Journal of Alloys and Compounds 615 (2014) 843–848

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Journal of Alloys and Compounds

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Photochemical decoration of silver nanoparticles on graphene oxidenanosheets and their optical characterization

http://dx.doi.org/10.1016/j.jallcom.2014.07.0420925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: tuan.leanh1@hust.edu.vn (A.-T. Le).

Nguyen Thi Lan a, Do Thi Chi a, Ngo Xuan Dinh a, Nguyen Duy Hung a, Hoang Lan a, Pham Anh Tuan b,Le Hong Thang c, Nguyen Ngoc Trung d, Nguyen Quang Hoa e, Tran Quang Huy f, Nguyen Van Quy g,Thanh-Tung Duong h, Vu Ngoc Phan a, Anh-Tuan Le a,⇑a Department of Nanoscience and Nanotechnology, Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), No. 1 Dai Co VietStreet, Hai Ba Trung District, Hanoi, Viet Namb Vietnam Metrology Institute, 08 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Viet Namc School of Materials Science and Engineering, Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hai Ba Trung District, Hanoi, Viet Namd School of Engineering Physics, Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hai Ba Trung District, Hanoi, Viet Name Department of Physics, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Namf Laboratory for Ultrastructure and Bionanotechnology (LUBN), National Institute of Hygiene and Epidemiology (NIHE), No. 1 Yecxanh Street, Hai Ba Trung District, Hanoi, Viet Namg International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hai Ba Trung District, Hanoi, Viet Namh Department of Materials Engineering, Chungnam National University, Daeduk Science Town, 305-764 Daejeon, Republic of Korea

a r t i c l e i n f o

Article history:Received 24 April 2014Received in revised form 13 June 2014Accepted 5 July 2014Available online 11 July 2014

Keywords:Ag-GO nanohybridGreen synthesisOptical properties

a b s t r a c t

Nanohybrid materials based on silver nanoparticles (Ag-NPs) and graphene oxide (GO) are attracting con-siderable research interest because of their potential many applications including surface-enhancedRaman scattering, catalysis, sensors, biomedicine and antimicrobials. In this study, we established a sim-ple and effective method of preparing a finely dispersed Ag-GO aqueous solution using modified Hummerand photochemical technique. The Ag-NPs formation on GO nanosheets was analyzed by X-ray diffrac-tion, transmission electron microscopy, Raman spectroscopy, and Fourier-transform infrared spectros-copy. The average size of Ag-NPs on the GO nanosheets was approximately 6–7 nm with nearlyuniform size distribution. The Ag-GO nanohybrid also exhibits an adsorption band at 435 nm becauseof the presence of Ag-NPs on the GO nanosheets. Photoluminescence emission of the Ag-GO nanohybridwas found at 400 and 530 nm, which can be attributed to the interaction between the luminescence ofexploited GO nanosheets and localized surface plasmon resonance from metallic Ag-NPs. The observedexcellent optical properties of the as-prepared Ag-GO nanohybrid showed a significant potential for opto-electronics applications.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Graphene, which consists of a one-atom-thick sheet of sp2-bonded carbon atoms in a hexagonal two-dimensional lattice, isattracting considerable research interests because of its remark-able physicochemical properties. Such properties include a highspecific surface area, mechanical strength, and thermal and electri-cal conductivities, as well as extraordinary electronic propertiesand electron transport capabilities [1]. These excellent propertiesmake graphene a promising nanomaterial for various technologicalapplications, ranging from biosensor, energy to optoelectronicdevices [1].

A specific class of graphene research deals with graphene oxide(GO), GO sheets are chemically synthesized graphene sheets thatare modified with oxygen-containing functional groups. Oxygen-ated groups in GO can strongly affect the electronic, mechanical,and electrochemical properties of GO, thereby resulting in differ-ences between GO and pristine graphene. In comparison with thepristine graphene, the existence of these oxygen functional groupscan also provide advantages such as hydrophilicity and controlla-ble electronic properties for using GO in various technologicalapplications [2–4].

Silver nanoparticles (Ag-NPs) are attractive objects for the sci-entific community in materials science because Ag-NPs possesmany advantages such as good conductivity, catalytic and wide-spectrum antimicrobial activity against various micro-organismsand localized surface plasmon resonance (LSPR) effect [5,6].

844 N.T. Lan et al. / Journal of Alloys and Compounds 615 (2014) 843–848

To explore the combined advantageous properties of Ag-NPsand GO sheets, Ag-GO nanohybrids have been intensively studied[7,8]. The Ag-NPs have an important role in many applications suchas surface-enhanced Raman scattering, catalyst, and sensors, aswell as biomedical and antimicrobial applications. Insertion ofAg-NPs into the GO nanosheet is important for further explorationof Ag-NPs properties and applications. For example, Wei et al. [9]reported that introduction of Ag-NPs into GO sheets indicate thatthe antibacterial performance of Ag-GO nanohybrids wereenhanced compared with Ag-NPs and GO materials alone. TheAg-GO nanohybrids also show non-toxic effect on rat skin [9].Other reports [10–12] showed excellent antimicrobial activity forAg-GO nanohybrids.

To date, several solution-based routes have been developed tosynthesize the Ag-NPs on the GO nanosheets such as microwaveirradiation, hydrogen reduction in supercritical CO2, surface-modification method using thiol groups, and citrate-modifiedchemical reduction, [7–12]. However, some challenges andproblems remain in preparing highly dispersed metallic Agnanoparticles of regular size on GO nanosheets and in controllingstable dispersions of Ag-GO suspension in aqueous solutionbecause of Ag-GO agglomeration. To overcome this problem, weintroduce a simple and effective method for preparation ofAg-GO nanohybrids via a two-step process, in which aqueousdispersions of GO nanosheets are produced using a modifiedHummer technique and the Ag-NPs are then decorated on GOnanosheets by a photochemical technique.

In this study, we demonstrate an easy synthesis method foreffective decoration of the Ag-NPs on the GO nanosheet using mod-ified Hummer and photochemical techniques. UV irradiation wasused to improve the uniform dispersions of Ag-NPs on the GOnanosheets during the reduction process by glucose with oleic acidas a capping agent. The analyzed results suggest that presence ofAg-NPs on the surface of GO nanosheets and the interactionbetween Ag-NPs and functional groups on the edge of the GOnanosheets were ascribed to the electron transfer from metallicAg to the GO nanosheets. Two emission peaks in photolumines-cence of Ag-GO nanohybrids were also observed at 400 and530 nm, which are attributed to the interaction between lumines-cence of exploited GO nanosheets and localized surface plasmonresonance from metallic Ag-NPs. Photoluminescence intensity ofAg-GO nanohybrid increased at peak �400 nm with increasingconcentration of Ag-NPs because of surface plasmon-enhancedluminescence.

2. Experimental procedures

2.1. Chemicals

Analytical-grade silver nitrate (AgNO3, 99.9%), sodium hydroxide (NaOH),ammonium hydroxide (NH3, 25%), potassium permanganate (KMnO4, 99.9%),hydrogen peroxide (H2O2, 30%), sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl,37%), nitric acid (HNO3, 63%), oleic acid, and glucose that were used in this studywere purchased from Shanghai Chemical Reagent Co. Ltd. Graphite (nature coalpowder) was fabricated from coal in Vietnam.

2.2. Synthesis of graphene oxide (GO) by modified Hummer method

First, GO nanosheets were synthesized from coal powder by modified Hummermethod as described previously [13]. Briefly, 1 g of coal powders were mixed withHNO3 and KMnO4 at a volume ratio of 1:2:1.5, respectively, and then the mixturewere converted to exploited graphite (EG) under microwave 800 W for 1 min. Inthis reaction, the mixture of 2 g of EG, 8 g of KMnO4, and 1 g of NaNO3 was addedslowly to 160 mL of 98% H2SO4 at 5 �C in ice-water bath and then stirred for 30 min.Ice-water bath was removed, and then temperature was increased slowly to 45 �Cand continuously stirred for 2 h. Deionized water was added slowly to the mixturewhich was stirred until purple fumes were inhibited. By increasing reaction tem-perature to 95 �C and stirring the mixture for 1 h, the resulting product of the GOnanosheets was obtained with yellow–brown color. The GO nanosheets were thentreated by H2O2 30% and HCl 10% solution to eliminate KMnO4, MnO2, and other

metal ions that remained in the GO solution. The final GO products were purifiedby filtering, washing several times by ultrasonic vibration, centrifugation withdeionized water, and removal of ultrafine carbon powder that was not oxidized.

2.3. Synthesis of Ag-GO nanohybrid by modified photochemical method

The Ag-NPs were then deposited on the GO nanosheets by modified Tollens pro-cess as reported elsewhere [14]. Fig. 1 shows the schematic of a two-step process tosynthesize the Ag-GO nanohybrid. In a typical experiment, 1.7 g (10 mmol) ofAgNO3 was dissolved in 100 mL of deionized water. The AgNO3 solution was thenprecipitated with 0.62 g (15.5 mmol) of sodium hydroxide (Aldrich, >99%). Theobtained precipitate, which is composed of Ag2O, was filtered and dissolved in100 mL of aqueous ammonia (0.4% w/w, 23 mmol) until a transparent solution ofsilver ammonium complex [Ag(NH3)2]+(aq) formed. Up to 2.5 g (8.9 mmol) of oleicacid was then added dropwise into the complex, and the resulting solution wasgently stirred for 2 h at room temperature until the complete homogeneity of thereaction mixture was achieved. As to the synthesis of Ag-GO nanohybrid, resultingcomplex mixture was mixed with GO suspension (3 mg/mL) while stirring for30 min and followed by the addition of 2 g (11.1 mmol) of glucose. The reductionprocess of the silver complex solution (in quartz glass) was initiated with UV irra-diation. A UV lamp (k = 365 nm, 35 W) was used as a light source to stimulate thereduction process. After 12 h of UV irradiation, the Ag-NPs were deposited on theGO nanosheets to form the Ag-GO nanohybrid.

2.4. Characterization techniques

Transmission electron microscopy (TEM, JEOL-JEM 1010) was conducted todetermine the morphology and distribution of the Ag-NPs on the GO nanosheets.The samples for TEM characterization were prepared by placing a drop of colloidalsolution on a formvar-coated copper grid that was dried at room temperature. Thecomposition of the Ag-GO nanohybrid was characterized by energy-dispersiveX-ray (5410 LV JEOL). The crystalline structure of the prepared Ag-NPs and Ag-GOnanohybrid was analyzed by X-ray diffraction (XRD, Bruker D5005) using Cu Karadiation (k = 0.154 nm) at a step of 0.02� (2h) at room temperature. The back-ground was subtracted using linear interpolation method.

The chemical functional groups of GO and Ag-GO were characterized using FTIRmeasurements, samples were collected with one layer coating in potassium bro-mide and compressed into pellets, and spectra in the range of 400–4000 cm�1 wererecorded with Nicolet 6700 FT-IR instrument. Raman measurement was conductedusing 633 nm of HeANe laser excitation.

The UV–vis absorbance spectra were recorded using a HP 8453 spectrophotom-eter, and the absorption spectrum of all suspension samples in 10 mm path lengthquartz cuvettes was 300–900 nm. The photoluminescence spectra of GO, Ag, andAg-GO were measured using Nanolog, Horiba. The photoluminescence spectra wereobtained with 300 nm excitation.

3. Results and discussion

3.1. Formation of GO nanosheet and Ag-GO nanohybrid

Fig. 2 shows the TEM images of (a) Ag-NPs and (b–d) Ag-GOnanohybrids at different magnifications. The Ag-NPs are finely dis-persed (Fig. 2a), the average size of the Ag-NPs was �5 nm (seeinset of Fig. 2a). No aggregation of silver particles was alsoobserved, indicating the important role of UV irradiation for con-trolling stably uniform dispersions in Ag-NP synthesis process.

Fig. 2b–d clearly show the presence of a large number of Ag-NPsthat are anchored to the GO surfaces. The adhered nanoparticleshave quasi-spherical morphologies and are dispersed uniformlyon the GO nanosheets. In these TEM images, most nanoparticlediameters are �7 nm (see inset of Fig. 2a). The wrinkles of theGO nanosheets (Fig. 2c) are also observed, revealing that the GOnanosheets are thin. Based on TEM analysis, no aggregation ofAg-NPs is found on the surface of GO nanosheets. The small sizesand fine dispersions of Ag-NPs on GO nanosheets enable potentialfor various technological applications.

The formation of the Ag-NPs on GO nanosheets is further con-firmed by XRD analysis. Fig. 3 shows the XRD patterns for GOnanosheets and GO-Ag nanohybrid samples. The GO nanosheetsexhibited a broad peak at 10.9� corresponding to the (002) inter-layer spacing of 0.81 nm, which indicates that the ordinal struc-tures of graphite have been exploited and that oxygen-containingfunctional groups have been inserted into the interspaces. After

Fig. 1. A schematic protocol for a two-step process to synthesize the Ag-GO nanohybrid.

Fig. 2. TEM images of (a) Ag-NPs and (b–d) Ag-GO nanohybrids at different magnifications.

N.T. Lan et al. / Journal of Alloys and Compounds 615 (2014) 843–848 845

decorating the Ag-NPs, three distinct diffraction peaks appear at2h = 38.2�, 44.4�, and 64.5�, which correspond to the (111),(200), and (220) crystalline planes of metallic Ag (JCPDS No. 04-0783). These observations confirm that the metallic Ag-NPs areeffectively anchored to the surface of GO nanosheets. TEM andXRD analyses revealed that the GO, Ag-NPs, and Ag-decorated GOnanosheets were formed. These obtained results suggest that theAg-NPs are successfully decorated on the GO nanosheets usingtwo-step process. In the present study, the mechanism for Ag-GOformation can be understood as follows: after mixing silver

ammonia complex with GO nanosheets, the positively chargedAg[(NH3)2]+ can be easily attached to the negatively charged oxy-gen functional groups on the GO. When Ag-GO formation occursby adding glucose to the mixture, the aldehyde groups of glucoserelease electrons to reduce silver ammonia complex into silvernanoparticles. The Ag-NPs can be deposited into the GO nano-sheets because of the electrostatic interaction between silverammonia complex and GO nanosheets. UV irradiation is performedduring the reduction process to control uniform dispersions ofAg-NPs on the GO nanosheets.

Fig. 3. XRD patterns for GO nanosheets and GO-Ag nanohybrid samples. Fig. 5. Raman spectra of (a) GO and (b) Ag-GO nanohybrids.

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On the basis of TEM and XRD studies as well as earlier reports[15,16], a possible mechanism of silver nanoparticle formationand growth under the applied experimental conditions was sug-gested. The UV irradiation causes excitation of [Ag(NH3)2]+ ions fol-lowed by electron transfer from the glucose molecule to Ag+, thusproducing Ag0 atoms which then form clusters and seeds:

½AgðNH3Þ2�þ þ RCHOH!hv

Ag0 þ 2NH3 þHþ þ R _COH

nAg0 ! ðAgnÞ0;

where RCHOH represents glucose in cyclic form. The use of UV irra-diation leads to the substantially simultaneous formation of a largeamount of silver nuclei which then started to grow. This situationresults in small dimensions and stably uniform dispersions of thefinally obtained silver NPs on the GO nanosheets. The remaining sil-ver ions are adsorbed on the surface of already formed nanoparti-cles and attract oppositely charged oxygen functional groups onthe GO sheets through an electrostatic interaction to keep thereduced silver nanoparticles staying on the GO [11].

3.2. Chemical groups in GO and Ag-GO nanohybrid

To elucidate the chemical attachment of Ag-NPs on the GOnanosheets, FTIR and Raman analyses were conducted. Fig. 4shows the FTIR spectra of (a) GO and (b) Ag-GO nanohybrids. Forthe case of GO (Fig. 4), the presence of adsorption bands at3493 cm�1 corresponds to the AOH stretching vibration. Otherpeaks of oxygen functional groups were also detected includingCO2 groups at 2359 cm�1, C@C bonding of aromatic rings of theGO carbon skeleton structure at 1647 cm�1, and OAH deforma-tions of the CAOH groups at 1383 cm�1. These oxygen functionalgroups could be located on both basal planes and edges of the

Fig. 4. FTIR spectra of (a) GO and (b) Ag-GO nanohybrids.

GO nanosheets [2,3]. However, a noticeable decrease in the inten-sity of the adsorption bands of the oxygenated functional groupswas found in the FTIR spectrum of the Ag-GO nanohybrid. Thisfinding results mainly from both presence of the Ag-NPs on thesurface of GO nanosheets and a slight reduction of GO by glucoseduring the synthesis process of Ag-GO. The decrease of OAHstretch absorption intensity in the hybrids is attributed to interac-tions between silver ions and hydroxyl group of GO. The variationof the other peak in the case of Ag-GO demonstrates the interactionbetween silver ions and oxygen functional groups on both basalplanes (hydroxyl group OH) and edges (carboxyl group CAOH) ofthe GO nanosheets through the formation of a coordination bondor through simple electrostatic attraction. The FTIR results demon-strate that the GO nanosheets have been successfully exfoliated,and strong interactions may exist between Ag-NPs and the remain-ing hydroxyl and carboxyl groups on the surface of the GO.

Fig. 5 shows the Raman spectra of (a) GO and (b) Ag-GO nano-hybrids. For the case of GO (Fig. 5), two characteristic prominentpeaks were observed at 1360 cm�1 (D band) and at 1591 cm�1 (Gband). Compared with GO, the Raman spectra of Ag-GO indicatesthat the D band and the G band are slightly shifted to 1338 and1595 cm�1, respectively. The D band represents edges, otherdefects, and disordered carbon because of vibration of sp3-bondedcarbon atoms and impurities, whereas the G band arises from thezone center E2g mode, assigning to the ordered sp2-bonded Catoms. A significant frequency shift (about 22 cm�1) toward asmaller wavenumber of the D-band is found in Ag-GO sample com-pared with the GO indicating a higher level of disorder of thegraphene layers and increased numbers of defects because ofthe partial reduction of GO by glucose during the synthesis of theAg-GO nanohybrid. The spectra showed that the carbon frameworkof GO is modified by reduction reaction process of Ag-NPs. Thisfinding is consistent with the FTIR result and that from previousworks [7,10].

Besides, the ratio of intensity of the D band to that of the G band(ID/IG) also increased. The ID/IG values are approximately 0.77 and0.92 for GO and GO-Ag respectively. The ratio value of ID/IG repre-sents the degree of disorder and the average size of the sp2

domains. The observed increasing ID/IG-value suggested a decreasein in-plane size of graphene upon the reduction process. The partialreduction of GO could cause fragmentation along the reactive sitesand might yield new graphitic domains, leading to smaller sizesand higher number of graphene than that of GO before the reduc-tion [7,11].

In the present study, the higher increased ID/IG-value of theAg-GO nanohybrid than that of the GO is likely attributed to thesurface-enhanced Raman scattering from the intense local electro-magnetic fields of Ag-NPs that accompanies plasmon resonanceeffect. The FTIR and Raman results suggest that the attachment

Fig. 7. (a) The PL emission spectra of GO, Ag-NPs, and Ag-GO nanohybrid and (b)the PL emission spectra of Ag-GO nanohybrid at different concentration of Ag.

N.T. Lan et al. / Journal of Alloys and Compounds 615 (2014) 843–848 847

of Ag-NPs on the GO nanosheets and the interaction between theAg and the functional groups of the GO nanosheets were ascribedto the electron transfer from the metallic Ag to the GO nanosheets.

3.3. Optical characterization of GO, Ag, and Ag-GO

To explore optical characterizations of the prepared Ag-GOnanohybrid, we conducted UV–vis and photoluminescence (PL)analyses.

Fig. 6 shows (a) the UV–vis spectra of GO, Ag-NPs, and Ag-GOnanohybrid and (b) the UV–vis spectra of Ag-GO nanohybrid at dif-ferent Ag concentrations. One peak (Fig. 6a) at 305 nm comes fromn ? p* transitions of C@O bond in sp3 hybrid regions, and anotherprominent peak at �393 nm is ascribed to the CAOH bond,whereas the presence of absorption peak at �726 nm is attributedto band edge absorption feature [17]. Obviously, the Ag-NP and Ag-GO samples display strong absorption peaks at 428 and 435 nm,respectively, because of the surface plasmon resonance (SPR) effectof Ag-NPs. The appearance of characteristic surface plasmon bandat 435 nm indicates the formation of Ag-NPs on GO nanosheets.The SPR phenomenon occurs when the incident light interacts withvalence electrons at the outer band of Ag-NPs, leading to oscillationof electrons along with the frequency of the electromagneticsource [18]. However, the absorption band is shifted to longerwavelength with increased concentration of Ag-NPs (Fig. 6b). Theshifting of the absorption peak toward longer wavelength forhigher concentration of Ag-NPs indicates the formation of largerAg nanoparticles with different shapes and sizes [8,18]. The surfaceplasmon band shifts are strongly dependent on particle size, shape,chemical surrounding, and adsorbed species on the surface anddielectric medium, whereas the plasmon peak and full width athalf maximum depends on the extent of colloid aggregation[18,19].

Fig. 7 shows (a) the PL emission spectra of GO, Ag-NPs, andAg-GO nanohybrid and (b) the PL emission spectra of Ag-GO

Fig. 6. (a) The UV–vis spectra of GO, Ag-NPs, and Ag-GO nanohybrid and (b) theUV–vis spectra of Ag-GO nanohybrid at different concentration of Ag.

nanohybrid at different Ag concentrations. The PL emission spectraof GO aqueous suspension show two emission peaks at 412 and530 nm. Previous experiments verified that GO fluorescence isdue to electron–hole recombination from conduction band bottomand nearby localized electronic states to wide-range valance band.In view of atomic structure, the GO emission is predominantlyresulting from electron transitions among/between the non-oxidized carbon region (AC@CA) and the boundary of oxidizedcarbon atoms (CAO, C@O, or O@CAOH) [15,20].

The Ag-NP aqueous suspension also displays a maximum emis-sion peak at 400 nm. The visible luminescence of Ag-NP colloid isascribed to excitation of electrons from occupied D bands intostates above the Fermi level. Subsequent electron–phonon andhole–phonon scattering processes lead to energy loss and finallyphotoluminescent–radiative recombination of an electron froman occupied sp band with the hole [18].

PL spectrum of Ag-GO showed two emission peaks at 400 and530 nm, which were attributed to the interaction between lumi-nescence of atomically layered GO and localized surface plasmonresonance from metallic Ag-NPs. Compared with the GO, the firstpeak position is shifted to 400 nm, and emission of peak at400 nm is attributed to the plasmon resonance of Ag-NPs, whereasthe second peak did not change at �530 nm. With increasing con-centration of Ag-NPs, the PL intensity of Ag-GO was also increasedat peak �400 nm because of surface plasmon-enhanced lumines-cence [21,22]. The effect of surface plasmon interaction of theAg-NPs with the GO surface was also probed by UV–vis observa-tion. The signal at about 332 nm in the PL spectra is the Raman sig-nal of water, whereas the signals at �606 and �668 nm areattributed to the second mode of lamp and water, respectively.

4. Conclusions

An easy and effective method of preparing Ag-GO aqueoussolution was presented. The fine dispersions of Ag-NPs on the GO

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nanosheets were altered by tuning the UV irradiation. The resultsalso indicated that attraction of Ag-NPs on the GO nanosheetsand the interaction between the Ag-NPs and the functional groupsof the GO nanosheets were ascribed to the electron transfer fromthe metallic Ag to the GO nanosheets. The Ag-GO emission waspredominantly caused by the interaction between luminescenceof atomically layered GO and localized surface plasmon resonancefrom the metallic Ag-NPs.

Acknowledgements

This work was supported by Vietnam’s National Foundation forScience and Technology Development (NAFOSTED) through a fun-damental research project (Code: 103.44-2012.60). The authorswould like to thank P.T. Huy at AIST for providing GO samplesand also thanks to N.D. Cuong at AIST for proof reading and usefuldiscussions.

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