Large-scale-pattern-growth-of-graphene-films-for-s

LETTERSLarge-scalepatterngrowthofgraphenefilmsforstretchabletransparentelectrodesKeunSooKim1,3,4,YueZhao7,HoukJang2,SangYoonLee5,JongMinKim5,KwangS.Kim6,Jong-HyunAhn2,3,PhilipKim3,7,Jae-YoungChoi5&ByungHeeHong1,3,4Problemsassociatedwithlarge-scalepatterngrowthofgrapheneconstituteoneofthemainobstaclestousingthismaterialindeviceapplications1.Recently,macroscopic-scalegraphenefilmswerepreparedbytwo-dimensionalassemblyofgraphenesheetschem-icallyderivedfromgraphitecrystalsandgrapheneoxides2,3.However,thesheetresistanceofthesefilmswasfoundtobemuchlargerthantheoreticallyexpectedvalues.Herewereportthedirectsynthesisoflarge-scalegraphenefilmsusingchemicalvapourdepositiononthinnickellayers,andpresenttwodifferentmethodsofpatterningthefilmsandtransferringthemtoarbitrarysub-strates.Thetransferredgraphenefilmsshowverylowsheetresis-tanceof280Vpersquare,with80percentopticaltransparency.Atlowtemperatures,themonolayerstransferredtosilicondioxidesubstratesshowelectronmobilitygreaterthan3,700cm2V21s21andexhibitthehalf-integerquantumHalleffect4,5,implyingthatthequalityofgraphenegrownbychemicalvapourdepositionisashighasmechanicallycleavedgraphene6.Employingtheoutstandingmechanicalpropertiesofgraphene7,wealsodemonstratethemac-roscopicuseofthesehighlyconductingandtransparentelectrodesinflexible,stretchable,foldableelectronics8,9.Graphenehasbeenattractingmuchattentionowingtoitsfasci-natingphysicalpropertiessuchasquantumelectronictransport4,5,atunablebandgap10,extremelyhighmobility11,highelasticity7andelectromechanicalmodulation12.Sincethediscoveryofthefirstiso-latedgraphenepreparedbymechanicalexfoliationofgraphitecrys-tals6,manychemicalapproachestosynthesizelarge-scalegraphenehavebeendeveloped,includingepitaxialgrowthonsiliconcarbide(refs13,14)andruthenium(ref.15)aswellastwo-dimensionalassemblyofreducedgrapheneoxides3,16–18andexfoliatedgraphenesheets2.Epitaxialgrowthprovideshigh-qualitymultilayergraphenesamplesinteractingstronglywiththeirsubstrates,butelectricallyisolatedmono-orbilayergraphenefordeviceapplicationshasnotbeenmade.Ontheotherhand,theself-assemblyofsolublegraphenesheetsdemonstratesthepossibilityoflow-costsynthesisandthefabricationoflarge-scaletransparentfilms.However,theseassembledgraphenefilmsshowrelativelypoorelectricalconductivityowingtothepoorinterlayerjunctioncontactresistanceandthestructuraldefectsformedduringthevigorousexfoliationandreduc-tionprocesses.Inthiswork,wedevelopatechniqueforgrowingfew-layergraphenefilmsusingchemicalvapourdeposition(CVD)andsuccessfullytransferringthefilmstoarbitrarysubstrateswithoutintensemechanicalandchemicaltreatments,topreservethehighcrystallinequalityofthegraphenesamples.Therefore,weexpecttoobserveenhancedelectricalandmechanicalproperties.Thegrowth,etchingandtransferringprocessesoftheCVD-grownlarge-scalegraphenefilmsaresummarizedinFig.1.Ithasbeenknownforover40yearsthatCVDofhydrocarbonsonreactivenickelortransition-metal-carbidesurfacescanproducethingraphiticlayers19–21.However,thelargeamountofcarbonsourcesabsorbedonnickelfoilsusuallyformthickgraphitecrystalsratherthangraphenefilms(Fig.2a).Tosolvethisproblem,thinlayersofnickelofthicknesslessthan300nmweredepositedonSiO2/Sisub-stratesusinganelectron-beamevaporator,andthesampleswerethenheatedto1,000uCinsideaquartztubeunderanargonatmosphere.Afterflowingreactiongasmixtures(CH4:H2:Ar550:65:200standardcubiccentimetresperminute),werapidlycooledthesamplestoroomtemperature(,25uC)attherateof,10uCs21usingflowingargon.Wefoundthatthisfastcoolingrateiscriticalinsuppressingformationofmultiplelayersandforseparatinggraphenelayersefficientlyfromthesubstrateinthelaterprocess20.Ascanningelectronmicroscope(SEM;JSM6490,Jeol)imageofgraphenefilmsonathinnickelsubstrateshowsclearcontrastbetweenareaswithdifferentnumbersofgraphenelayers(Fig.2a).Transmissionelectronmicroscope(TEM;JEM3010,Jeol)images(Fig.2b)showthatthefilmmostlyconsistsoflessthanafewlayersofgraphene.Aftertransferofthefilmtoasiliconsubstratewitha300-nm-thickSiO2layer,opticalandconfocalscanningRamanmicroscope(CRM200,Witech)imagesweremadeofthesamearea(Fig.2c,d)22.ThebrightestareainFig.2dcorrespondstomonolayers,andthedarkestareaiscomposedofmorethantenlayersofgraphene.BilayerstructuresappeartopredominateinbothTEMandRamanimagesforthisparticularsample,whichwaspreparedfrom7minofgrowthona300-nm-thicknickellayer.Wefoundthattheaveragenumberofgra-phenelayers,thedomainsizeandthesubstratecoveragecanbecon-trolledbychangingthenickelthicknessandgrowthtimeduringthegrowthprocess(SupplementaryFigs1and2),thusprovidingawayofcontrollingthegrowthofgraphenefordifferentapplications.Atomicforcemicroscope(AFM;NanoscopesIIIaandE,DigitalInstruments)imagesoftenshowtheripplestructurescausedbythedifferencebetweenthethermalexpansioncoefficientsofnickelandgraphene(Fig.2c,inset;seealsoSupplementaryFig.3)19.Webelievethattheseripplesmakethegraphenefilmsmorestableagainstmech-anicalstretching23,makingthefilmsmoreexpandable,aswewilldiscusslater.Multilayergraphenesamplesarepreferableintermsofmechanicalstrengthforsupportinglarge-areafilmstr