Observation of Distinct Electron-Phonon Couplings in Gated B

A Raman study of a back gated bilayer graphene sample is presented. The changes in the Fermi level induced by charge transfer splits the Raman G-band, hardening its higher component and softening the lower one. These two components are associated with the

ObservationofDistinctElectron-PhononCouplingsinGatedBilayerGraphene

L.M.Malard,D.C.Elias,E.S.AlvesandM.A.Pimenta

DepartamentodeF´ sica,UniversidadeFederaldeMinasGerais,30123-970,BeloHorizonte,Brazil

(Dated:August18,2008)ARamanstudyofabackgatedbilayergraphenesampleispresented.ThechangesintheFermilevelinducedbychargetransfersplitstheRamanG-band,hardeningitshighercomponentandsofteningthelowerone.Thesetwocomponentsareassociatedwiththesymmetric(S)andanti-symmetricvibration(AS)oftheatomsinthetwolayers,thelateronebecomingRamanactiveduetoinversionsymmetrybreaking.ThephononhardeningandsofteningareexplainedbyconsideringtheselectivecouplingoftheSandASphononswithinterbandandintrabandelectron-holepairs.

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2 gPACSnumbers:63.20.Kd,78.30.Na,81.05.Uw

uA Theinteractionofelectronsandphononsisafunda-8mentalissueforunderstandingthephysicsofgraphene,1 resultingintherenormalizationofphononenergyand ]thebreak-downoftheadiabatic(Born-Oppenheimer)ap-llproximation[1,2,3,4].Althoughmanynewphysicalin-asightsabouttheelectron-phononinteractionwerefoundhinmonolayergraphenethebilayergraphene-sisauniquesystemtostudydistinctcouplingsbetweeneelectronsandphononsthathavedi erentsymmetriesmInthispaperweshowthattheapplicationofagatevolt-.tageinbilayergraphenesplitsthesymmetricandanti-asymmetricopticalphononcomponents,con rmingare-mcenttheoreticalpredictionforthedistinctinteractions-ofthesephononswithintrabandandinterbandelectron-dnholepairs[7].Thisresultisspeciallyrelevantsincetheodevelopmentofbilayergraphenedeviceswithtunablegapcdependsonthedetailedunderstandingoftheinteraction[ betweenelectronsandphononsinthismaterial 1ThezonecenterE2gphononmodeofmonolayervgraphene,2 1whichgivesrisetotheRamanG-bandaround1580cm,exhibitsaverystrongcouplingwithelectron-5holepairs.Thisinteractionrenormalizesthephononen-4ergy ω0,givingrisetotheso-calledKohnanomalyatthe2.zonecenterofthephonondispersion[1].Moreover,theo-8reticalmodelshavepredictedaninterestingbehaviorfor0thephononenergywhentheFermilevelεFischangedby8varyingtheelectronorholeconcentration:thephonon0:energysoftenslogarithmicallyforvaluesofthechemicalvpotentialsmallerthanhalfofthephononenergy,anditiXhardensotherwise.Thishardeningisascribedtothesup-rpressionoftheinteractionbetweenphononsandelectron-aholepairsfor| εF|> ω0/2.Besidesthis,anincreaseineitherelectronofholedensityincreasesthephononlife-time[2,3,4,5,6]duetotheinhibitionoftheprocessofphonondecayintoelectron-holepairs,thusreducingtheG-bandlinewidth.RecentRamanstudiesofmonolayergraphenehavecon rmedthetheoreticalpredictioncon-cerningthehardeningandnarrowingofthetheG-band,bydopingthesamplewithelectronsorholes[4,5,6].Inthecaseofbilayergraphene,theE2gphononmodesplitsintotwocomponents,associatedwiththesymmet-ric(S)andanti-symmetric(AS)displacementsoftheatomsinthetwo layers.Moreover,duetothesplittingoftheπandπbandsinthismaterial,phononscancou-

FIG.1:(a)Opticalmicroscopeimageofgatedgraphenebi-layersample(theblackscalebarcorrespondsto15µm).(b)RamanspectrumoftheG′bandofbilayergraphene.(c)Ra-manspectrumoftheGbandfordi erentvaluesoftheappliedgatevoltage.

plewithelectron-holepairsproducedininterbandorin-trabandtransitions.T.Ando[7]calculatedrecentlytheself-energyoftheSandASphononsforvaryingFermien-ergiesandpredictedthehardeningandsofteningoftheSandASphonons,respectively,inducedbyelectronorholedoping.Inthisworkwepresentexperimentalresultsthatcon rmthetheoreticalpredictionbyT.AndothusshowingthattheG-bandofbilayergraphenehasindeedtwocomponentswhichexhibitoppositedependenceastheFermilevelistuned.

Figure1(a)showsanopticalmicroscopeimageofthebilayergraphenedeviceobtainedbyexfoliatingbulkgraphiteona300nmthicknesssiliconoxidelayeronthetopofaheavilypdopedsiliconsubstrate.Anelec-tricalcontactwasmadebysolderinganindiummicrowiredirectlyonthetopofthe ake[11].ChargecarrieswereinducedinthesamplebyapplyingagatevoltageVgtotheSisubstratewithrespecttothegraphenecon-tact.TheRamanmeasurementswerecarriedoutatroomtemperatureusingatripleDilorXYspectrometerwitharesolutionsmallerthan1cm 1,an80xobjectivewith

A Raman study of a back gated bilayer graphene sample is presented. The changes in the Fermi level induced by charge transfer splits the Raman G-band, hardening its higher component and softening the lower one. These two components are associated with the

FIG.2:(coloronline)(a)RamanG-bandofthebilayergraphenefor-80V,-40V,-20Vand+40Vgatevoltages.TwoLorentziancurvesareneededto ttheG-bandfor-80V,-40Vand-20V.(b)Displacementoftheatomsforthesymmetricandanti-symmetrichighestenergyphononmodesintheΓpointofbilayergraphene.

spotsizeof～1µmandthe2.41eVlaserexcitationwith1mWpower.

Thecharacterizationofthenumberoflayersinourgraphene′samplewasdeterminedbyanalyzingtheshapeof

theGbandaround～2700cm 1whichisshowninFig.1(b)(thisbandisalsocalledasD*or2Dbyotherauthors).TheG′bandofoursampleiscomposedoffourpeaksandexhibitsatypicalshapeofabilayergraphene[12,13,14,15].

Figure1(c)showstheRamanspectratakenatdi erentappliedgatevoltages.Weobserveinthis gurethatboththepositionandtheshapeoftheG-banddependontheappliedgatevoltage,inagreementwithpreviousRamanstudiesofgatedbilayergraphene[16,17].However,asourexperimentsweredoneatroomtemperature,wewerenotabletoobservetheinitialsofteningoftheG-bandfor| εF|< ωG/2,asobservedatlowtemperaturesbyYanetal.[16].

Figure2(a)showsthe ttingsoftheRamanG-bandofthebilayergraphenedevicefordi erentvaluesofappliedgatevoltage.TheG-bandsobservedinthespectratakenatVgequalsto-80V,-40Vand-20Vclearlyexhibittwopeaks.Allspectratakenatpositivevaluesofgatevoltagecouldbe ttedbyasingleLorentzian.

Figures3(a)and3(b)showtherelativeshift(withre-specttoG-bandpositioninthe …… 此处隐藏：14324字，全部文档内容请下载后查看。喜欢就下载吧 ……