to extend the length of the

wires to make network junctions between the AgNWs Moreover smaller diameter

AgNWs directly decrease the haze in the visible wavelength range because of the

decreased scattering of light [25 26] Decreasing the haze of AgNWs whilst retaining

the conductivity as well as transparency has been a great challenge In general

decreasing the diameter and extending the length of wires have been expected to be a

promising strategy to achieve low haze with high conductivity and high transparency

Synthesis of AgNWs by the polyol process is now widely known as a scalable and a

7

simpler method when compared to other techniques [27 28] Recently AgNWs with

lengths typically below 20 μm were elongated to several hundred micrometers by a

multistep repetitive process involving the polyol method [14 29] Transparent

electrodes consisting of long AgNWs has been shown to possess superior electrical

properties at high transmittances ranging to up to 95 when compared to carbon

nanomaterials and other nanometals However AgNWs with diameters above 100 nm

are considered to lead to an increase in haze [25 26] In this report low-haze AgNW

electrodes with high conductivity were obtained when the length of the AgNWs was

increased while maintaining their large diameter A modified one-step polyol method

was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to

100 μm (the maximum length achieved was 230 μm) by adjusting the reaction

temperature and stirring speed AgNW electrodes based on these u-long NWs showed

lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high

optical transmittance of 94ndash97 The sheet resistance was comparable to or even

better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]

Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a

transmittance of 80 just after drying at room temperature without any post-treatments

which was six orders of magnitude lower than the values shown by typical AgNWs

8

1 Experiment

11 Synthesis of AgNWs and fabrication of the electrodes

The AgNWs were synthesized by the reduction of silver nitrate in the presence of

polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from

Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)

was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600

μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene

glycol) were added rapidly into the PVP solution within a minute The mixture was

subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm

for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm

After the reaction the solutions were washed in ethanol thrice centrifuged and the

product was dispersed in ethanol for further use To break the ultralong AgNWs their

suspension was ultrasonicated at 195 KHz and 300 W for 10 min

A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the

suspension was streamed along the longitudinal and transverse directions on the

substrate surface followed by air drying Then the AgNW electrode was prepared with

9

and without heating at 200 degC for 10 min The sheet resistance of the transparent

electrode fabricated by this method showed no large deviation at different locations of

the film

12 Characterization of the AgNW electrodes

The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was

measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co

Ltd) with the substrate as the reference The haze and Tp were measured under D65

illumination haze meter with a strong visible light source (HZ-V3 Suga Test

Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter

light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference

substrate was 92 and 01 respectively under D65 illumination Many researchers

consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis

as the generally accepted data Here we compared the Tp measured with the UV-Vis

spectra at 550 nm and the haze meter The results were identical Hence the measuring

the Tp with the haze meter is also an acceptable method to determine the transmittance

of the AgNW electrode (Figure S-1)

The sheet resistance of the electrode of 25 mm times 30 mm was measured using the

10

four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The

AgNWs on the substrate were observed using an optical microscope (VH-Z500

VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)

(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working

distance of 8 mm

2 Results and Discussion

Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)

exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)

The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It

has been known that lower reaction temperatures used in the polyol method results in

longer wires and the length was about 20 μm at 130 degC [30] In order to extend the

length we attempted to further decrease the reaction temperature to 110 degC When the

temperature was decreased to 110 degC at the same stirring speed the AgNWs

(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative

frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of

74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of

typical AgNWs However the low-temperature sample included some nanoparticles too

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

7

simpler method when compared to other techniques [27 28] Recently AgNWs with

lengths typically below 20 μm were elongated to several hundred micrometers by a

multistep repetitive process involving the polyol method [14 29] Transparent

electrodes consisting of long AgNWs has been shown to possess superior electrical

properties at high transmittances ranging to up to 95 when compared to carbon

nanomaterials and other nanometals However AgNWs with diameters above 100 nm

are considered to lead to an increase in haze [25 26] In this report low-haze AgNW

electrodes with high conductivity were obtained when the length of the AgNWs was

increased while maintaining their large diameter A modified one-step polyol method

was developed to form ultra-long (u-long) AgNWs with lengths ranging from 20 μm to

100 μm (the maximum length achieved was 230 μm) by adjusting the reaction

temperature and stirring speed AgNW electrodes based on these u-long NWs showed

lowest haze values of 33ndash16 with lower sheet resistance of 24ndash109 Ω at a high

optical transmittance of 94ndash97 The sheet resistance was comparable to or even

better than that of a monolayered graphene sheet (125 Ω at 97 transmittance) [6]

Furthermore the sheet resistance of the u-long AgNWs electrodes was 19 Ω at a

transmittance of 80 just after drying at room temperature without any post-treatments

which was six orders of magnitude lower than the values shown by typical AgNWs

8

1 Experiment

11 Synthesis of AgNWs and fabrication of the electrodes

The AgNWs were synthesized by the reduction of silver nitrate in the presence of

polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from

Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)

was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600

μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene

glycol) were added rapidly into the PVP solution within a minute The mixture was

subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm

for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm

After the reaction the solutions were washed in ethanol thrice centrifuged and the

product was dispersed in ethanol for further use To break the ultralong AgNWs their

suspension was ultrasonicated at 195 KHz and 300 W for 10 min

A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the

suspension was streamed along the longitudinal and transverse directions on the

substrate surface followed by air drying Then the AgNW electrode was prepared with

9

and without heating at 200 degC for 10 min The sheet resistance of the transparent

electrode fabricated by this method showed no large deviation at different locations of

the film

12 Characterization of the AgNW electrodes

The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was

measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co

Ltd) with the substrate as the reference The haze and Tp were measured under D65

illumination haze meter with a strong visible light source (HZ-V3 Suga Test

Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter

light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference

substrate was 92 and 01 respectively under D65 illumination Many researchers

consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis

as the generally accepted data Here we compared the Tp measured with the UV-Vis

spectra at 550 nm and the haze meter The results were identical Hence the measuring

the Tp with the haze meter is also an acceptable method to determine the transmittance

of the AgNW electrode (Figure S-1)

The sheet resistance of the electrode of 25 mm times 30 mm was measured using the

10

four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The

AgNWs on the substrate were observed using an optical microscope (VH-Z500

VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)

(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working

distance of 8 mm

2 Results and Discussion

Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)

exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)

The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It

has been known that lower reaction temperatures used in the polyol method results in

longer wires and the length was about 20 μm at 130 degC [30] In order to extend the

length we attempted to further decrease the reaction temperature to 110 degC When the

temperature was decreased to 110 degC at the same stirring speed the AgNWs

(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative

frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of

74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of

typical AgNWs However the low-temperature sample included some nanoparticles too

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

8

1 Experiment

11 Synthesis of AgNWs and fabrication of the electrodes

The AgNWs were synthesized by the reduction of silver nitrate in the presence of

polyvinylpyrrolidone (PVP) in ethylene glycol All the reagents were purchased from

Wako Pure Chemical Industries Ltd PVP (average molecular weight 360 k 0163 g)

was mixed with 22 g of ethylene glycol Subsequently 28 g of FeCl3 solution (600

μmolL in ethylene glycol) and silver nitrate (AgNO3) solution (018 g in 3 g of ethylene

glycol) were added rapidly into the PVP solution within a minute The mixture was

subjected to one of the following treatments heated at 150 degC with stirring at 600 rpm

for 1 h heated at 110 degC for 12 h at 600 rpm or heated at 110 degC for 12 h at 60 rpm

After the reaction the solutions were washed in ethanol thrice centrifuged and the

product was dispersed in ethanol for further use To break the ultralong AgNWs their

suspension was ultrasonicated at 195 KHz and 300 W for 10 min

A suspension of the AgNWs in ethanol was drop-coated onto a glass substrate and the

suspension was streamed along the longitudinal and transverse directions on the

substrate surface followed by air drying Then the AgNW electrode was prepared with

9

and without heating at 200 degC for 10 min The sheet resistance of the transparent

electrode fabricated by this method showed no large deviation at different locations of

the film

12 Characterization of the AgNW electrodes

The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was

measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co

Ltd) with the substrate as the reference The haze and Tp were measured under D65

illumination haze meter with a strong visible light source (HZ-V3 Suga Test

Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter

light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference

substrate was 92 and 01 respectively under D65 illumination Many researchers

consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis

as the generally accepted data Here we compared the Tp measured with the UV-Vis

spectra at 550 nm and the haze meter The results were identical Hence the measuring

the Tp with the haze meter is also an acceptable method to determine the transmittance

of the AgNW electrode (Figure S-1)

The sheet resistance of the electrode of 25 mm times 30 mm was measured using the

10

four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The

AgNWs on the substrate were observed using an optical microscope (VH-Z500

VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)

(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working

distance of 8 mm

2 Results and Discussion

Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)

exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)

The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It

has been known that lower reaction temperatures used in the polyol method results in

longer wires and the length was about 20 μm at 130 degC [30] In order to extend the

length we attempted to further decrease the reaction temperature to 110 degC When the

temperature was decreased to 110 degC at the same stirring speed the AgNWs

(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative

frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of

74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of

typical AgNWs However the low-temperature sample included some nanoparticles too

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

9

and without heating at 200 degC for 10 min The sheet resistance of the transparent

electrode fabricated by this method showed no large deviation at different locations of

the film

12 Characterization of the AgNW electrodes

The parallel transmittance (Tp) in the wavelength range 300 nmndash900 nm was

measured using a UVndashvisible (vis)ndashnear infrared spectrophotometer (V670 JASCO Co

Ltd) with the substrate as the reference The haze and Tp were measured under D65

illumination haze meter with a strong visible light source (HZ-V3 Suga Test

Instruments Co Ltd) The haze meter detected the average haze and Tp using y filter

light at wavelengths ranging from 400 nm to 700 nm The Tp and haze of the reference

substrate was 92 and 01 respectively under D65 illumination Many researchers

consider the Tp of the AgNW electrode measured at a wavelength of 550 nm by UV-Vis

as the generally accepted data Here we compared the Tp measured with the UV-Vis

spectra at 550 nm and the haze meter The results were identical Hence the measuring

the Tp with the haze meter is also an acceptable method to determine the transmittance

of the AgNW electrode (Figure S-1)

The sheet resistance of the electrode of 25 mm times 30 mm was measured using the

10

four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The

AgNWs on the substrate were observed using an optical microscope (VH-Z500

VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)

(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working

distance of 8 mm

2 Results and Discussion

Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)

exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)

The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It

has been known that lower reaction temperatures used in the polyol method results in

longer wires and the length was about 20 μm at 130 degC [30] In order to extend the

length we attempted to further decrease the reaction temperature to 110 degC When the

temperature was decreased to 110 degC at the same stirring speed the AgNWs

(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative

frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of

74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of

typical AgNWs However the low-temperature sample included some nanoparticles too

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

10

four-probe method (Loresta GP T610 Mitsubishi Chemical Analytech Co Ltd) The

AgNWs on the substrate were observed using an optical microscope (VH-Z500

VHX-600 Keyence Co) and a field emission scanning electron microscope (FE-SEM)

(JSM-6700F JEOL Ltd) operated at an acceleration voltage of 15 kV at a working

distance of 8 mm

2 Results and Discussion

Typical AgNWs synthesized at 150 degC using a high stirring speed (of 600 rpm)

exhibited lengths below 20 μm with over 97 cumulative frequency (Figure 1a and d)

The average length and diameter of the AgNWs were 11 μm and 68 nm respectively It

has been known that lower reaction temperatures used in the polyol method results in

longer wires and the length was about 20 μm at 130 degC [30] In order to extend the

length we attempted to further decrease the reaction temperature to 110 degC When the

temperature was decreased to 110 degC at the same stirring speed the AgNWs

(low-temperature AgNWs) grew to over 60 μm in length with over 99 cumulative

frequency (Fig 1b and d) The low-temperature AgNWs showed an average diameter of

74 nm similar to that of typical AgNWs and an average length of 19 μm twice that of

typical AgNWs However the low-temperature sample included some nanoparticles too

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

11

The two reaction temperatures affording different AgNW lengths can be reasoned as

follows High concentrations of silver nuclei upon being subjected to high temperatures

during the nucleation stage led to a large number of seeds (namely a limited amount of

the Ag+ source) that tended to develop into short AgNWs In contrast low reaction

temperatures produced limited silver nuclei which were stable in solution When new

silver nuclei were reduced in the same solution slowly due to the low temperature these

fresh nuclei tended to accumulate on the previously present silver nuclei which led to

long wires However with the severe disturbance caused by high stirring speeds some

silver nuclei tended to grow into new seeds resulting in nanoparticles (Fig 1b)

In order to observe the effect of the stirring speed the stirring speed was maintained at

60 rpm to synthesize AgNWs at 110 degC U-long AgNWs with lengths ranging to 230 μm

(Figure 1g) and with an average diameter of 91 nm were obtained as shown in Figure 1

(c) (d) The cumulative frequency of the length of the u-long AgNWs decreased to 10

for lengths below 20 μm and 88 of the wires were 20 μmndash100 μm in length (Figure 1

d) The other 2 of the wires were 100 μmndash230 μm in length To the best of our

knowledge these lengths have never been realized with a one-step polyol process until

now The average length of the u-long wires was 44 μm which is over four times longer

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

12

than typical AgNWs At 110 degC and at high stirring speeds the Ag+ ions and the fresh

nuclei are in rapid motion and are dispersed by the high flow velocity It is challenging

to collect these fresh and free nuclei to form stable and long wires (Figure 1e) On the

contrary low stirring speeds and low temperatures ie low flow velocities and low

growth rates always resulted in the collection of Ag+ ions and fresh nuclei together to

form stable and long wires under a relatively quiet growth environment (Figure 1f)

Figure 2 (a) shows the variation of the sheet resistance of the AgNW electrodes

fabricated at room temperature with only a coat-drying process The sheet resistances of

the AgNW electrodes increased with transparency The electrodes with the u-long

AgNWs showed low sheet resistances of 19 Ωndash680 Ω at a Tp of 80ndash91 The

typical AgNW electrodes showed higher sheet resistances exceeding 107 Ω and were

over the measurement range With only coating at room temperature without any

post-treatment the sheet resistance of the electrodes comprising of u-long AgNWs was

six orders lower than that of typical AgNWs Figures 2 (b) and (c) show the typical

SEM images of the two kinds of AgNW electrodes at the same transmittance The

number of AgNWs in the electrodes consisting of the typical and u-long AgNWs was 2

times 1010 and 3 times 109 per square meter respectively with a Tp of 98 (The values were 7

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

13

times 1010 and 1 times 1010 per square meter respectively at a Tp of 95) According to Bergin

et al [30] it can be assumed that the difference remains at the same level as observed in

the case under a Tp of 80ndash95 Since the AgNW electrodes exhibited a network

structure because of the contact points between the wires the number of contact points

was completely different in the two kinds of wires In addition a contact resistance

always existed between the wires The higher the number of contact points the higher is

the resistance With the typical AgNWs the electrodes did not show high conductivity

because the network included a large number of contact points with excessively high

contact resistance The small diameter also caused high contact resistance because of

the small contact area In contrast in the case of the u-long wires the contact resistance

was low because of the limited number of contact points and the large contact area

derived from the longer lengths and larger diameters which resulted in high

conductivities even without heat treatment (Figure 2a) The results indicated that the

contact resistance was crucial for achieving high-performance transparent AgNW

electrodes without post-treatments like heating pressing or illumination by a flash

lamplaser The u-long AgNW electrodes can thus be applied in plastic electronics

which require low-temperature production with just air drying

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

14

The relationship between the sheet resistance and transparency after heating are shown

in Figure 3 In the transparent range the sheet resistance of the electrodes consisting of

u-long AgNWs was always lower than the electrodes consisting of typical wires

especially over 90 transparency The sheet resistance was in the range 7 Ω ndash18 Ω

for the two electrodes with transparencies below 90 At Tp ranging from 90 to 94

the sheet resistance of the u-long AgNWs was maintained below 15 Ωndash24 Ω while

in the case of the typical AgNWs the sheet resistances were in the range 18 Ωndash112

Ω More interestingly the haze of the electrodes containing u-long AgNWs decreased

to below 4 (Fig 3) With increase in the Tp from 94 although the electrodes with

typical AgNWs were not conductive at over 96 transparency because of the lack of

junctions only the u-long AgNW electrodes showed a conductive network and

exhibited lower sheet resistances of 24 Ωndash1200 Ω The sheet resistance of 109 Ω

at a high optical transmittance of 97 was comparable or even better than monolayer

graphene sheet which shows a sheet resistance of 125 Ω [6] More importantly with

the u-long wires the haze dramatically decreased to 34ndash12 with an increased Tp of

94ndash98 which was identical haze value that shown by ITO films The haze of the

AgNW films will be discussed further subsequently

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

15

It is known that the haze of the AgNW films closely depends on the light scattering in

wires caused by the large diameters as mentioned before [25 26] The differences in

the diameters between the u-long AgNWs and typical AgNWs were clearly observed in

the UV-Vis spectra Figure 4 shows the Tp spectra of the transparent electrodes on glass

substrates at different transparencies All the spectra showed a wide flat region from 450

nm to 900 nm with the exception of two strong dips at 355 nm and 400 nm in addition

to a weak dip at 700 nm The wide flat region is advantageous for display applications

to see clearly under visible light and in solar cells as well However the strong dips at

around 355 nm and 400 nm were detected with the excitation of the localized surface

plasmon resonance due to oscillations of free electrons in a direction transverse to the

individual AgNW [31 32] In addition the dipped peak of the u-long AgNWs was

red-shifted especially around 400 nm and was broad when compared to the spectrum

of typical AgNWs The red-shift and peak broadening supported the increase in the

diameter of the AgNWs [31 32] The other weak dip at 700 nm in the Tp spectra was

most likely caused by the excitation of the propagating surface plasmon polaritons

along the longitudinal direction of the AgNWs [31] According to Groep et al the dip

should have been blue-shifted and intensified with increase in the diameter which is in

agreement with our results A difference in the diameter (about 20 nm from SEM)

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

16

between the typical and u-long AgNW was detected from the UV-Vis spectra

According to the simulation results if the diameter of the wires is small enough low

haze films with performances similar to ITO films can be achieved [26] Figure 5 (a)

shows the relationship between the haze and transparency of the AgNW electrodes The

haze of the electrodes was identical before and after heating The two kinds of AgNW

electrodes showed the same trend in terms of the haze which always linearly decreased

with increase in transparency The haze shown by the typical wires was always lower

than that shown by the u-long wires when the transparency was below 95 because the

diameter of the typical wires was only 68 nm while the diameter of the u-long wires

was larger (91 nm) The increase in the probability of light scattering owing to an

increase in diameter can be expected to be more exaggerated resulting in an increase in

haze [26] which corresponds to the slightly high haze shown by the u-long wires below

95 Tp However at Tp values exceeding 95 the haze of the u-long AgNWs

electrodes gradually evened up to the level (below 3) shown by the typical AgNWs

and was close to the values shown by traditional ITO films The result seems to imply

that the light scattering drastically decreased because of the excessively small number of

wires in these almost transparent AgNW electrodes (Fig 2c) In our case the u-long

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

17

AgNWs electrodes still maintained a network structure to achieve high conductivities

with the high Tp (Figure 3)

It is worth noting that AgNW electrodes that exhibit high performance at low haze

values have been achieved with the u-long wires In order to examine the relationship

between the haze and the length the haze shown by broken u-long AgNWs is shown in

Fig 5a The broken u-long AgNWs were of the same diameter as the u-long AgNWs

but were shorter in length (under 10 μm) as shown in Fig 5b U-long AgNWs as is and

broken showed almost the same trend however the slope observed for the typical

AgNWs was smaller than the other two The nearly identical slope shown by the u-long

and broken u-long wires suggests that the haze is independent of the length Meanwhile

these coefficients in the large diameter wires are always larger than that of the typical

wires with smaller diameter which indicated that the dependence of the scattering effect

on the diameter is a key aspect Importantly comparing the haze of AgNWs with same

diameter and different lengths (u-long AgNWs vs broken u-long AgNWs in Fig 5a) it

was found that the haze was very slightly lower in the case of the longer wires because

the less number of contact points between wires may be less exaggerate scattering

Similarly when the Tp was over 95 the relationship between haze and Tp in the

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

18

broken u-long wires followed a nearly similar trend like other two type wires However

a conductive electrode consisted by the broken u-long AgNWs was not achieved with

Tp over 95 due to less contacts between short wires Hence enough length in these

AgNWs is also an important factor as well as the diameter for low haze Therefore the

low haze of AgNWs electrodes can be achieved with small diameter or with sufficient

length ie u-long wires in the present work

Figure 5 (c) shows the model of the random network in a grid square in which five

wires were placed in a square to make eight contact points The area coverage of these

wires Ac was analyzed with a model which involved wires of the same diameter and

length The model was determined by observation of a network of real AgNWs network

in which the ratio of the number of contact points to the number of wires (designated as

M) is assumed to be 16 as nearly limit of making conductive networks The value of M

was found to be 16 at 98 Tp for the electrode of u-long AgNWs while the value was

17 at 95 Tp for an electrode with typical AgNWs (Fig 2) Also these electrodes

showed a sheet resistance of over 100 Ω The sheet resistance dramatically increased

with decrease in the M value in the two even transparent electrodes because of the small

number of wires After determining the model the value of Ac can be analyzed by

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

19

counting the black colored parts which is fitted by an expanded scale over a square grid

of several centimeters and with a high resolution bitmap image of 2000 dpi Then Ac

was used to recalculate Tp using the following equation [30]

AcC 100Tp (1)

In the above equation C is a coefficient to determine the slop for liner relationship

between Tp and Ac Ac is also related to the wire length L wire diameter D and number

of wires per unit area N by the following equation [30]

DLNAc (2)

The value of C in eq (1) was determined to be 103 and 125 for typical and u-long

AgNWs respectively by fitting an approximate line to the Tp vs Ac plot using eq (2)

and the data of N L and D which have already been shown in Fig 1 and 2 Combining

eq (1) and the approximate formula stated in Fig 5 (a) a relationship between haze and

length was obtained and is shown in Fig 5 (d) Several kinds of structures were made

with the minimum number of wires to obtain the value of Ac A small difference below

1 for Ac was observed in each various structure In fact the difference was not larger

than the wobbly borders at the edge of wires in the bitmap image which have been

shown as error bars calculated for the haze value in Fig 5 (d) The diameters of 68 nm

and 91 nm were assumed for the typical and u-long AgNWs respectively In order to

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

20

decrease the haze below 4 the length of the grid ie the wire length was dramatically

increased The calculation implied that the long wires were required to improve the haze

while maintaining a network structure For the typical AgNWs with a Tp of 96 to

achieve a haze value below 15 a wire length over 11 microm was required This is the

reason why the typical AgNWs could not achieve a network with only 11 microm in real

average length On the contrary the u-long AgNWs could accomplish the lowest haze of

09 with a Tp of 98 because the real average length of 44 microm was sufficient to make

a network when compared to the required length of 32 microm Even with the large diameter

a haze below 1 which is superior to that of ITO can be realized with sufficiently long

wires Hence transparent electrodes with a low haze can be obtained even with large

diameter AgNWs by ensuring that the wires are long enough to create a network

structure

3 Conclusions

U-long silver nanowires with lengths up to 230 μm (mainly distributed in the range of

20ndash100 μm) and a diameter of 91 nm were successfully synthesized by modifying the

one-step polyol method The synthesis was carried out at 110 degC at a stirring speed of 60

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

21

rpm The transparent electrodes with the u-long AgNWs overcame the disadvantageous

high haze confronted with the widely reported typical AgNWs In the Tp range

94ndash97 the electrode with u-long AgNWs showed a low sheet resistance of 24

Ωndash109 Ω and a low haze of 34ndash16 under visible light which surpasses the

performance of ITO The sheet resistance of 109 Ω at a high optical transmittance of

97 was comparable or even better than that of monolayer graphene sheet at 125 Ω

In general decreasing the diameter and extending the length of the wires is expected to

be the only effective strategy to fabricate highly conductive AgNW electrodes with low

haze However the lowest haze of 12 and a sheet resistance of 1200 Ω achieved at

a Tp of 98 and a further lowered haze of 09 that can potentially be achieved

according the calculations by using large diameter highly conductive AgNWs is

extremely encouraging Furthermore the electrodes fabricated with the u-long AgNWs

at room temperature without post-treatment by a simple drying process showed a sheet

resistance of 19 Ω at a transmittance of 80 which was six orders of magnitude

lower than shown by electrodes of typical AgNWs The advantages of using u-long

AgNWs of high electricaloptical performance and the room temperature processing

involved for the fabrication can be used to realize soft and flexible electrodes for plastic

electronics

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

22

Acknowledgments

T Araki acknowledges the financial support from a Grant-in-Aid for JSPS Fellows

The authors would like to thank the members of Showa Denko K K for constructive

discussions and encouragement

Electronic Supplementary Material Supplementary material (Comparison of Tp

measured by UV-Vis spectroscopy and by haze meter) is available in the online version

of this article at http

dxdoiorg101007

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

23

References

[1] Wu Z C Chen Z H Du X Logan J M Sippel J Nikolou M Kamaras K

Reynolds J R Tanner D B Hebard A F Rinzler A G Transparent

Conductive Carbon Nanotube Films Science 2004 305 1273ndash1276

[2] Cao Q Zhu Z T Lemaitre M G Xia M G Shim M Rogers J A

Transparent Flexible Organic Thin-Film Transistors That Use Printed

Single-Walled Carbon Nanotube Electrodes Appl Phys Lett 2006 88 113511

[3] Dan B Irvin G C Pasquali M Continuous and Scalable Fabrication of

Transparent Conducting Carbon Nanotube Films ACS Nano 2009 3 835ndash843

[4] Eda G Fanchini G Chhowalla M Large-area ultrathin films of reduced

graphene oxide as a transparent and flexible electronic material Nat Nanotechnol

2008 3 270ndash274

[5] Wang X Zhi L Mullen K Transparent Conductive Graphene Electrodes for

Dye-Sensitized Solar Cells Nano Lett 2008 8 323ndash327

[6] Bae S Kim H Lee Y Xu X Park J Zheng Y Balakrishnan J Lei T

Kim H Song Y Kim Y Kim K S Oumlzyilmaz B Ahn J Hong B Iijima S

Roll-to-roll production of 30-inch graphene films for transparent electrodes Nat

Nanotechnol 2010 5 574ndash578

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

24

[7] Tvingstedt K Inganas O Electrode grids for ITO-free organic photovoltaic

devices Adv Mater 2007 19 2893ndash2897

[8] OrsquoConnor B Haughn C An K H Pipe K P Shtein M Transparent and

conductive electrodes based on unpatterned thin metal films Appl Phys Lett

2008 93 223304

[9] Kang M G Kim M S Kim J S Guo L J Organic solar cells using

nanoimprinted transparent metal electrodes Adv Mater 2008 20 4408ndash4413

[10] Hu L Kim H Lee J Peumans P Cui Y Scalable Coating and properties of

transparent flexible silver nanowire electrodes ACS Nano 2010 4 2955ndash2963

[11] De S Higgins T M Lyons P E Doherty E M Nirmalraj P N Werner J

B Boland J J Coleman J N Silver Nanowire Networks as Flexible

Transparent Conducting Films Extremely High DC to Optical Conductivity

Ratios ACS Nano 2009 3 1767ndash1774

[12] Hu L Wu H Cui Y Metal nanogrids nanowires and nanofibers for

transparent electrodes MRS Bulletin 2011 36 760ndash765

[13] Madaria A R Kumar A Zhou C Large scale highly conductive and patterned

transparent films of silver nanowires on arbitrary substrates and their application

in touch screens Nanotechnology 2011 22 245201

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

25

[14] Lee J Lee P Lee H Lee D Lee S Ko S Very long Ag nanowire

synthesis and its application in a highly transparent conductive and flexible metal

electrode touch panel Nanoscale 2012 4 6408ndash6414

[15] Tokuno T Nogi M Karakawa M Jiu J Nge T Aso Y Suganuma K

Fabrication of silver nanowire transparent electrodes at room temperature Nano

Res 2011 4 1215ndash1222

[16] Mayousse C Celle C Moreau E Mainguet J Carella A Simonato J

Improvements in purification of silver nanowires by decantation and fabrication

of flexible transparent electrodes Application to capacitive touch sensors

Nanotechnology 2013 24 215501

[17] Coskun S Ates E S Unalan H E Optimization of silver nanowire networks

for polymer light emitting diode electrodes Nanotechnology 2013 24 125202

[18] Gaynor W Lee J Peumans P Fully solution-processed inverted polymer solar

cells with laminated nanowire electrodes ACS Nano 2009 4 30ndash34

[19] Gaskell J M Sheel D W Deposition of indium tin oxide by atmospheric

pressure chemical vapour deposition Thin Solid Films 2012 520 4110ndash4113

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

26

[20] Kim T Canlier A Kim G Choi J Park M Han S Electrostatic Spray

Deposition of Highly Transparent Silver Nanowire Electrode on Flexible

Substrate ACS Appl Mater Interfaces 2013 5 788minus794

[21] Cronin J P Trosky M Agrawal A Reduction of haze in tin oxide transparent

conductive coatings on glass US6268059 B1 2001

[22] Hecht D S Thomas D Hu L Ladous C Lam T Park Y Irvin G Drzaic

P Carbon-nanotube film on plastic as transparent electrode for resistive touch

screens Journal of the SID 2009 17 941ndash946

[23] Yamada T Ishihara M Hasegawa M Large area coating of graphene at low

temperature using a roll-to-roll microwave plasma chemical vapor deposition

Thin Solid Films 2013 532 89ndash93

[24] Wu J Agrawal M Becerril H A Bao Z Liu Z Chen Y Peumans P

Organic light-emitting diodes on solution-processed graphene transparent

electrodes ACS Nano 2010 4 43ndash48

[25] Katagiri K Hunakubo T Metal Nanowires method for producing same

transparent conductor and touch panel US 20120255762 A1 2012

[26] Preston C Xu Y Han X Munday J N Hu L Optical haze of transparent

and conductive silver nanowire films Nano Res 2013 in press

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

27

[27] Sun Y Gates B Mayers B Xia Y Crystalline silver nanowires by soft

solution processing Nano Lett 2002 2 165ndash168

[28] Jiu J Murai K Kim D Kim K Suganuma K Preparation of Ag nanorods

with high yield by polyol process Materials Chemistry and Physics 2009 114

333ndash338

[29] Lee J Lee P Lee D Lee S Ko S Large-scale synthesis and characterization

of very long silver nanowires via successive multistep growth Cryst Growth Des

2012 12 5598minus5605

[30] Bergin S M Chen Y Rathmell A R Charbonneau P Li Z Wiley B J

The effect of nanowire length and diameter on the properties of transparent

conducting nanowire films Nanoscale 2012 4 1996minus2004

[31] Groep J Spinelli P Polman A Transparent conducting silver nanowire

Networks Nano Lett 2012 12 3138minus3144

[32] Kottmann J P Martin O J F Smith D R Schultz S Plasmon resonances of

silver nanowires with a nonregular cross section Phys rev B 2001 64 235402

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

28

Figure 1 Optical micrographs of (a) typical AgNWs synthesized at 150 degC at 600 rpm

(b) low temperature AgNWs synthesized at 110 degC at 600 rpm and (c) u-long AgNWs

synthesized at 110 degC at 60 rpm The scale bars of the inset SEM images correspond to

300 nm (d) Relative frequencies of the length of typical low temperature and u-long

AgNWs indicating major elements under 20 μm 60 μm and 20ndash100 μm respectively

Schematic illustration of the growth of the AgNWs during synthesis (e) at high and (f)

at low stirring speeds involved in the movement and cohesion of Ag+ ions (g) The

maximum length of u-long AgNWs observed after the reaction was 230 μm

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

29

Figure 2 (a) Sheet resistance of the transparent electrode without heating SEM images

at identical Tp of (b) typical AgNWs and (c) u-long AgNWs The u-long AgNWs

revealed lesser number of wires leading to lesser number of contact points The scale

bars of the SEM images correspond to 5 μm

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

30

Figure 3 The relationship between the sheet resistance haze and Tp for typical and

u-long AgNWs The electrodes with u-long AgNWs showed lower electrical resistance

below 5 haze and over 90 Tp

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

31

Figure 4 UV-Vis spectra of typical and u-long AgNWs indicating a red-shift around the

wavelength of 400 nm in u-long AgNWs because of their larger diameters

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs

32

Figure 5 (a) The relationship between haze and Tp with an approximate formula

expressed using percentages (b) Optical micrographs of short wires fabricated by

breaking the u-long AgNWs by ultrasonication The scale bar of the image corresponds

to 10 μm (c) The model for the random network of AgNWs (d) The relationship

between haze and length of the wires obtained by using the model and eq (1) The plot

demonstrates that low haze can be realized by increasing the length of AgNWs