Improved Electrical Conductivity of Graphene Films Integrated with Metal Nanowires

Improved Electrical Conductivity of Graphene Films Integrated with Metal Nanowires

Iskandar N.Kholmanov,?,?Carl W.Magnuson,?Ali E.Aliev,§Huifeng Li,?Bin Zhang,?Ji Won Suk,?Li Li Zhang,?Eric Peng,?S.Hossein Mousavi,∥Alexander B.Khanikaev,∥Richard Piner,?

Gennady Shvets,∥and Rodney S.Ruo?*,?

?Department of Mechanical Engineering and the Materials Science and Engineering Program,The University of Texas at Austin,1 University Station C2200,Austin,Texas78712,United States

?CNR-IDASC Sensor Lab Department of Chemistry and Physics,University of Brescia,via Valotti,9,Brescia25133,Italy

§Alan G.MacDiarmid NanoTech Institute,University of Texas at Dallas,Richardson,Texas75083,United States

∥Department of Physics and Center for Nano and Molecular Science and Technology,The University of Texas at Austin,Austin, Texas78712,United States

*Supporting Information

graphene/NW?lms into EC devices demonstrates their

electrochromic devices

excellent transport properties with theoretical values of charge carrier mobility higher than200000cm2/V·s.1In addition,single layer graphene absorbs about2.3%of visible light.2,3The combination of these unique properties makes graphene an excellent candidate for transparent conductive ?lms(TCF).Chemical vapor deposition(CVD)of hydro-carbon gases on metal surfaces allows scaling graphene?lms to large sizes that can be transferred onto arbitrary substrates.4,5 These characteristics open the possibility to replace indium tin oxide(ITO)by graphene as the TCF,particularly for?exible and large-area device applications.

However,the sheet resistance(R s)of CVD-grown monolayer graphene(>1kΩ/sq)6is signi?cantly higher than ITO-based TCFs.The charge carrier mobility in these graphene?lms is much lower6than mechanically exfoliated graphene7,8as well as theoretically calculated values.1Defects in?uence the transport properties of CVD-grown graphene. Large-area CVD-grown graphene is a polycrystalline material with topological defects such as dislocations and grain boundaries.9?11Grain boundaries in graphene are line defects at the interfaces between two domains with di?erent crystallo-graphic orientations.9?12Depending on the detailed atomic structure,these defects can disrupt the sp2delocalization ofπelectrons in graphene and e?ectively scatter the charge carriers.10,12,13This potential formation of highly resistive grain boundaries may lead to the carriers being trapped periodically in domains.Nanoripples,another line feature in CVD grown?lms,can also scatter charge carriers by the out-of-plane?exural phonons con?ned within the defects.1,14In addition to grain boundaries and nanoripples,a higher(than theoretical values1)electrical resistivity of the graphene can arise from other defects,such as point defects,wrinkles,folds, tears and cracks,and so forth,that can scatter the charge carriers resulting in decreased ballistic transport path length and carrier mobility.15?17

The electrical properties of graphene can be improved by minimizing the role of di?erent defects.Some nonlinear structural defects in graphitic structures can be healed by high-temperature processing.18,19Growing a larger grain size graphene may result in some improvement in transport properties due to the lower density of grain boundaries.13,20 However,to date these approaches have not yielded large-area single layer graphene?lms with a sheet resistance<100Ω/sq

Received:August2,2012

Revised:October2,2012

required for some device applications.Recently,Jeong et al.21theoretically predicted that elimination of the detrimental e ?ect of line defects can be achieved through the integration of CVD-grown graphene with one-dimensional (1D)metal nanowires (NWs).We demonstrate here experimentally the assembly of monolayer graphene with 1D metal NWs with the goal of minimizing the in ?uence of line defects and line disruptions (the latter describes the wrinkles,ripples,and folding)on the transport properties of graphene ?lms.Graphene/metal NW hybrid ?lms with TCF characteristics comparable to that of ITO ?lms (typically,R s =30?80Ω/sq for an optical transmittance at λ=550nm (T 550)=90%)were obtained and also tested as transparent electrodes in electrochromic devices to evaluate their possibly replacing the traditionally used ITO ?lms.

Monolayer graphene was grown on polycrystalline Cu foil using a CVD technique described elsewhere.6A scanning electron microscope (SEM)image of a typical single layer graphene that continuously spans steps and facets of the Cu substrate is shown in Figure 1a.Line disruptions such as

wrinkles,formed due to the di ?erence in thermal contraction between graphene and the Cu substrate upon cooling,4can be easily seen in the SEM image.Transfer of graphene onto SiO 2/Si substrate substrates using a wet transfer method 6,22(see also Supporting Information)results in a higher density of line disruptions (Figure 1b),indicating that the transfer process produces additional line disruptions in graphene ?lms.

The line disruptions in the transferred graphene were also observed by Raman spectroscopy (WITEC Alpha300,λ=532

nm,100×objective).The Raman D band (～1365cm ?1)of graphene is activated by the defects that cause an intervalley double resonance involving transitions near two inequivalent K points at neighboring corners of the ?rst Brillouin zone of graphene.23,24A Raman map (1300?1400cm ?1)around the D mode of graphene on a SiO 2/Si substrate shows bright lines corresponding to the line disruptions (Figure 1c).The spectra obtained on the bright lines (blue and green circles in Figure 1c)show the presence of the D peak (blue and green spectra in Figure 1d),in addition to the G and 2D modes centered at ～1575cm ?1and ～2680cm ?1,respectively.The intensity ratio of G and 2D modes in these spectra are di ?erent (I (2D)/I (G)≈1.4for blue and 0.9for green),indicating the diversity of line disruptions.No detectable D peak was observed in the spectrum taken on the areas without bright lines (red circle in Figure 1c corresponding to red spectrum in Figure 1d).The latter spectrum is characterized by the intensity ratio of G and 2D modes (I (2D)/I (G)≈2)and the full width at half-maximum (fwhm)of the 2D band (≈27cm ?1)associated with single layer graphene.23

The sheet resistance of the graphene transferred onto glass substrates is as high as about 1.35±0.14k Ω/sq but decreases to about 1.05±0.11k Ω/sq after thermal treatment at 170°C for 1h in a vacuum chamber (p <2×10?2Torr).Integration of the graphene with Ag NWs is shown schematically in Figure 2a.Ag NW (average length and diameter of 5?25μm and 100?130nm,respectively)?lms on glass and SiO 2/Si substrates were obtained by spin coating NW dispersions in isopropyl alcohol with three di ?erent concentrations:0.2mg/mL,0.6mg/mL,and 1.0mg/mL 25(see Supporting Information).The corresponding ?lms were denoted as NW1,NW2,and NW3,respectively.The ?lms possess high optical transparency (T 550of about 98.6%,97.2%,and 96.0%,respectively)that decreases with increasing NW concentration used to make the ?lm.All of the NW ?lms (NW1,NW2,and NW3)used in this work are nonconductive due to the subpercolation network of the NWs.Ag NW ?lms above percolation may possess TCF characteristics comparable to ITO ?lms.26,27Here,we targeted the subpercolation regime,where NWs can individually and locally improve the conductivity of graphene platelets but not provide their own global conductive path(s).This excludes the electrical conductivity of pure Ag NW ?lms in the hybrid systems and allows considering the contribution of individual NWs (no network of NWs)in altering the electrical conductivity of the hybrid ?lms.

Transfer of graphene onto Ag NW ?lms was ?rst performed by a dry transfer technique 22(top processes shown in Figure 2a)that avoids trapping of the solutions (used in the transfer processes)near NWs (see Supporting Information).However,this transfer yielded graphene/Ag NW hybrid ?lms in which most of the NWs were surrounded by suspended graphene (Figure 2b)that can be easily torn during integration into devices that would result in worsened transport properties of the hybrid ?lm.To avoid the formation of the suspended graphene,a small amount of poly(methyl methacrylate)(PMMA)solution was drop-coated on top of the precoated PMMA/graphene/NW ?lm (bottom processes in Figure 2a,denoted as “modi ?ed dry transfer ”).This results in dissolving of the precoated PMMA and allows the PMMA/PMMA/graphene to better conform to the surface morphology of the underlying Ag NWs.After curing at room temperature for about 30min,the PMMA was dissolved by acetone.In

the

Figure 1.Characterization of CVD-grown graphene.(a)SEM image of graphene monolayer continuously grown across the grain boundaries and steps of a polycrystalline Cu substrate.(b)SEM image of graphene transferred onto a SiO 2/Si substrate.Dark islands,one of which is shown in the “squared ”area,are bilayer graphene.(c)Raman map (1300?1400cm ?1)centered on the D mode (1365cm ?1).The arrows in a,b,and c show the line disruptions (such as wrinkles,ripples,and folding).Scale bars are 10μm.(d)Raman spectra corresponding to the areas shown in the Raman map in (c)by blue,red,and green circles.

obtained graphene/NW hybrid ?lms no suspended graphene around the NWs has been observed as illustrated by the SEM image (Figure 2c).The graphene layer follows the curvature of the underlying NWs,providing larger interfacial contact area between graphene and NWs.This may enhance charge transfer between these two nanostructures thus improving the conductivity of the hybrid ?lm.

A typical SEM image of the hybrid ?lms produced by modi ?ed dry transfer method (Figure 3a)shows randomly oriented individual Ag NWs covered with a continuous 2D graphene layer.Figure 3b shows a NW crossing several line disruptions of the graphene layer.NW/line disruption crossings can also be seen in the Raman map (Figure 3d,corresponding to the dashed area in the optical microscopy image in Figure 3c).A Raman map (1500?1620cm ?1)around the G mode of graphene on the SiO 2/Si substrate shows bright lines that correspond to the line disruptions and dark lines corresponding to the NWs (Figure 3c).The latter shows a lower intensity of Raman signal of graphene on top of the NWs.

The optical transmittance spectra of monolayer graphene and graphene/NW hybrid ?lms presented in Figure 3e show higher than 90%transparency for all the ?lms,which satis ?es requirements for optical properties of transparent electrodes.The sheet resistance R s was measured using the van der Pauw

method after annealing the ?lms at 170°C for 1h in a vacuum chamber (p <2×10?2Torr).Figure 3f shows that the R s of the hybrid ?lms decreases signi ?cantly with increasing concen-tration of Ag NWs for the ?lms from NW1to NW3.The lowest R s =64±6.1Ω/sq with T 550=93.6%was obtained for the graphene/NW3?lms,signi ?cantly lower than that of pure graphene (R s =1.05±0.11k Ω/sq).The obtained sheet resistance of the graphene/NW3hybrid ?lms is comparable to the intrinsic sheet resistance of “perfect ”graphene (30Ω/sq for graphene/SiO 2system)that is due to solely electron ?phonon scattering.16Taking into account the nonpercolative concen-tration of Ag NWs in the hybrid system and the role of the line defects (such as grain boundaries)12and line disruptions 14to the graphene sheet resistance,the low R s values obtained demonstrate that the Ag NWs bridge line defects and line disruptions and thus strongly reduces the electrical resistance of graphene.Metal NWs crossing the line disruptions (Figure 3b)and line defects 21thus provide new conductive pathways for charge carriers in polycrystalline monolayer graphene.The SEM and Raman map (Figure 3b and 3d)show the Ag NWs bridging line disruptions;however some or many of the Ag NWs are also bridging the graphene line defects,as explained next.The length of Ag NWs reaches some tens of micrometers (Figure S1in the Supporting Information),and the average

size

Figure 2.Fabrication of graphene/Ag NW ?lms.(a)Schematic illustration of hybrid ?lm fabrication.(b)SEM image of graphene dry transferred onto the NW ?lm.Suspended graphene around the NW is shown by arrows.(c)Graphene/NW ?lm after modified dry transfer process showing graphene conformal to the underlying NWs with no observable suspended graphene.Scale bars are 1μ

m.

Figure 3.(a)Typical SEM image of graphene/NW ?lms.(b)SEM image of a NW crossing several line disruptions shown by arrows.(c)Optical microscopy image of the hybrid ?lms with a dashed line corresponding to the Raman map (1560?1620cm ?1)showing a NW crossing with a line disruption in d.Scale bars in a,b,c,and d are 6μm.(e)Optical transmittance spectra of graphene and graphene/NW ?lms.(f)R s versus optical transmittance for graphene and graphene/NW ?lms.

of graphene grains is about 10?12μm (Figure S2in Supporting Information).Thus,many of the Ag NWs covered by monolayer graphene bridge the graphene grain boundaries,and therefore,the obtained low sheet resistance values are due to the bridging by Ag NWs of both line disruptions and grain boundaries present in these graphene ?lms.

The conductivity of the obtained graphene/NW ?lms can be further improved while maintaining the T 550>90%.For example,adding a second graphene layer onto the graphene/NW3yields a ?lm with R s =24(±3.6)Ω/sq for an optical transmittance of T 550≈91%(graphene ×2/NW3in Figure 3f)that is better than doped four layer graphene-based (R s =30Ω/sq with T 550=90%)5?lms and ITO-based (R s =30?80Ω/sq with T 550=90%)?lms.Alternatively,NW ?lms above percolation are likely to yield graphene/NW ?lms with conductivity exceeding the intrinsic limit of ideal graphene,as has been demonstrated for graphene/metal grid systems.28These ?lms are fundamentally di ?erent from that of the monolayer graphene/subpercolation NW ?lms studied and described https://www.sodocs.net/doc/2914506707.html,ly,in this work the graphene ?lm is the only conductive ?lm component and the Ag NWs are an additional globally nonconductive component used to improve the conductivity of the graphene ?lms.In contrast,in the hybrid systems with the percolated NW ?lms or metal grids,the metal-component ?lms are the main conductive constituent,and graphene is used as an additional conductive component to enhance the conductivity of the metal-component structures.Doping has been studied as an alternative route to improve the conductivity of graphene,5,29,30as it can increase carrier densities,but doping does not directly address the adverse e ?ects of line defects on conductivity.Doping of monolayer graphene ?lms to achieve R s <100Ω/sq has not to our knowledge been achieved.Doped graphene ?lms are often of limited stability,such as ?lms that have about a 40%increase in graphene sheet resistance within a few days.31,32

The graphene/NW ?lms were tested as a transparent electrode in electrochromic (EC)devices.Typical EC devices are composed of an EC material and an electrolyte that are placed between two TCFs.During the electrochemical intercalation,induced by an external electric ?eld applied between the two TCF electrodes,the injection and extraction of electrons and metal cations results in a modulation of optical properties of the EC layer.33,34In our experiments one of the two ITO TCFs was replaced by the graphene/NW TCFs on glass substrates (Figure 4a).Tungsten trioxide (WO3)?lms were used as the EC layer,and a propylene/ethylene carbonate

solution (1:1)containing 1M LiClO 4was used as a Li-conductive electrolyte.It has been reported that sol ?gel prepared nanostructured WO 3?lms exhibit improved perform-ance of EC devices with fast kinetics of the optical modulation.34?36However,direct deposition of sol ?gel prepared WO 3onto graphene/NW TCFs yields inhomoge-neous ?lms with poor morphologies.Therefore,a bu ?er layer of EC WO 3with a thickness of approximately 100nm was deposited onto the graphene/NW hybrid TCFs by thermal evaporation of WO 3powder.Sol ?gel WO 3?lm with more attractive morphology for intercalation was spin coated on top of the bu ?er layer.The ?nal double layer WO 3thin ?lms had a total thickness of about 500nm.The complete EC device consisting of WO 3EC layers and electrolyte placed between graphene/NW and ITO transparent electrodes on glass substrates is schematically shown in Figure 4a (see also Supporting Information).

Electrochemical reduction of WO 3induced by external voltage (?3.0V to the graphene/NW TCF)is accompanied with injection of electrons and intercalation of Li +ions into the EC layers and generation of W 5+sites.This results in an intense electrochromic absorption band due to the optically driven intervalence charge transfer between the W 6+and W 5+states and yields blue coloration of the EC ?lms.37The “coloration ”reaction can be written as:37

++→+??++x WO [Li e ]Li W W O

x x x 3(1)653

where x is the fractional number of sites in the WO 3lattice that are ?lled by Li cations.The application of a reverse external ?eld applied to the TCF electrodes extracts the Li cations and restores the bleached state of the EC ?lm.The optical transmittance of the whole sandwich structure during coloration/bleaching cycles changes from T 550=77.2%(bleached state)to T 550=31.3%(colored state)as shown in Figure 4b.Stable coloration/bleaching processes are achieved after several initial cycles.The coloration and bleaching time for 90%transmittance change are 115s and 205s,respectively.These values are close to that of a EC device using the same sol ?gel EC ?lm,same electrolyte,but with two ITO electrodes.35The photograph images in Figure 4c show homogeneously bleached and colored states of the EC device.Repeatable cycling and homogeneous optical modulation of the tested devices demonstrates the successful performance of the graphene/NW ?lms as a transparent electrode in EC devices.In summary,line defects and line disruptions in transferred CVD-grown polycrystalline graphene ?lms degrade

their

Figure 4.(a)Schematic illustration of an EC device structure.(b)Optical transmittance spectra of bleached and colored states of the EC device.(c)Photographs of the EC device in bleached and colored states with a background “Graphene ”.The graphene/NW transparent electrode with a conductive silver paste (yellow area)on top of the electrode is shown.

transport properties.We experimentally showed that graphene/ NW assembly yields transparent conductive?lms with a sheet resistance(64Ω/sq)slightly higher than the calculated intrinsic resistance of ideal graphene.The results demonstrate that the combination of graphene with1D metal NWs can strongly reduce the overall resistance of the?lms.These hybrid?lms were successfully tested as a transparent electrode in electro-chromic devices that show the coloration/bleaching character-istics comparable with EC devices using only ITO electrodes. The integration of such graphene/NW TCFs into EC devices demonstrates their potential for replacing ITO in a broad range of applications including displays,photovoltaics,and organic

light-emitting diodes.

■ASSOCIATED CONTENT

*Supporting Information

Details of Ag NW?lm deposition,transfer of graphene,and EC device fabrication including the deposition of WO3?lms.This material is available free of charge via the Internet at http://

https://www.sodocs.net/doc/2914506707.html,.

■AUTHOR INFORMATION

Corresponding Author

*E-mail:r.ruo?@https://www.sodocs.net/doc/2914506707.html,.

Notes

The authors declare no competing?nancial interest.■ACKNOWLEDGMENTS

This work was supported by a Tokyo Electron Ltd(TEL)-customized Semiconductor Research Corporation award (Project No.:2009-OJ-1873 Development of graphene-based transparent conductive?lms for display applications). S.H.M.,A.B.K.,and G.S.would like to acknowledge the support from the Air Force O?ce of Scienti?c Research(FA9550-08-1-

0394).

■REFERENCES

(1)Morozov,S.V.;Novoselov,K.S.;Katsnelson,M.I.;Schedin,F.; Elias,D.C.;Jaszczak,J.A.;Geim,A.K.Phys.Rev.Lett.2008,100, 016602.

(2)Kuzmenko,B.;van Heumen,E.;Carbone,F.;van der Marel,D. Phys.Rev.Lett.2008,100,117401.

(3)Nair,R.R.;Blake,P.;Grigorenko,A.N.;Novoselov,K.S.;Booth, T.J.;Stauber,T.;Peres,N.M.R.;Geim,A.K.Science2008,320,1308.

(4)Li,X.;Cai,W.;An,J.;Kim,S.;Nah,J.;Yang,D.;Piner,R.; Velamakanni,A.;Jung,I.;Tutic,E.;Banerjee,S.K.;Colombo,L.; Ruoff,R.S.Science2009,324,1312?1314.

(5)Bae,S.;Kim,H.;Lee,Y.;Xu,X.;Park,J.-S.;Zheng,Y.; Balakrishnan,J.;Lei,T.;Kim,H.R.;Song,Y.I.;Kim,Y.-J.;Kim,K.S.; O zyilmas,B.;Ahn,J.-H.;Hong,B.H.;Iijima,S.Nat.Nanotechnol. 2010,5,574?578.

(6)Li,X.S.;Zhu,Y.W.;Cai,W.W.;Borysiak,M.;Han,B.Y.;Chen,

D.;Piner,R.D.;Colombo,L.;Ruoff,R.S.Nano Lett.2009,9,4359?4363.

(7)Bolotin,K.I.;Sikes,K.J.;Jiang,Z.;Klima,M.;Fudenberg,G.; Hone,J.;Kim,P.;Stormer,H.L.Solid State Commun.2008,146,351?355.

(8)Du,X.;Skachko,I.;Barker,A.;Andrei,E.Y.Nat.Nanotechnol. 2008,3,491?495.

(9)Huang,P.Y.;Ruiz-Vargas,C.S.;van der Zande,A.M.;Whitney, W.S.;Levendorf,M.P.;Kevek,J.W.;Garg,S.;Alden,J.S.;Hustedt, C.J.;Zhu,Ye.;Park,J.;McEuen,P.L.;Muller,D.A.Nature2011,469, 389?393.

(10)Yazyev,O.V.;Louie,S.G.Nat.Mater.2010,9,806?809.

(11)Kim,K.;Lee,Z.;Regan,W.;Kisielowski,C.;Crommie,M.F.; Zettl,A.ACS Nano2011,5,2142?2146.

(12)Song,H.S.;S.L.Li,S.L.;Miyazaki,H.;Sato,S.;Hayashi,K.; Yamada,A.;Yokoyama,N.;Tsukagoshi,K.Sci.Rep.2012,2,337; DOI:10.1038/srep00337.

(13)Yu,Q.;Jauregui,L.A.;Wu,W.;Colby,R.;Tian,J.;Su,Z.;Cao,

H.;Liu,Z.;Pandey,D.;Wei,D.;Chung,T.F.;Peng.,P.;Guisingers, N.P.;Stach,E.A.;Bao,J.;Pei,S.-S.;Chen,Y.P.Nat.Mater.2011,10, 443?449.

(14)Ni,G.X.;Zheng,Y.;Bae,S.;Kim,H.R.;Pachoud,A.;Kim,Y. S.;Tan,C.L.;Im,D.;Ahn,J.H.;Hong,B.H.;Ozyilmaz,B.ACS Nano 2012,6,1158?1164.

(15)Kim,K.;Park,H.J.;Woo,B.-C.;Kim,K.J.;Kim,G.T.;Yun,W. S.Nano Lett.2008,8,3092?3096.

(16)Chen,J.H.;Jang,C.;Xiao,S.D.;Ishigami,M.;Fuhrer,M.S. Nat.Nanotechnol.2008,3,206?209.

(17)Gannett,W.;Regan,W.;Watanabe,K.;Taniguchi,T.; Crommie,M.F.;Zettl,A.Appl.Phys.Lett.2011,98,242105. (18)Kholmanov,I.N.;Edgeworth,J.;Cavaliere,E.;Gavioli,L.; Magnuson,C.;Ruoff,S.R.Adv.Mater.2011,23,1675?1678. (19)Karoui,S.;Amara,H.;Bichara,C.;Ducastelle,F.ACS Nano 2010,4,6114?6120.

(20)Li,X.S.;Magnuson,C.W.;Venugopal,A.;An,J.;Suk,J.W.; Han,B.;Borysiak,M.;Cai,W.;Velamakanni,A.;Zhu,Y.;Fu,L.; Vogel,E.M.;Voelkl,E.;Colombo,L.;Ruoff,R.S.Nano Lett.2010,10, 4328?4334.

(21)Jeong,C.;Nair,P.;Khan,M.;Lundstrom,M.;Alam,M.A.Nano Lett.2011,11,5020?5025.

(22)Suk,J.W.;Kitt,A.;Magnuson,C.W.;Hao,Y.;Ahmed,S.;An, J.;Swan,A.K.;Goldberg,B.B.;Ruoff,R.S.ACS Nano2011,5,6916?6924.

(23)Ferrari,A.C.Solid State Commun.2007,143,47?57.

(24)Ding,F.;Ji,H.X.;Chen,Y.H.;Herklotz,A.;Dorr,K.;Mei,Y.

F.;Rastelli,A.;Schmidt,O.

G.Nano Lett.2010,10,3453?3458.

(25)Kholmanov,I.N.;Stoller,M.;Edgeworth,J.;Lee,W.H.;Li,H.; Lee,J.;Barnhart,C.;Potts,J.;Piner,R.;Akinwande,D.;Barrick,J.E.; Ruoff,R.S.ACS Nano2012,6,5157.

(26)Hu,L.;Kim,H.S.;Lee,J.Y.;Peumans,P.;Cui,Y.ACS Nano 2010,4,2955?2963.

(27)Garnett,E.C.;Cai,W.;Cha,J.J.;Mahmood,F;Connor,S.T.; Christoforo,M.G.;Cui,Y.;McGehee,M.D.;Brongersma,M.L.Nat. Mater.2012,11,241?249.

(28)Zhu,Y.;Sun,Z.;Yan,Z.;Jin,Z.;Tour,J.M.ACS Nano2011,5, 6472?6479.

(29)Blake,P.;Brimicombe,P.D.;Nair,R.R.;Booth,T.J.;Jiang,D.; Schedin,F.;Ponomarenko,L.A.;Morozov,S.V.;Gleeson,H.F.;Hill,

E.W.;et al.Nano Lett.2008,8,1704?1708.

(30)Han,T.H.;Lee,Y.;Choi,M.R.;Woo,S.H.;Bae,S.H.;Hong,

B.H.;Ahn,J.H.;Lee,T.W.Nat.Photon.2012,6,105?110.

(31)Ni,G.X.;Zheng,Y.;Bae,S.;Tan.,C.Y.;Kahya,O.;Wu,J.; Hong,B.H.;Yao,K.;Ozyilmaz,B.ACS Nano2012,6,3935?3942.

(32)Yan,C.;Kim,K.S.;Lee,S.K.;Bae,S.H.;Hong,B.H.;Kim,J.

H.;Lee,H.J.;Ahn,J.H.ACS Nano2012,6,2096?2103.

(33)Granqvist,C.G.;Niklasson,G.A.;Azens,A.Appl.Phys.A: Mater.Sci.Process.2007,89,29?35.

(34)Deepa,M.;Srivastava,A.K.;Kar,M.;Agnihotry,S.A.J.Phys.D: Appl.Phys.2006,39,1885?1893.

(35)Aliev,A.E.;Shin,H.W.Solid State Ionics2002,154?155,425?431.

(36)Kras o vec,U.O.;Topic,M.;Georg,A.;Georg,A.;Draz i c,G.J. Sol-Gel Sci.Technol.2005,36,45?52.

(37)Somani,P.R.;Radhakrishnan,S.Mater.Chem.Phys.2002,77, 117?133.