bDepartmentofPhysics,BilkentUniversity,Bilkent,06800,Ankara,Turkey
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Articlehistory: Received25April2017
Receivedinrevisedform5July2017 Accepted13July2017
Availableonline18July2017 Keywords:
X-Rayphotoelectronspectroscopy(XPS) Graphene
Polymerinduceddoping Ramanspectroscopy Work-function
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Tunabilityofelectronicpropertiesofgrapheneisoneofthemostpromisingpropertiestointegrateitto highefficiencydevicesinthefieldofelectronics.Herewedemonstratethesubstrateinduceddopingof CVDgraphenedevicesusingpolymerswithdifferentfunctionalgroups.BothX-Raysecondaryelectron cut-offandRamanspectraconfirmp-typedopingofaPVC-Graphenefilmwhencomparedtoa PMMA-Grapheneone.Wealsosystematicallyanalyzedthereversibledopingeffectofacid-baseexposureandUV illuminationtofurtherdope/undopethepolymersupportedgraphenedevices.TheshiftsintheRaman 2Dbandtowardslowerandthentohigherwavenumbers,withsequentialexposuretoammoniaand hydrochloricacidvapors,suggestn-typedoingandrestorationofgraphenetoitsoriginalstate.Finally, then-typedopingwithUVirradiationonhalf-coveredsampleswasutilizedandshownbybothXPS andRamantocreatetworegionswithdifferentelectronicpropertiesandresistances.Thesetypeof con-trolledandreversibledopingroutesoffernewpathsforelectronicdevicesespeciallytowardsfabricating graphenep-njunctions.
©2017ElsevierB.V.Allrightsreserved.
1. Introduction
Graphene,thetwodimensionalsp2hybridizedhexagonal
hon-eycomb shaped network of pure carbon atoms, has received
tremendousinterestfromresearchersduetoitsunique proper-tiesespeciallyinthefieldofelectronics[1–5].Thiscanmostlybe attributedtothetunabilityofelectronicpropertiesofgraphene. Oneofthemostwidelyusedwayfortuningtheelectronic proper-tiesisdoping[6–12].Graphenedopingcanbemadebyelectrical, chemical, contact or optical means. In our previous study, we showedthatapplyinga gatevoltage,hence creatinganelectric field,inducesfreechargecarriersongrapheneandshiftstheFermi energy,whichcanbemonitoredbygate tunablephotoemission ofXPSpeaks[13].Contactdopingissimilar,butinthiscasethe electricfieldisformedduetodifferencesintheworkfunctionsof grapheneandcontactmaterials[14,15].Manyothersuse chem-icaldoping[6,8,16],whichis generallyreliedonsubstitutionof someof heteroatoms[17–19]tothecarbonnetworkor by sur-facetransferdoping[20,21].Althoughthesubstitutionaldoping resultsindopedandstablegraphene,thesp2 carbonnetworkis
irreversiblydisrupted duetodefectformation,which alsoleads
∗ Correspondingauthor.
E-mailaddress:suzer@fen.bilkent.edu.tr(S.Suzer).
todecrease in conductivityand mobility. Chemicaldoping can alsobeachievedbyexposing thegraphenesamplestogasesor liquidswhich adsorbonthesurfacehencecausechangesinthe FermiEnergy[22].Additionally,ultravioletirradiationmodulated graphenedopingisalsoreportedbymanyothers[9,11,12,23–25]. Opticallydrivenmolecularstructuretransformation ofadsorbed speciesthatcausereversibledopingofgraphenehasrecentlybeen reportedusingpyrene[7],spiropyran[25],and azo-based chro-mophores[9,12].Luoetal.,showedthatareversibleandcontrolled n-typedopingwascausedbyUVilluminationofCVDgraphene[11] whiledeepUVilluminationunderoxygenorozoneenvironment wasreportedtoinducep-typedoping[23,24].
Ramanspectroscopyiscommonandwellestablishedtechnique foranalysisofgrapheneanditsderivatives.Bythismethod,notonly thequalityofthelayercanbeassessed,butalsothenumberof lay-ersofgraphenematerialscanbeidentified[26–28].Forgraphene, Ramanspectrashowtwomainfeatureswhicharetheso-calledG and2Dbands,andthepositionofthesebandsareusedto deter-minethetypeofdoping[29,30].Additionally,presenceofDandD’ bandsintheRamanspectrum,whicharealsocalleddefectactivated bands,indicativeofthechangesinthesp2hybridizedstructureof
graphene,havealsobeenusedtoprobethepresenceofimpurities ordefects[28,31,32].
X-Rayphotoelectronspectroscopyisalsofrequentlyusedfor elementalanalysisandalsoforelucidatingthenatureofthedoped http://dx.doi.org/10.1016/j.apsusc.2017.07.097
Fig.1. ASurveyXPspectrumofatypicalgrapheneonsiliconwafertogetherwiththeO1s,C1sandSi2plevelsrecordedatahigherresolution.O1sandC1sregionsconsist ofasinglepeakwhereasSi2pconsistsoftwopeaks,assignabletolowerbindingenergySi0andhigherbindingenergySi+4.
heteroatoms[10,17,33].InXPS,thesampleisirradiatedbyaknown energyofX-rayswhichresultsincreationofphotoelectronswhose kineticenergiesaredeterminedandconverted tobinding ener-giesusingtheenergyoftheX-rayphotonsandthespectrometer workfunction.Bindingenergyofaspecificlevelisdifferentforeach atom,whichalsovarieswithitschemicalenvironment.Therefore, XPSgives chemicalaswellaselementalinformation.XPSurvey andhighresolutionSi2p,C1sandO1sspectraforatypicalsample ofgrapheneonSiO2/SisubstrateareshowninFig.1.SinceXPSis
asurfacesensitivetechniqueandgiveinformationonlyaboutthe topfewnanometersdepthofthesample’ssurface,andgrapheneis asingle(orafew)layerofcarbonatoms,siliconsubstrateiswithin theanalysisdepthandisobservablebothinthesurveyandinthe highresolution(intheenergyscale)scans.Forexample,whilethe Si2p1/2peakappearsaround100eVforSi0wealsoobserveasmall
intensitypeakaround104eVcorrespondingtotheSi4+ofthethin
nativeoxide(SiO2)layer.Inconventionalusage,XPSisusedto
har-vestinformationon;(i)aselectedpoint,(ii)alongaline(line-scan), or(iii)ontheentirearea(arealscan)withalateralresolutionof 30–400m.However,inallthedatareportedinthiswork,only 100mspotand100mstepsizesareused.Inlinescanorareal modes,sincethenumberofspectraarelarge,thespectral infor-mationisusuallycondensedanddisplayedasvariationseitherin; (i)theintensityor(ii)thebindingenergypositionsoftheselected regions.
Inone alternativeuseofXPS,applicationof externalbiasto thesampleisusedtocausedevelopmentofadditionalchargeson thesurface,whichallowstheusertoextractadditionalelectrical propertiesaboutthesample,becausethepositionof photoelec-tronpeaksalsoshiftsundertheappliedpotential.Forexample, thebindingenergyofAu4f7/2peakis84.0eV,correspondingtothe
zerooxidationstategoldatomswithinthemetal.Whenweapply anexternal biastothegoldsubstratethepositionof photoelec-tronpeaksshiftswiththesameamplitudeastheappliedbiasdue toitsexcellentconductivity,seeFig.2.However,shiftswith dif-ferentmagnitudesareusuallyobservedforpoorly-conductiveor non-conductivematerials.
Inoneofourpreviouswork,weshowedthatvariationsinthe electricalpotential,derivedfromthepositionsoftheC1sbinding energythroughoutthesampleduetoexternalbiasarelinearly cor-relatedwiththebiasappliedforapristinegraphene[34].However, byanalysisofpeakpositions,itwasalsodemonstratedthatthe
externalbiasreflectsandamplifiestheappearanceof morphologi-caldefectsonthegraphenelayer,seeFig.3b[35].
Aswediscussedearlierinthispaper,XPSisachargesensitive techniquesothatthepeakpositionsarealsoaffectedandshiftdue tochargingonthesurface.ThismakesXPSagoodcandidateto ana-lyzedopingintheory.However,aswehadpreviouslyshownthat theenergydifferencebetweenthep-typeandn-typeSi2ppeaks onHterminatedSiwafersurfacesismuchsmaller(0.18eV)due tobandbending,henceitisdifficulttofullyevaluatetheeffectof doping,asshowninFig.4.
Anotherpossibilitythatphotoelectronspectroscopyoffersisthe determinationoftheworkfunctionofthesamplethroughanalysis ofthelow energycut-off region,dominatedbysecondary elec-trons.Suchdeterminationsarealsousedforassessingthetypeand theextentofdoping.Theonsetofthesecondaryelectroncut-off givesinformationabouttheworkfunctionofthesampleusingthe relation;
sample=h−
EK,max−EK,min
whereEK,maxisthemaximummeasuredkineticenergyofan elec-tron emitted from theFermi level, and EK,min is theminimum measuredkineticenergywhichislocatedintheregionofthe sec-ondaryelectroncut-off[36,37].Eventhough,UPSisgenerallyused for workfunctiondeterminationsdue toitshigher photonflux [38,39],XPShasalsobeenusedintheliterature[37,40,41]. How-ever,experimentallyitischallengingtomeasuretheelectronswith nearlyzerokineticenergyusingaconventionalXPSspectrometer. Thatiswhy,generallyanegativebiasisappliedtodetectand ana-lyzethisregion,becausethenegativebiasaccelaratesthekinetic energyofallelectronstothespectrometer.Inourcase,toanalyze systematicallythesecondarycut-offregiona−30Vexternalbias isappliedfromthebottomofthesiliconwafers.Sincethisapplied biasgivesanoffsettotheentireenergyscaleinthespectrum,the aboveequationisstillvalid.ThehighresolutionSi2pandsecondary electroncut-offregionareshowninFig.4forbothann-typeanda p-typeH-terminatedSiwaferswhichshowsa∼0.30eVdifference inthecut-offedge,asopposedto0.18eVdifferenceintheSi2p peak.
Mainlyfortheelectronicapplicationsofgraphenethebiggest limitationistheabsenceofasemiconductingbandgap.Forthis reason,manyresearchershaveinvestigatedthedopingofgraphene whichisaneffectivemethodtotuneitselectronicproperty.This
Fig.2. (a)SchematicrepresentationofatypicalgraphenedeviceandbehaviorofAu4fpeaksofthesourceandthedrainelectrodesunderexternalbias.(b)2-Dimensional (areal)contourplotoftheAu4f7/2peakintensity.(c)3-DimensionalplotofthebindingenergypositionofAu4f7/2withinthedesignatedareawherexandyaxisaredistances,
whilethezandcolorbarbothrepresentsthebindingenergyscale.Dataisrecordedwith100mofX-raysspotsizeandwith100mstepsinbetweentwopoints.
Fig.3. VariationsintheC1sbindingenergypositionsoftheentireareaofthedevicefor;(a)a330Ohmpristinegraphenesample,and(b)the4kOhmoxidizedgraphene, under+6Vexternalbiastooneoftheelectrodes,whiletheotherisgrounded.Theirschematicrepresentationsaregivenatthebottom.
workpresentsa controlleddoping offlexible graphenedevices achievedbyvariousdopingmethodsandbringsanewperspective tocharacterizeit. In this work,byusingXPS andRaman Spec-troscopyascomplementarytechniquesweinvestigatedsubstrate induceddopingoftwopolymerswithdifferentfunctionalgroups onCVDgraphenelayers.Inaddition,weusedacidandbase expo-sureandalsoultraviolet illuminationinorder tocreatefurther doping.Fabrication ofadevicewithtworegions,each havinga differenttypeofdoping,isalsodemonstrated.
2. Experimental
2.1. Devicepreparation
Thegrapheneusedtofromdevicesis producedona copper foilby chemical vapor depositionat 1035◦C and 10Torr pres-sure using flowing methane and hydrogen gases. 2% solutions
of PMMA and PVC were prepared in acetone and THF
respec-tively.Afterthegraphenegrowth,PMMAandPVCsolutionsare depositedonthegraphenesidebydropcasting.Afterthecoating,
Fig.4.XPspectraof(a)Si2pand(b)Cut-Offregionsrecordedofap-typeandann-typeH-terminatedSisamples.
Fig.5.Schematicillustrationofthedevicefabricationinthesource−draingeometryusingthepolymersupportedgrapheneandAuelectrodes.(a)CVDgrapheneisprepared onCufoil.(b)AlayerofPMMA(orPVC)iscoatedonthegraphenefacethroughdropcasting.(c)CufoilisetchedawayusingFeCl3solutiontoobtainfree-standing
Polymer/Graphenefilms.(d)Polymer-graphenesampleisreversetransferredtotheSiwafer.(e)Goldelectrodesaredepositedoneachsideofthesample.
thepolymerlayerwiththegrapheneoncopperisheatedto80◦C. Afterwards,thepolymer/graphenefilmisisolatedbydissolvingthe CufoilusingFeCl3solution.Remainingpolymer-graphenesample
isreversetransferredtotheSiwafer,withgraphenefacingupward. Finally,goldelectrodesaredepositedoneachsideofthesample. TheprocessissummarizedinFig.5.
2.2. Characterizationmethods
AllXPSmeasurementsarecarriedoutusingaThermoFisher K-Alpha X-Ray Photoelectron Spectrometer with an energy of 1486.6eVmonochromaticsource.Theinstrumentismodifiedfor applicationofexternalbiaswhichiscontrolledbyaKeithley2400 Source-meter.Forsecondaryelectroncut-offmeasurementsboth
Fig.7. (a)SchematicsoftheprocessshowingtheacidandbasevaporexposurestepsofPVC/Graphenedevice.(b)RamanspectraofPVC/Grapheneasprepared,afterNH3
andthenHClexposure.(c)ShiftofRaman2Dbandand(d)Gbandpositionbetweentreatmentsforasprepared,afterexposuretoHClvapor,andthentoNH3vapor.
electrodesareconnectedtothesampleholderinordertoapplya −30Vbiastotheentireholder.Ontheotherhand,forthe Volt-ageContrastXPSmeasurementsoneoftheelectrodesisgrounded whiletheexternalbiasisappliedtotheotherone,inducingcurrent flowacrossthedevice.RamanspectraareobtainedwithaJobin
YvonLABRAMRamanSpectrometerequippedwith532nmgreen
laser.Thelaserbeamisfocusedbya50xobjectivelens.The scat-teredradiationiscollectedbythesameobjectivelensandsentto aCCDdetector.
3. Resultsanddiscussion
3.1. Substrateinducedeffects
Ramanspectra for PMMA and PVC supported graphene are
showninFig.6a.SincethecharacteristicfingerprintsofPVCand PMMAappearintheregionbelow1500cm−1weonlyfocusedon
theGand2DpeakscomingfromthegraphenelayerwhereGband isduetoRamanactivedoublydegenerateinplanestretchingofsp2 carbonscorrespondingtoE2gmode,and2DisanovertoneofDband
whichisthein-planebreathing-likemodeofcarbonrings[26,28]. Thepositionofthesetwofeaturesgivesinformationaboutdoping. AnupshiftoftheGbandusuallydemonstratestheoccurrenceof doping(holesorelectrons)ingraphenefilms,whiletheshiftin2D indicatesthetypeofdoping.Downshiftin2Dbandindicatesan n-typeandupshiftindicatesap-typedoping[29,42].Itisalso well-knownthat,moleculeswithelectronwithdrawinggroupsadsorbed onthesurfaceofgraphenewillleadtop-typedopingofgraphene, andmoleculeswithelectrondonatinggroupswillleadton-type doping[7,43,44].Inthiscase,sinceboththeGandthe2Dbands shifttohigherwavenumbers,thePVC-Graphenefilmexhibits p-typeshiftingcomparetothePMMA-Graphenefilm.Ascanbeseen ashiftof∼11cm−1isobserved,whichisreproducible.Intherecent workofSrivastavaLuoetal.[49],dopingofmonolayergraphene
Fig.8.(a)SchematicrepresentationofUVirradiationonhalfofthesampletogetherwiththeRamanspectraof(b)PMMA/Grapheneand(c)PVC/Graphenefromboththe maskedandUV-irradiatedregions.
throughthechargetransferbythetrappingorganicmoleculessuch astoluene,chlorobenzene,acetone,DMF,andPCwasreported.In ordertocheckanyeffectcomingfromthesolventthatweused dur-ingthesamplepreparation,wedipedPMMA/GrsampletotheTHF andPVC/GrsampletotheacetoneandcheckedtheRamanSpectra whichdoesnotshowanypeakshiftingontheGand2Dbands,see
SupportingInformation.
AswasshowninFig.4forn-andp-dopedSiwafers,thework functionissensitivetothedopinglevelandcanbeprobedbythe secondaryelectroncut-offmeasurementsunder−30Vbias.Similar behaviorcanalsobeobservedforthepolymersupportedgraphene samples,whicharenowshowninFig.6b,togetherwiththe cor-respondingshiftsdetectedthroughRamanmeasurementsshown in Fig.6a. Hence,thesecondary electroncut-off measurements alsosupportthemorep-typebehaviorof thegrapheneonPVC, comparedtothegrapheneonPMMA,whichhas∼0.6eVhigher cut-offenergy.Notealsothatmorethanthreedifferentsamples arepreparedandanalyzedforensuringthereproducibilityofthese findings.
3.2. Reversibledopingwithacid/baseexposure
Havingdemonstratedthatthesubstratesdoinducecontrollable andstabledopingontheCVDgraphene,wenowturntoinvestigate theeffectofexposingthegraphene-PVCsampletovaporsofacid andbasesolutions.Inthistypeofdopingtheexposedmoleculesare adsorbedontothesurfaceandactlikeacceptorsordonors.While avarietyofacidsandbaseshavebeenusedinliterature,nitricacid isthecommonlyusedone,whichisknowntoreproduciblycause p-typedoingonthepristinegraphenesamples[45,46].However,to ourknowledge,dopingbyacid/basevaporsasafurther doping/un-dopingofthealreadydopedsampleshasnotbeenreportedbefore. Tochecktheapplicabilityonthegraphenesamplesinthepresence ofapolymerasasubstrate,wesequentiallyexposedthesampleto thevaporsofammoniaandthenhydrochloricacidandrecorded
quicklytheRamanspectraaftereachstep.Ramanspectraofthe graphenesampleonPVC;(i)asprepared,(ii)afterNH3,and(iii)
HClexposureareshowninFig.7b,andtheshiftsintheGandthe 2DbandsaredepictedinFig.7c.
SincethechangesintheGbandpositiondonotreveal informa-tionaboutthetypeoftheinduceddoping,itismoreinformative toevaluatethe2Dshifts. ExposingthegrapheneonPVCtothe ammoniavaporscausesablueshiftinthespectrumhenceindicates inducedn-dopingwhichcanbereversedbackwithfurther expo-suretoHCl.Wealsoobservethatthistypeofdopingisreversible andturnstoitsoriginalstateafterafewhours.Thatisthereason whywedonotshowthesecondaryelectroncut-offspectrausing XPS,whichisperformedunderultra-highvacuumconditions,and afterprolongedpump-times.
3.3. UVirradiationandvoltagecontrastXPS
AlthoughUVirradiationisextensivelyusedtoinducedopingin differentwaysintheliterature,fromourpreviousstudiesweknow thatUValsoaffectsthepolymersubstrate.Forexample,in Refer-ences[47]and[48],wehadshownthatUVirradiationofPVCcauses photo-degradationandreleasesHCl,i.e.acidvapors,whichwealso expecttoeffecttheFermienergyofthegrapheneoverlayer.For thisreason,weirradiatedonlyhalfofbothtypesofpolymer sup-portedgraphenesamplesforadurationof5hwithUVlamp(mostly 254nmHg-Line)bymaskingtheotherhalf.ThistypeofUV irradi-ationreducesopticaltransparencyofthesamplesandblocksthe RamanmeasurementbyinducingasaturationintheRaman sig-nal.Therefore,onlythesecondaryelectroncut-offmeasurements under−30VontheUVirradiatedandmaskedregionareshownin Fig.8band8cforPMMAandPVCsupportedCVDgraphenefilms, respectively.Whilethecut-offedgeclearlyshowsthen-type dop-ingupontheUVirradiationonPMMAsupportedsamplewhichis alsoconsistentwiththepreviousstudyofLuoetal.,whoshowed thereversibleandcontrolledn-typedopingbyUVilluminationof
Fig.9. XPspectraof(a)Cut-Offand(b)C1sregions,recordedinthelinescanmodefromtheoneelectrodetowardtheotherunder−30Vgateand+6Vsourcebiasrespectively.
CVDgraphene[11],there is nomeasurableshift(hencedoping effect)inducedbytheUVirradiationonthePVCsupportedone. Webelievedthatthemainreasonisthenearcancelationofthe twoopposingeffects;(i)n-typedopingoftheUV,and(ii)p-type dopingoftheHClformationduringtheirradiation.
Foradditionalinformation,werecordedthesecondaryelectron cut-offspectraonalinestartingfromtheUVirradiatedtowardsthe maskedregionofthePMMAsupportedgraphenesample,where theclearindicationoftworegionswithdifferentworkfunctions areobservedasshown in Fig.9a.Furthermore,we fabricateda deviceonthesamesamplebydepositinggoldelectrodesinthe source-draingeometryandappliedanexternalbiasfromoneend whiletheotheroneisgrounded,andacquiredspectraona sim-ilarlineasinFig.9a,whicharedepictedinFig.9b.Thistypeof measurementsiscalledVoltageContrastXPSandhasextensively beenusedbyourgroup[35].Forexample,asweshowedearlier inFig.3athat;(i)forthedefectfreepristinegraphenesamplethe C1sbindingenergydropwaslinearalongtheline(IRdrop),and(ii) inthepresenceofdefectsthisdropdeviatedstronglyfrom linear-ity[22].However,verydifferentfromthepreviousfindingsisthat, forthecaseofhalfUVirradiatedPMMAsupportedgraphene,two differentregionswithtwodifferentslopesareobserved,indicating thatthetworegionshavedifferentresistancesandcarrier concen-trations,probedbyboththecut-offandalsothecharge-contrast measurements.
4. Conclusions
AsimpleandeasymethodfordopingofCVD-growngraphene isperformedusingpolymersubstrateshavingdifferentfunctional groups.Thismethodcanbeappliedtoanypolymerwhichisnot solubleinwater.Thefunctionalgroupsoftheunderlyingpolymer layerinducedopinginthegraphenelayer.Thedoping characteris-ticsanalyzedfortwotypesofpolymer,PMMAandPVC,byRaman SpectroscopyandXPS.Thismethodcausesthepermanentdoping onthegraphenelayerswithoutintroducingfurtherdefects. How-ever,bycontrolledexposuretoacid/basevapors orUVonecan createregionswithdifferentdopingtypesand/orlevelsinthesame film,whichmightevenleadtofabricationofp-njunctions, electri-calpropertiesofwhichcaneasilybeassessedbytheintroduced simplevariantsofXPSmeasurements.
Acknowledgements
ThisworkwaspartiallysupportedbytheScientificand Tech-nological Research Councilof Turkey (TUBITAK)[grant number 215Z534]andC.K.alsoacknowledgesthesupportfromthe Euro-peanResearchCouncil(ERC)Consolidator[GrantNoERC.682723 SmartGraphene].
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