Embed Size (px)
Citation preview
저 시-비 리- 경 지 2.0 한민
는 아래 조건 르는 경 에 한하여 게
l 저 물 복제, 포, 전송, 전시, 공연 송할 수 습니다.
다 과 같 조건 라야 합니다:
l 하는, 저 물 나 포 경 , 저 물에 적 된 허락조건 명확하게 나타내어야 합니다.
l 저 터 허가를 면 러한 조건들 적 되지 않습니다.
저 에 른 리는 내 에 하여 향 지 않습니다.
것 허락규약(Legal Code) 해하 쉽게 약한 것 니다.
Disclaimer
저 시. 하는 원저 를 시하여야 합니다.
비 리. 하는 저 물 리 목적 할 수 없습니다.
경 지. 하는 저 물 개 , 형 또는 가공할 수 없습니다.
공학석사 학위논문
Liquid Crystalline Behaviors of
Reduced Graphene Oxide
환원된 그래핀 옥사이드의 액정 거동 특성
2016 년 2월
서울대학교 대학원
융합과학부 나노융합전공
김 민 재
공학석사 학위논문
Liquid Crystalline Behaviors of
Reduced Graphene Oxide
환원된 그래핀 옥사이드의 액정 거동 특성
2016 년 2월
서울대학교 대학원
융합과학부 나노융합전공
김 민 재
Liquid Crystalline Behaviors of
Reduced Graphene Oxide
지도교수 김연상
이 논문을 공학석사학위논문으로 제출함
2015 년 12월
서울대학교 대학원
융합과학부 나노융합전공
김 민 재
김민재의 석사학위논문을 인준함
2015 년 12월
위 원 장 송 윤 규 (인)
부 위 원 장 김 연 상 (인)
위 원 박 원 철 (인)
위 원 Giusy Scalia (인)
I
Abstract
Liquid crystalline behaviors of
reduced graphene oxide
Min Jae Kim
Department of Transdisciplinary Studies
Program in Nano Science and Technology
Seoul National University
Graphene, a honeycomb lattice made of carbon atoms has a large
surface area, high mechanical strength, high intrinsic mobility,
extreme thermal conductivity, and many other ultimate properties for
prospective applications such as transparent electrodes, flexible
devices, energy storage, biomedicine, etc. It is generally recognized
as a hydrophobic material and has poor solubility in any solvent. On
the other hand, graphene oxide (GO) that can serve as the precursor
II
of graphene is well-dispersed in aqueous environments because
graphene sheets are covered with carboxyl, epoxides and hydroxyl
groups. GO flakes would form liquid crystals (LCs) in water and
organic solvents because of its tremendously high aspect ratio and
high solubility. In the recent studies, GO LC can also be controlled by
electric fields and has a very large Kerr coefficient. Therefore they
can be very useful for electro-optical devices, such as low-power
consuming display device that is a crucial component for next
generation wearable IT devices. However, it is generally
acknowledged that GO undergoes spontaneous modification and
reduction in the room condition. So, it is remain challenge that GO LC
also undergoes reduction while they are using in electro-optical
device.
Here, we show that aqueous reduce graphene oxide (r-GO)
dispersions also can have the liquid crystalline behavior. In this study,
r-GO was synthesized by reducing GO with L-ascorbic acid which
generally called vitamin C as a reducing agent. And we applied
hexadecyltrimethylammonium bromide (CTAB) as a surfactant to
prevent the aggregation of r-GO during the reduction process. R-
GO dispersion medium with surfactant can also have LC phase that
react by external electric field and they could be a good LC phase for
III
electro-optical devices. So we can call r-GO dispersion medium with
surfactant as r-GO LC. Optical birefringence induced by external
electric field was observed in the r-GO LC and the birefringence of
r-GO LC was higher than GO LC in same concentrations. This means
that to get the retardation point that some electro-optical devices
wanted, less electrical energy needed when the r-GO LC used than
GO ones.
And it was shown that the color of the GO LC was changed gray at
the first time to black in the room condition after 1 month, meaning
that reduction of GO LC. But in case of r-GO LC, since r-GO LC was
already reduced and well dispersed with surfactant, they were very
stable in the room conditions. So there was no color change by time
in case of r-GO LC. It is apparent that staying stable state is required
for making high quality electro-optic device so r-GO LC are more
adequate material than GO LC.
Keywords: Liquid Crystals, Graphene, Graphene Oxide, Surfactant,
Birefringence, Liquid Crystalline Behaviors
Student Number: 2014-24819
IV
Table of Contents
Abstract ...................................................................Ⅰ
Table of Contents .....................................................Ⅳ
List of Figures ..........................................................Ⅵ
List of Tables ...........................................................Ⅷ
Chapter 1. Introduction .............................................1
1.1 Current Display Material Issues for Display Panels ...........1
1.2 Introduction to Liquid Crystals ............................................3
1.3 Graphene Oxide Liquid Crystal ............................................7
Chapter 2. Experiments ............................................9
2.1 Preparation of GO .................................................................9
2.2 Preparation of r-GO ...........................................................11
2.3 Fabrication of ITO patterned Cells ....................................13
2.4 Equipment for the Measurements ......................................16
Chapter 3. Characteristics of GO and r-GO..........17
3.1 Morphology characteristics.................................................17
3.2 Synthesis and Characterization...........................................24
3.3 Qualitative characteristics ..................................................26
Chapter 4 Liquid crystalline behaviors of r-GO...30
4.1 Shear force induced birefringence of r-GO.......................30
4.2 Electric field induced birefringence of r-GO ....................32
V
4.3 The transmittance of r-GO LC dependence on azimuthal
Angle ............................................................................35
4.4 Electric field induced birefringence of r-GO LCs that
used several type of surfactant ...................................38
4.5 Electro-optic switchability of r-GO LC without alignment
layer ..............................................................................41
4.6 Electro-optic measurements of GO and r-GO LC..........42
4.7 Time stability of GO and r-GO LC...................................47
Chapter 5. Conclusion ..........................................49
References ...........................................................50
요약(국문초록) ......................................................54
VI
List of Figures
Figure 1.1 Orders of liquid crystals by temperature
Figure 1.2 Common direction of liquid crystals
Figure 1.3 Optical birefringence of liquid crystals
Figure 2.1 R-GO suspension (a) with CTAB, (b) without CTAB
Figure 2.2 Photo-lithography masks for ITO patterning
Figure 2.3 ITO patterned cells that were used in this research
Figure 3.1 SEM image of (a) GO pristine and (b) r-GO pristine
Figure 3.2 SEM image of (a) 1min sonicated GO and (b) 1min
sonicated r-GO
Figure 3.3 Size distributions of (a) Pristine GO and (b) Pristine r-
GO
Figure 3.4 Size distributions of (a) 1min sonicated GO and (b) 1min
sonicated r-GO
Figure 3.5 (a) AFM image of GO and (b) height profiles of GO
Figure 3.6 (a) AFM image of r-GO and (b) height profiles of r-GO
Figure 3.7 Difference of the r-GO dispersibility depending on the
type of surfactant
Figure 3.8 XPS data: (a) before reduction and (b) after reduction
Figure 3.9 Optical spectroscopy spectra of GO an r-GO with CTAB
Figure 4.1 Shear force induced birefringence of r-GO
Figure 4.2 The transmittance of r-GO LC dependence on azimuthal
angle. (a) If the azimuthal angle is 0° or 90°, the light didn’t pass
through the crossed polarizers and cell even if the electric field was
VII
applied. (b) When the azimuthal angle is not 0° or 90°, the light is
transmitted.
Figure 4.3 Electric field induced birefringence of r-GO LCs that used
(a) CTAB, (b) SDS and (c) Brij as a surfactant.
Figure 4.4 R-GO LC using nonionic surfactant aggregations that were
aimed to electric direction when electric fields were applied.
Figure 4.5 Effective birefringence of (a) r-GO LC and (b) GO LC
at 475㎚.
Figure 4.6 Effective birefringence of (a) r-GO LC and (b) GO LC
at 550㎚.
Figure 4.7 Effective birefringence of (a) r-GO LC and (b) GO LC
at 650㎚.
Figure 4.8 Stability of (a) r-GO LC and (b) GO LC in room condition
VIII
List of Tables
Table 4.1 Electric field induced birefringence of r-GO
1
Chapter 1. Introduction
1.1 Current Display Material Issues for Display Panels
Over the past decades, display panel technology has been rapidly
developed. The quality and productivity of display panels has
enhanced dramatically while simultaneously costs have gradually cut
down. Display panels are shown in products as small and light as
smart watches and as large as 80-inch public display panels. In the
near future, next generation display will be developed such as
transparent and foldable display.1
In recent years, Korean electronics companies, Samsung Display
and LG Display, have played a leading role in display industry.2 They
have made over 50% of total world display panel sales and had a good
impact on Korean exports.3 However, important materials for display
panel have been monopolized by a few Japanese and European
companies. Kuraray is a leading Japanese chemicals firm that began
as a chemicals and textiles producer. Its share of the global market
for the PVA film used in LCDs is 80 percent.4 Fujifilm and Konica is
also Japanese chemicals firm, Fujifilm now boasts an 80 percent
global share for the polarizing film used in LCDs (TAC film), with the
remaining 20 percent held by Konica.4 Alignment layer used in LCDs
are also monopolized by Japanese companies.5 Liquid crystals which
are the most important material in LCD panel and determine response
2
time and viewing angle of the panel have also been monopolized
Merck which is German and Japanese firms, Chisso.6 These
companies have made a profit under any circumstance even though
panel makers showed a great loss and recorded a trade deficit
because they are monopolized and have own unique technology. So
Korea government and managers of panel makers have wanted to
have own these materials or develop domestic company to supply
them. Especially, liquid crystals of display panels have the largest
market size of the display materials and determine properties of
panels so it cannot be emphasize enough to develop own material. In
these circumstance, researchers in Korean university have shown
that graphene oxide is an attractive novel liquid crystal material for
the excellent electro-optic performance and could be a good
alternative last year.7-9
3
1.2 Introduction to Liquid Crystals
Liquid crystals (LCs) are intermediate states between liquids and
solids, LCs mean mesomorphic phases not materials. Mechanical
properties and symmetry properties of LCs are intermediate between
those of liquid and those of solid, so LCs have fluidity and partial
ordering.10 LCs can be classified in thermotropic and lyotropic
depending on the factors determining the phase formation.
Thermotropic LCs are determined their phase by temperature and
lyotropic LCs are by concentrations. Thermotropic LCs can be
classified by temperature from crystalline, smectic, nematic and
liquid as shown in figure 1.1. LC phases have common direction
indicated by ‘n’ means averaged direction of LC molecules as shown
in figure 1.2. In the nematic phase, LC molecules have an orientation
order that aligned by the common direction n. At the lower
temperature range between the melting point and nematic phase
region, smectic phase are found in certain compounds. This phase
have orientation order and moreover positional order. Nematic LCs
have been widely used in display devices such as LCD panels because
of their anisotropic properties and responsiveness to external
electric fields. 11
The degree of liquid crystal’s orientation order is expressed by the
order parameter, S.
4
S = 2
3 <cos2θ -1>
where, θ is the angle represents the fluctuation of LC molecules
away from the common direction n.12 The order parameter of
crystalline is 1 and liquid is 0. LCs have intermediate value between
0 and 1.12
Liquid crystals have optically birefringence (Δn), the difference
between refractive index of two optical axes, ordinary and
extraordinary, as shown in figure 1.3.
Δn = ne - no
If Δn > 0, the LC has positive birefringence and if Δn < 0, it has
negative birefringence.13
The optical path difference (Δ) in LCs can be expressed by the
thickness (d) of the LCs and the birefringence (Δn).14
Δ= d Δn
From this expression, the magnitude of the phase difference can
be expressed
Δφ= (2𝜋
𝜆)dΔn
where λ is the wavelength of the incident light.14 Normally, light
cannot pass through the crossed polarizer but if there are liquid
crystal phase existing, the optical path difference of the liquid
crystals makes the light pass through the crossed polarizers. 14
5
Figure 1.1 Orders of liquid crystals by temperature
Figure 1.2 Common direction of liquid crystals
6
Figure 1.3 Optical birefringence of liquid crystals
7
1.3 Graphene Oxide Liquid Crystal
Graphene, a honeycomb lattice made of carbon atoms is known for
its large surface area and outstanding properties having high
mechanical strength, high intrinsic charge mobility, extreme thermal
conductivity, and many other ultimate properties for prospective
applications such as transparent electrodes, flexible devices, energy
storage, biomedicine, etc.15-21 It is generally recognized as a
hydrophobic material and has poor solubility in any solvent.22
However, graphene oxide (GO) which can serve as precursor of
graphene, is well-dispersed in aqueous environments because
graphene sheets are covered with carboxyl, epoxides and hydroxyl
groups.23 The dispersability of GO in water is very useful for an easy
handling, unlike reduced graphene, making it very suitable for large-
scale applications and integration in devices or materials. In fact GO
has already been used in a large number of applications such as bio-
devices,24, 25 composites,26, 27 drug delivery materials28, 29 and
optoelectronics.30, 31
Interestingly, GO flakes can form liquid crystal phases in water and
organic solvents because of its tremendously high aspect ratio and
high solubility.32, 33 GO LC can also be switched by electric fields of
very low strength due to a very large Kerr coefficient.8 The Kerr
effect is a second-order electro-optic effect.34 In a Kerr medium,
the electric-field-induced birefringence is proportional to the
8
electric field (E) as
Δn = λKE2 ,
where λ is the wavelength and K is the Kerr coefficient.34
Therefore they can be very useful for next generation displays
such as wearable IT device because low-power consuming is a
crucial component for them.7-9 However, it is generally
acknowledged that GO undergoes spontaneous modification and
reduction in the room condition.35-38 Since the reduced graphene
oxide is not dispersable in water its flakes aggregate during reduction,
deteriorating the suspension quality. So, it is a problem that GO LC
undergoes reduction during the operation in electro-optical devices.
In this thesis it is shown that r-GO dispersions with surfactants
not only are very stable but can also form liquid crystals (LCs) in
water by shearing and can be controlled by electric field. R-GO LC
doesn’t undergo chemical modification and reduction in the room
temperature and have superior electro-optic performance with much
higher retardation induced by external electric filed than GO LC.
9
Chapter 2. Experiments
2.1 Preparation of GO
GO was obtained by chemically oxidizing and exfoliating natural
graphite according to Hummers methods.39 Natural expendable
graphite was purchased from Qingdao Xinghe Graphite co. The
characteristics of GO such as dimension are determined by the type
of the graphite.40 Expendable graphite enables easy exfoliation with
few damages of the layers. All reagents except graphite were
purchased from Sigma Aldrich. The graphite (1g) and sodium nitrate
(NaNO3, 1g) as an oxidant were added into a flask at 0℃. Sulfuric
acid (H2SO4, 48㎖) was added very slowly to the flask and stirred for
20 min with the speed of 400 rpm. Potassium permanganate (KMnO4,
6g) as a strong oxidant were slowly added to the flask. The color of
mixture turned green as the formation of the MnO3+ which is an
oxidizing agent. As lots of heat can be produced during the process,
the flask must be maintain around 0℃ by putting the flask in ice bath.
The reaction continued 2hr in the ice bath then distilled water (40㎖)
were added to the mixture very slowly and carefully in drop by drop
for 30 minutes to prevent violent reaction until the mixture turned
from green to purple. Distill water (100㎖) were added into the
mixtures then the color of mixtures changed to dark red brown.
In order to remove bi-product like as unreacted oxidant MnO2,
10
hydrogen peroxide solution (H2O2, 5㎖) were added. The mixtures
have many yellow-greenish bubbles and finally turned bright green
by the reduction of MnO2 to Mn2+ by hydrogen peroxide solution.
Then the mixtures were centrifuged for 10 minutes with a speed of
1200 rpm and the upper transparent part of mixtures were removed
and added the distilled water, repetitively. By this step, the Mn2+ and
H+ were washed out. The yellow part of mixtures appeared and they
turned to dark brown, finally, which means GO suspension by
repeating the centrifugation. This step repeated until the upper
solution reached around 7 pH meaning that GO suspension has few
bi-product.
11
2.2 Preparation of r-GO
R-GO was obtained by reducing GO with L-ascorbic acid generally
called vitamin C. Commonly, the aggregation of reduced GO occurs
during the reduction process to diminish the entropy of the system
by decreasing exposed hydrophobic surfaces to the water and van
der Waals attraction force.41, 42 To prevent aggregation of r-GO we
have used bromide (CTAB) surfactant during the reduction process.
CTAB as a surfactant can cover the hydrophobic surface and help to
form stable dispersions in water. On the specifics, we mixed
0.1mg/ml GO suspension 50mg of 2wt% of CTAB aqueous solution
and then 50mg of L-Ascorbic acid was added then everything was
stirred for 72hr. As a result, well dispersed r-GO LC was obtained.
As shown in figure 2.1, suspension r-GO with CTAB was well
dispersed and stable but only r-GO in water aggregated.
12
Figure 2.1 R-GO suspension (a) with CTAB, (b) without CTAB
(a) (b)
13
2.3 Fabrication of ITO patterned Cells
We made several types of cells that were made with ITO patterned
glass to measure electro-optic properties of LCs. ITO patterned
glass was made by chemical etching process by following these steps.
ITO covered glasses were covered with photoresist (GXR601), they
were baked on the hot plate at 110℃ for 1min, then exposed to UV
rays for 10 seconds with being covered with a photo-lithography
mask. Several types of photo-lithography mask were used as shown
in figure 2.2. Then they were dipped into the developer (DPD-200)
then UV rays exposed photoresist were removed and non-exposed
photoresist by being shielded with mask were existed. They were
baked on the hot plate at 110℃ for 10 minutes. And they were dipped
into the etchant (LCE-12) for 25 minutes then the non-protective
parts of ITO by photoresist were eliminated. ITO patterned glass
were covered with bare glass maintaining the cell gap, the distance
between ITO patterned glass and glass, was 3㎜ as shown in figure
2.3. In case of the cell that made by using mask figure 2.2-(b), four
cells were made by one etching process as dividing into 4 parts the
glass. The shortest distance between ITO patterns in the cells,
figure 2.3-(a), (b), (c), was 4㎜, 2㎜ and 3㎜, respectively.
14
Figure 2.2 Photo-lithography masks for ITO patterning
(a)
(b)
(c)
15
Figure 2.3 ITO patterned cells that were used in this research
(a)
(b)
(c)
16
2.4 Equipment for the Measurements
The flakes of GO an r-GO was identified X-ray photoelectron
spectroscopy (XPS; theta probe base system, Thermo Fisher
Scientic Co.) and UV-VIS spectrometer (Lambda 35, Perkin Elmer).
The flakes of GO an r-GO flakes were observed for examination of
size distribution by field-emission scanning electron microscopy
(FE-SEM; S-4300, Hitachi). The dispersion state of GO and r-GO
observed by atomic force microscopy (AFM; XE100, PSIA). The
centrifuge (Smart R17, Hanil science industrial) was used for phase
separation of GO and r-GO and tip sonotrode (UIS250L, Hielscher
ultrasonics) was used to break the GO and r-GO flakes.
For electro-optic experiments, the electric fields were applied by
function generator (33500B, Agilent) with amplifying by Voltage
amplifier (A400, FLC electronics) and they were checked by
oscilloscope (TDS 2014B, Tektronics). In these experiments,
Spectrometer (ULS2048, Avaspec) was used as photo-receiver.
17
Chapter 3. Characteristics of GO and r-GO
3.1 Morphology characteristics
Scanning Electron Microscope (SEM) was used to examine the
dimension of the GO and r-GO flakes. The flakes were deposited on
SiO2 by bubble deposition method that is a useful method to get
uniform and unwrinkled flakes.43 The size distributions of GO and rGO
are shown in figure 3.3 and 3.4. Large number of flakes has diameters
of about 10-20 micrometers (figures 3.1 and 3.2) with a slight shift
in value and broadening of the size distribution after reduction.
Smaller flakes were obtained breaking the larger flakes by ultra-
sonication with tip sonotrode (Hielscher ultrasonics GmbH, UIS250L)
applying 90% amplitude and 0.5 cycle for 1min. The diameter of
sonicated GO and r-GO is drastically reduced to a value of about 3㎛.
The reason for using also smaller flakes is that the larger flakes tend
to aggregate during reduction while smaller show better performance.
Figure 3.5 and 3.6 show atomic force microscopy (AFM) image of
GO and r-GO sample. Thickness profiles along the white line on AFM
images show that the thickness of GO and r-GO flakes is about 1㎚,
indicating that r-GO and GO flakes were single layered. They
confirm that there were no aggregations during the process.
18
Figure 3.1 SEM image of (a) GO pristine and (b) r-GO pristine
(a)
(b)
19
Figure 3.2 SEM image of (a) 1min sonicated GO and (b) 1min
sonicated r-GO
(a)
(b)
20
0 10 20 30 40 50 60 700
10
20
30
40
50
60F
req
ue
ncy(%
)
Diameter (um)
Pristine GO
0 10 20 30 40 50 60 70 800
10
20
30
40
fre
qu
en
cy(%
)
Diamter(um)
Prisitne r-GO
Figure 3.3 Size distributions of (a) Pristine GO and (b) Pristine r-
GO
(a)
(b)
21
0 2 4 6 8 10 12 140
10
20
30
40
50
60
70
80
Fre
qu
en
cy(%
)
Diameter(um)
r-GO 1min sonic
Figure 3.4 Size distributions of (a) 1min sonicated GO and (b) 1min
sonicated r-GO
0 2 4 6 8 10 12 14 16 180
10
20
30
40
50
60
70F
req
ue
ncy(%
)
Diameter(um)
GO 1min sonic(a)
(b)
22
Figure 3.5 (a) AFM image of GO and (b) height profiles of GO
(a)
(b)
23
Figure 3.6 (a) AFM image of r-GO and (b) height profiles of r-GO
(a)
(b)
24
3.2 Synthesis and Characterization
Surfactants are classified as nonionic, anionic and cationic according
to the charge in the head groups. We applied several type of
surfactant during the reduction process and turned out cationic
surfactant such as CATB is the best type of surfactant to make r-
GO LC. All type of surfactant were able to produce well dispersed r-
GO LC for a while, but anionic surfactants such as Sodium dodecyl
sulfate (SDS) did not have long term stability and started to
aggregate after 1 week. In contrast, cationic and nonionic surfactant
such as Brij L4 (Polyethylene glycol dodecyl ether, Polyoxyethylene
lauryl ether) didn’t show aggregation after 1 year as shown in the
figure 3.7. R-GO LC having nonionic surfactant has stability in room
condition, but it started to aggregation with electric filed as shown
later part in figure 4.4.
25
Figure 3.7 Difference of the r-GO dispersibility depending on the
type of surfactant
26
3.3 Qualitative characteristics
X-ray photoelectron spectroscopy (XPS) was employed to confirm
that reduction of graphene oxide by L-ascorbic acid occurs also in
presence of CTAB. Figure 3.8 shows the C 1s XPS spectra of GO
before and after the reduction with L-ascorbic acid and CTAB.
Before reduction and the addition of CTAB (figure 3.8-(a)), the
sample showed four different peaks centered at 284.5, 286.6, 287.5
and 288.6 eV, corresponding to C=C/C-C in aromatic rings, C-O-C,
C=O and O=C-OH groups respectively. After the reduction with L-
ascorbic acid in presence of CTAB, the intensities of C 1s peaks of
the carbons binding to oxygen, particularly C-O-C, decreased
dramatically, showing that most oxygen including functional groups
were removed after the reduction. A new peak appeared at 285.8eV
corresponding to C-N bond. Although XPS sample was made by
centrifuging r-GO LC and replacing supernatant liquid with deionized
distilled water and repeating this process several times to remove
CTAB in r-GO LC, some CTAB molecules remained in the r-GO.
This is revealed by the presence of the peak at 285.8 eV connected
to the C-N (the head part of the CTAB) as shown in figure 3.8. The
observed increase and decrease in intensity of the different peaks
connected to the functional groups indicates the removal of the
groups and recovery of the carbon network thus confirming that GO
can be reduced to r-GO with L-ascorbic in presence of CTAB
27
The optical spectroscopy by UV-VIS spectrometer (Lambda 35,
Perkin Elmer) was also conducted to confirm the reduction. As seen
in figure 3.9, the peak in the 227-232 ㎚ region determines the
grade of the remaining conjugation according to the C-C and C=C
bonds of graphene oxide. The peak at around 300 ㎚ corresponds to
the carbonyl groups in graphene oxide. The right shift observed in
r-GO peak around 290 ㎚ after reduction means that the carbonyl
groups in graphene oxide decreased and C-C and C=C bonds
increased after the reduction.
28
290 288 286 284 282
Inte
nsity (
a.u
.)
Binding Energy (eV)
Raw data
Fitting line
288.6 eV
287.5 eV
286.6 eV
285.8 eV
284.5 eV
Figure 3.8 XPS data: (a) before reduction and (b) after reduction
290 288 286 284 282
Binding Energy (eV)
Inte
nsity (
a.u
.)
Raw data
Fitting line
288.6 eV
287.5 eV
286.6 eV
284.5 eV
(a)
(b)
29
Figure 3.9 Optical spectroscopy spectra of GO an r-GO with CTAB
200 300 400 500 600 700 800 900
0.00
0.25
0.50
0.75
1.00
1.25
Ab
so
rba
nce
Arb
. U
.
Wave length(nm)
GO
rGO
30
4 Liquid crystalline behaviors of r-GO
4.1 Shear force induced birefringence of r-GO
Shear force induced birefringence observed when a bottle containing
0.2wt% r-GO dispersion was shaken. Before the shaking r-GO
dispersion appeared dark in the bottle between the crossed polarizers
illuminated by a white light source, indicating an isotropic phase.
When the bottle was shaken, however, bright lines showed indicating
shear force induced birefringence as shown in the figure 4.1 and
meaning that r-GO dispersion has anisotropic state by shear force.
They disappeared within few seconds after the shaking stopped.
Shear force induced birefrigence was best seen in 0.2wt% and r-GO
dispersion became too dark to observe birefringence over the 0.2wt%.
31
Figure 4.1 Shear force induced birefringence of r-GO
Shear force s
32
4.2 Electric field induced birefringence of r-GO
There are no reports about the shear force or electric field induced
birefringence in r-GO dispersions so far due to the instability of the
dispersions. Here we show that using surfactants it is possible to
switch the optical state even of graphene suspensions good for
electro-optical devices. So an isotropic state also can turn
birefringent with characteristics, explained below, that suggest the
induction of a liquid crystal phase. R-GO LC was an isotropic state
without electric field, but turned birefringent with electric field. To
observe the occurrence of birefringence in the r-GO LC, cells with
substrates with patterned indium-tin-oxide (ITO) electrodes, filled
with r-GO LC, were placed between crossed polarizers. The design
of the cells is shown in figure 2.3, where the distance between two
ITO electrodes is 1000㎛ and the cell gap is 300㎛. When the cell
was placed between the crossed polarizers illuminated by a white
light source, no light passed through the crossed polarizers and cell.
But if an electric field of 10Vmm-1 and frequency of 100 kHz was
switched on light passed through the system as visible in the insets
of table 4.4. This is an immediate and straightforward way to detect
the macroscopic alignment of graphene in suspensions. In several
designs of cells, r-GO LC was switchable by electric fields as shown
in the table 4.1. The r-GO LC was electro-optically responsive
33
meaning that an external field had an aligning effect. When the
electric field weren’t applied, r-GO flake’s directions were
randomly but when the electric fields were applied, r-GO flakes were
aimed to electric filed direction and this direction order made the r-
GO dispersions to act as liquid crystal in electro-optic device like
the liquid crystal display.
34
Table 4.1 Electric field induced birefringence of r-GO
35
4.3 The transmittance of r-GO LC dependence on
azimuthal angle
The transmittance T of light passing through an LC film placed
between crossed polarizers has the expression:
where α is the azimuthal angle of the LC that means the angle
between transmittance axis of polarizer and the LC average alignment
direction, λ is the incidence wavelength, neff the effective index of
the LC, no the ordinary index of the LC and d is the cell gap
corresponding to the geometrical path of light within the LC film.44
Also in case of r-GO LC, analogously to standard LC, the
transmittance depends on the azimuthal angle. When rotating the cell
while applying the electric field the light transmission changes
depending if the LC director is aligned along one of the polarizers (α
= 0° or 90°) as shown in figure 4.2-(b) or at an angle α ≠ 0 as
shown in figure 4.2-(c). If the azimuthal angle is 0° or 90°, the
light didn’t pass through the crossed polarizers and cell even if the
electric field was applied. When the azimuthal angle is not 0° or
90°, the light is transmitted.
On the basis of this experiment, we can say that all graphene flakes
of r-GO LC aimed to electric field direction. When we applied the
])([sin2sin 22 dnnT oeff
36
electric field to r-GO LC, if there were any r-GO flakes to another
direction except electric field direction, a little bit of light had to pass
through the crossed polarizers even through the cell were aligned to
electric field direction which meant that the azimuthal angle of the
cell is 0°. But as shown in figure 4.2-(b), no light passed through
the crossed polarizers, so there were no flake that was aligned
another direction except electric field direction. Electric field
controls the direction of r-GO flakes. Without electric field r-GO
flakes aimed randomly, so this state meaning isotropic state of r-GO
as shown in figure 4.2-(a), so no light can pass through the crossed
polarizers. With electric field all of r-GO flake aims one direction, so
this state make r-GO anisotropic state, so light can pass through the
crossed polarizer depended on azimuthal angle.
37
Figure 4.2 The transmittance of r-GO LC dependence on azimuthal
angle. (a) If the azimuthal angle is 0° or 90°, the light didn’t pass
through the crossed polarizers and cell even if the electric field was
applied. (b) When the azimuthal angle is not 0° or 90°, the light is
transmitted.
(a) (b)
(c)
38
4.4 Electric field induced birefringence of r-GO LCs
that used several type of surfactant
R-GO LCs that used nonionic, anionic and cationic surfactant,
respectively, were applied electric field in the cells. All of the r-GO
LC was able to observe the occurrence of birefringence. Even r-GO
LC using nonionic surfactant was able to have switching property,
meaning that force of aiming r-GO flake to electric filed direction is
not charge of the surfactant but polarization of the r-GO itself.
In case of r-GO LC using anionic, some aggregation of r-GO flakes
was shown in the cell, so it was not proper surfactant to make r-GO
LC. R-GO LC using nonionic surfactant didn’t show r-GO flake
aggregation before electric field applied but r-GO flake aggregations
that were aimed to electric direction, interestingly. R-GO LC using
cationic surfactant was stable and did not show any aggregation while
electric field applied, so cationic type surfactant must be used to
make r-GO LC.
This experiment meaning that force of aiming r-GO flake to electric
filed direction is not the charge of the surfactant but polarization of
the r-GO itself, because r-GO LC that use nonionic surfactant
showed the occurrence of birefringence.
39
Figure 4.3 Electric field induced birefringence of r-GO LCs that used
(a) CTAB, (b) SDS and (c) Brij as a surfactant.
CTAB SDS Brij
(b) (a) (c)
40
Figure 4.4 R-GO LC using nonionic surfactant aggregations that were
aimed to electric direction when electric fields were applied.
41
4.5 Electro-optic switchability of r-GO LC without
alignment layer.
In general, liquid crystals in LCDs change their state anisotropic to
another anisotropic by electric field changing the direction of liquid
crystal molecules so they need to stay a certain anisotropic state
without electric field using alignment layer. However, the modulation
of the transmission of r-GO LC is obtained from a field-free dark
state that is isotropic thus doesn’t need any external force to bright
state that is anisotropic maintained by electric field. So r-GO LC
doesn’t need alignment layer. It is an advantage that electro-optic
device can be made without alignment layer because skipping one
process step in the manufacturing of displays. And the materials of
alignment layer also have been monopolized by foreign companies,
skipping the layer can give us the great cost reduction.
42
4.6 Electro-optic measurements of GO and r-GO LC
The electro-optical responses of GO LC and r-GO LC were
measured using identical values of external electric field, cells still
with patterned indium-tin-oxide (ITO) electrode were used but with
a different design as before. The sketch of the cells is shown in figure
2.3-(b), where the electrodes are placed on the same side for
obtaining field lines parallel to the substrates across the electrode
gap not to have gap variance between the electrodes. If the
electrodes are different sides, the gap between the electrodes can be
different by how two electrodes were set up, so making uniform gap
of many cells is very difficult. However the electrodes are placed on
the same side, the gap between the electrodes is decide by the mask
design that are using when ITO electrode is etched, so uniform
electrodes gaps of the many cells are easily made. The distance
between the two ITO electrodes was 2㎜ and the cell gap was 300㎛.
An external voltage with a square waveform and a frequency of 100
kHz and varying amplitudes was applied to the cells.
The effective birefringence (Δn) of GO and r-GO were measured
by these cells. The effective birefringence was calculated as
Here, I and I0 are the light intensity under crossed polarizers with
electric fields and parallel polarizers without electric fields. As
)(sin 2
0
ndII
43
apparent in figure 4.5, 4.6 and 4.7, r-GO LC had much higher induced
birefringence, than GO LC at same concentration in the various
wavelengths. For low concentrations, GO LC almost didn’t react by
electric field, but in contrast r-GO LC clearly responded to the field
even for the lowest concentration (0.1mg/ml). For high
concentrations, r-GO LC had almost twice higher induced
birefringence than GO LC. This means that to get a specific
retardation value required by certain electro-optical devices, less
electrical energy is needed when the r-GO LC is used instead of GO.
R-GO has higher electrical conductivity and high intrinsic charge
mobility than GO. Therefore r-GO has more easily polarizable than
GO and r-GO flakes are more easily aimed to electric filed direction
than GO, so r-GO LC has higher birefringence than GO LC. And these
properties maintained several wavelength, 475㎚, 550㎚ and 650㎚.
44
0 2 4 6 8 10 120.0
0.5
1.0
1.5
2.0
2.5
n(×
10
-5)
at 475nm
Electric filed(Vmm-1)
r_GO 0.1
r_GO 0.25
r_GO 0.5
r_GO 0.75
r_GO 1.0
0 2 4 6 8 10 120.0
0.5
1.0
1.5
2.0
2.5
n(×
10
-5)
at 475nm
Electric filed(Vmm-1)
GO 0.1
GO 0.25
GO 0.5
GO 0.75
GO 1.0
Figure 4.5 Effective birefringence of (a) r-GO LC and (b) GO LC
at 475㎚.
(a)
(b)
45
0 2 4 6 8 10 120.0
0.5
1.0
1.5
2.0
2.5 r-GO 0.1
r-GO 0.25
r-GO 0.5
r-GO 0.75
r-GO 1.0
n(×
10
-5)
at 550nm
Electric filed(Vmm-1)
0 2 4 6 8 10 120.0
0.5
1.0
1.5
2.0
2.5 r-GO 0.1
r-GO 0.25
r-GO 0.5
r-GO 0.75
r-GO 1.0
n(×
10
-5)
at 550nm
Electric filed(Vmm-1)
Figure 4.6 Effective birefringence of (a) r-GO LC and (b) GO LC
at 550㎚.
(a)
(b)
46
0 2 4 6 8 10 120.0
0.5
1.0
1.5
2.0
2.5
n(×
10
-5)
at 650nm
Electric filed(Vmm-1)
r_GO 0.1
r_GO 0.25
r_GO 0.5
r_GO 0.75
r_GO 1.0
0 2 4 6 8 10 120.0
0.5
1.0
1.5
2.0
2.5
n(×
10
-5)
at 650nm
Electric filed(Vmm-1)
GO 0.1
GO 0.25
GO 0.5
GO 0.75
GO 1.0
Figure 4.7 Effective birefringence of (a) r-GO LC and (b) GO LC
at 650㎚
(a)
(b)
47
4.7 Time stability of GO and r-GO LC
According to recent studies, 35-38 it has been proved that GO
undergoes spontaneous modification and reduction in the room
conditions. Also in our experiments, the color of the GO LC changed
becoming first gray than black in the room condition after 1 month,
indicating reduction of GO LC. The reduction of GO LC with time can
cause aggregations and deteriorate optical switching properties. But
in case of our controlled reduction in presence of surfactant, the well
dispersed r-GO LC were very stable even in the room conditions and
it was verified that there were no color changes with time. It is
apparent that having stable states is an essential requirement for
making high quality electro-optic device so r-GO LC is a more
adequate material than GO LC.
48
Figure 4.8 Stability of (a) r-GO LC and (b) GO LC in room condition
49
Chapter 5. Conclusion
GO flakes can form liquid crystal phases and also be switched by
electric fields of very low strength due to a very large Kerr
coefficient. GO LC can also be switched by electric fields of very low
strength due to a very large Kerr coefficient. Therefore they can be
very useful for next generation displays such as wearable IT device
because low-power consuming is a crucial component for them. We
found out another graphene based LC. That is r-GO dispersions with
cationic surfactant, they can also have LC phase that react by
external electric field. They showed much high effective retardation
by external electric filed than GO LC and doesn’t undergo chemical
modification and reduction in the room temperature so it is more
stable than GO LC. R-GO LC have another awesome advantage that
is electro-optic device can be made without alignment layer with r-
GO LC. There it can give cost save and skip the alignment layer
process in the factory.
.
50
References
1. Fujikake, H.; Kikuchi, H.; Fujisawa, T.; Kobayashi, S. Next-Generation
Liquid Crystal Displays. In The Liquid Crystal Display Story, Springer: 2014; pp
207-239.
2. Tabata, M. Risk and Mobility: A Case Study of the Thin-Film Transistor
Liquid-Crystal Display Industry in East Asia. East Asian Science, Technology and
Society 2015, 9, 151-166.
3. Lee, Y.-S.; Heo, I.; Kim, H. The role of the state as an inter-scalar mediator
in globalizing liquid crystal display industry development in South Korea. Review of
International Political Economy 2014, 21, 102-129.
4. Junjiro, S.; Youngwon, P. Japan's Position in East Asia's IT Industrial
Networks. SERI Quarterly 2012, 5, 39.
5. Kobayashi, S.; Kuroda, K.; Matsuo, M.; Nishikawa, M. Alignment Films
for Liquid Crystal Devices. In The Liquid Crystal Display Story, Springer: 2014; pp
59-80.
6. Jasper, C.; Sage, I. Liquid Crystals for Display Application. Ullmann's
Encyclopedia of Industrial Chemistry 2013.
7. Shen, T. Z.; Hong, S. H.; Song, J. K. Effect of centrifugal cleaning on the
electro-optic response in the preparation of aqueous graphene-oxide dispersions.
Carbon 2014, 80, 560-564.
8. Shen, T. Z.; Hong, S. H.; Song, J. K. Electro-optical switching of graphene
oxide liquid crystals with an extremely large Kerr coefficient. Nat Mater 2014, 13,
394-9.
9. Hong, S. H.; Shen, T. Z.; Song, J. K. Electro-optical Characteristics of
51
Aqueous Graphene Oxide Dispersion Depending on Ion Concentration. J Phys Chem
C 2014, 118, 26304-26312.
10. Yeh, P.; Gu, C. Optics of liquid crystal displays. John Wiley & Sons: 2010;
Vol. 67.
11. Gin, D. L.; Pecinovsky, C. S.; Bara, J. E.; Kerr, R. L. Functional lyotropic
liquid crystal materials. In Liquid Crystalline Functional Assemblies and Their
Supramolecular Structures, Springer: 2008; pp 181-222.
12. Priestly, E. Introduction to liquid crystals. Springer Science & Business
Media: 2012.
13. Li, J.; Gauza, S.; Wu, S.-T. Temperature effect on liquid crystal refractive
indices. Journal of applied physics 2004, 96, 19-24.
14. El-Dessouki, T. A.; Roushdy, M.; Hendawy, N. I.; Naoum, M. M.; Zaki, A.
A. Optical Measurements of Thermotropic Liquid Crystals. 2013.
15. Yin, Z.; Zhu, J.; He, Q.; Cao, X.; Tan, C.; Chen, H.; Yan, Q.; Zhang, H. Adv.
Energy Mater. 2014.
16. Huang, X.; Zeng, Z.; Fan, Z.; Liu, J.; Zhang, H. Adv. Mater. 2012, 24, 5979.
17. Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Ji, W.; Potts, J. R.; Ruoff, R. S. Adv.
Mater. 2010, 22, 3906.
18. He, Q.; Wu, S.; Yin, Z.; Zhang, H. Chem. Sci. 2012, 3, 1764.
19. Novoselov, K. S.; Fal’ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.;
Kim, K. Nature 2012, 490, 192.
20. Geim, A. K. Graphene: status and prospects. Science 2009, 324, 1530-4.
21. Liu, J.; Lin, Z.; Liu, T.; Yin, Z.; Zhou, X.; Chen, S.; Xie, L.; Boey, F.; Zhang,
H.; Huang, W. Small 2010, 6, 1536.
52
22. Behabtu, N.; Lomeda, J. R.; Green, M. J.; Higginbotham, A. L.; Sinitskii,
A.; Kosynkin, D. V.; Tsentalovich, D.; Parra-Vasquez, A. N.; Schmidt, J.; Kesselman,
E.; Cohen, Y.; Talmon, Y.; Tour, J. M.; Pasquali, M. Spontaneous high-concentration
dispersions and liquid crystals of graphene. Nat Nanotechnol 2010, 5, 406-11.
23. Eigler, S.; Hirsch, A. Chemistry with graphene and graphene oxide-
challenges for synthetic chemists. Angew Chem Int Ed Engl 2014, 53, 7720-38.
24. Wang, Y.; Li, Z. H.; Hu, D. H.; Lin, C. T.; Li, J. H.; Lin, Y. H.
Aptamer/Graphene Oxide Nanocomplex for in Situ Molecular Probing in Living
Cells. J. Am. Chem. Soc. 2010, 132, 9274-9276.
25. Chang, Y. L.; Yang, S. T.; Liu, J. H.; Dong, E.; Wang, Y. W.; Cao, A. N.;
Liu, Y. F.; Wang, H. F. In vitro toxicity evaluation of graphene oxide on A549 cells.
Toxicology Letters 2011, 200, 201-210.
26. Qi, X.; Pu, K. Y.; Li, H.; Zhou, X.; Wu, S.; Fan, Q. L.; Liu, B.; Boey, F.;
Huang, W.; Zhang, H. Angew. Chem., Int. Ed. 2010, 49, 9426.
27. Nguyen, S. T.; Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas,
K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Ruoff, R. S. Nature 2006, 442, 282.
28. Sun, X. M.; Liu, Z.; Welsher, K.; Robinson, J. T.; Goodwin, A.; Zaric, S.;
Dai, H. J. Nano-Graphene Oxide for Cellular Imaging and Drug Delivery. Nano
Research 2008, 1, 203-212.
29. Liu, Z.; Robinson, J. T.; Sun, X. M.; Dai, H. J. PEGylated nanographene
oxide for delivery of water-insoluble cancer drugs. J. Am. Chem. Soc. 2008, 130,
10876.
30. Eda, G.; Chhowalla, M. Chemically Derived Graphene Oxide: Towards
Large-Area Thin-Film Electronics and Optoelectronics. Advanced Materials 2010,
53
22, 2392-2415.
31. Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C.
Carbon nanomaterials for electronics, optoelectronics, photovoltaics, and sensing.
Chemical Society Reviews 2013, 42, 2824-2860.
32. Xu, Z.; Gao, C. ACS Nano 2011, 5, 2908.
33. Kim, J.; Han, T. H.; Lee, S. H.; Kim, J. Y.; Ahn, C. W.; Yun, J. M.; Kim, S.
O. Angew. Chem., Int. Ed. 2011, 50, 3043.
34. Kerr, J. XL. A new relation between electricity and light: Dielectrified
media birefringent. The London, Edinburgh, and Dublin Philosophical Magazine
and Journal of Science 1875, 50, 337-348.
35. Krishnan, D.; Kim, F.; Luo, J. Y.; Cruz-Silva, R.; Cote, L. J.; Jang, H. D.;
Huang, J. X. Energetic graphene oxide: Challenges and opportunities. Nano Today
2012, 7, 137-152.
36. Dimiev, A. M.; Alemany, L. B.; Tour, J. M. Graphene Oxide. Origin of
Acidity, Its Instability in Water, and a New Dynamic Structural Model. ACS Nano
2013, 7, 576-588.
37. Zhou, S.; Bongiorno, A. Origin of the chemical and kinetic stability of
graphene oxide. Sci Rep 2013, 3, 2484.
38. Kim, S.; Zhou, S.; Hu, Y.; Acik, M.; Chabal, Y. J.; Berger, C.; de Heer, W.;
Bongiorno, A.; Riedo, E. Room-temperature metastability of multilayer graphene
oxide films. Nat Mater 2012, 11, 544-9.
39. Hummers, W. S.; Offeman, R. E. PREPARATION OF GRAPHITIC
OXIDE. J. Am. Chem. Soc. 1958, 80, 1339-1339.
40. Kim, J. E.; Han, T. H.; Lee, S. H.; Kim, J. Y.; Ahn, C. W.; Yun, J. M.; Kim,
54
S. O. Graphene oxide liquid crystals. Angewandte Chemie International Edition
2011, 50, 3043-3047.
41. Gao, X.; Jang, J.; Nagase, S. Hydrazine and Thermal Reduction of
Graphene Oxide: Reaction Mechanisms, Product Structures, and Reaction Design. J
Phys Chem C 2010, 114, 832-842.
42. Zhang, J.; Yang, H.; Shen, G.; Cheng, P.; Zhang, J.; Guo, S. Reduction of
graphene oxide via L-ascorbic acid. Chem Commun (Camb) 2010, 46, 1112-4.
43. Azevedo, J.; Costa-Coquelard, C.; Jegou, P.; Yu, T.; Benattar, J. J. Highly
Ordered Monolayer, Multilayer, and Hybrid Films of Graphene Oxide Obtained by
the Bubble Deposition Method. J Phys Chem C 2011, 115, 14678-14681.
44. Ahmad, R. T.; Hong, S. H.; Shen, T. Z.; Song, J. K. Optimization of particle
size for high birefringence and fast switching time in electro-optical switching of
graphene oxide dispersions. Opt Express 2015, 23, 4435-40.
55
요약(국문초록)
탄소들이 벌집 모양의 육각형 그물처럼 배열된 평면들이 층으로 쌓여 있
는 구조인 그래핀은 높은 기계적 강도를 가지고 높은 열전도도 및 전기
이동도 특성 등의 여러 가지 우수한 특성을 가지기 때문에 투명 전극,
휘어지는 디바이스, 에너지 저장 장치, 바이오의약품 분야 등에 널리 사
용되고 있다. 그래핀은 일반적으로 소수성의 물질로 알려져 있고 용매에
잘 용해되지 않는 특성을 가진다. 반면, 그래핀의 전구체인 그래핀 옥사
이드(Graphene Oxide, GO)의 경우 그래핀의 표면을 카보닐, 에폭사이
드, 하드로옥시 그룹 등이 덮고 있기 때문에 수용액에 잘 용해된다. GO
분자의 경우 매우 높은 종횡비를 가지고 높은 용해도를 가지기 때문에
물이나 유기물에 분산되어 액정상(Liquid Crystal, LC)을 형성할 수 있
다. 최근의 연구에 따르면 GO LC는 외부 전기장에 의하여 제어될 수 있
고 매우 높은 Kerr 상관계수를 가지는 것으로 알려졌다. 그러므로 GO
LC는 착용 가능한 차세대 IT 제품의 필수 기술인 저소비전력 디스플레
이와 같은 전기광학 소자에 매우 유용하게 사용될 수 있다. 하지만 GO
는 상온에서 점차 환원이 되는 것으로 알려져 있으며 따라서 전기광학
소자에 응용되어 사용될 때에도 점차 환원이 되는 문제는 GO LC가 극
복하여야 할 도전 과제 중의 하나이다.
본 연구에서는 환원된 그래핀 옥사이드(reduce Graphene Oxide, r-GO)
또한 액정상을 가질 수 있음을 밝혀내었다. 본 연구에 사용된 r-GO는
비타민C로 불리어지는 L-ascorbic acid를 환원제로 사용하여 GO를 환
원시켰다. 환원과정에서 r-GO가 응집되는 것을 방지하기 위하여
56
hexadecyltrimethylammonium bromide (CTAB)를 계면활성제로서 첨
가하였다. 계면활성제를 적용한 r-GO 분산매는 액정상을 가질 수 있고
외부 전기장에 반응을 하며 전기광학 소자에 사용응용될 수 있는 좋은
액정상이다. 따라서 계면활성제가 적용된 r-GO 분산매를 r-GO LC라
정의하였다. 본 연구에서 r-GO LC가 외부 전기장에 의해 굴절률 이방
성을 가지는 것을 밝혀내었고 동일한 농도에서 GO LC 대비 더 높은 굴
절률 이방성을 가지는 것을 관찰하였다.
GO LC의 경우 상온에서 한 달 동안 방치하였을 경우 그 색이 갈색에서
검은색으로 변화하는 것을 관찰하였고 이것은 GO LC가 점차 환원되는
것을 의미한다. 하지만 r-GO LC의 경우 이미 환원이 된 상태이고 계면
활성제에 의해 잘 분산되어 있는 상태이기 때문에 상온에서 매우 안정적
이다. 따라서 r-GO LC의 경우 시간에 따른 색의 변화가 관찰되지 않았
다. 고품질의 전기광학 소자에는 안정된 물질이 요구되는 것은 당연하며
따라서 본 연구결과는 r-GO LC가 GO LC보다 전기 광학소자에 더 적
합한 재료임을 의미한다.
주요어: 액정, 그래핀, 그래핀 옥사이드, 계면활성제, 복굴절, 액정 거동
특성
학번: 2014-24819
