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NANOEXPRESSOpenAccessThinningandfunctionalizationoffew-layergraphenesheetsbyCF4plasmatreatmentChaoShen1,GaoshanHuang1*,YingchunCheng2*,RonggenCao1,FeiDing3,UdoSchwingenschlgl2andYongfengMei1*Abstract:Structuralchangesoffew-layergraphenesheetsinducedbyCF4plasmatreatmentarestudiedbyopticalmicroscopyandRamanspectroscopy,togetherwiththeoreticalsimulation.
Experimentalresultssuggestathicknessreductionoffew-layergraphenesheetssubjectedtoprolongedCF4plasmatreatmentwhileplasmatreatmentwithshorttimeonlyleadstofluorinefunctionalizationonthesurfacelayerbyformationofcovalentbonds.
Ramanspectrarevealanincreaseindisorderbyphysicaldisruptionofthegraphenelatticeaswellasfunctionalizationduringtheplasmatreatment.
TheF/CF3adsorptionandthelatticedistortionproducedareprovedbytheoreticalsimulationusingdensityfunctionaltheory,whichalsopredictsp-typedopingandDiracconesplittinginCF4plasma-treatedgraphenesheetsthatmayhavepotentialinfuturegraphene-basedmicro/nanodevices.
PACS:81.
05.
ue;73.
22.
Pr;52.
40.
Hf.
Keywords:Graphenesheets,Plasma,Thinning,Functionalization,Ramanspectroscopy,Densityfunctionaltheory,DiracconesplittingBackgroundGrapheneisonelayerofCatoms,arrangedinahex-agonallattice[1-3].
Sinceitwasfirstproducedbymech-anicalexfoliationin2004[4],graphenehasbeenstudiedboththeoreticallyandexperimentally,anddemonstrateshighlyattractiveproperties[5-10].
Especially,theDiracequationpredictsthatuniqueelectronicpropertiesshouldarisefromthehexagonalhoneycomblatticestructure,makingtheelectronsbehaveasmasslessrela-tivisticfermions[1,5,9].
Thecorrespondinghighmobilityandvelocityhavegreatpotentialinfutureelectronics[11,12].
However,severalproblemsneedtobesolvedbe-foreitcanbeultimatelyemployedinpracticalapplica-tions.
Forinstance,thezerobandgapaswellasbadwettabilityofgraphenemightcauseproblemsindevicefabrication[13-15].
Undersuchcircumstances,plasmatreatmentisconsideredtobeoneofthetrickstoover-comethedifficulties.
Thecorrespondingsurfacefunctio-nalizationchangesnotonlythesurfacestatus,butalsothestructureofgraphenesheets[16,17].
Previousresearcheshavealreadydemonstratedthatoxygenplasmahastheabilitytotunethepropertiesofgraphenesheets[14,17].
Ontheotherhand,fluorineplasma,whichmayprovideadditionaladvantages[16,18],issel-domexperimentallyinvestigatedindetail,althoughastrongp-dopingbehaviorwaspredicted[19,20].
Inthiswork,structuralchangesoffew-layergraphenesheetsinducedbyCF4plasmatreatmentarestudiedbyopticalmicroscopyandRamanspectroscopy.
Ourresultssug-gestanobviousthicknessreductioneffectinfew-layergraphenesheetstreatedwithCF4plasmaaswellassur-facefluorinefunctionalizationbyformationofcovalentbondsbetweenthetopgraphenelayerandtheions.
TheproduceddisorderingraphenelatticeiswellreflectedintheRamanspectra,andthecorrespondingmechanismisstudiedtheoretically.
Theresultspresentedinthisworkprovideapossibledirectiontoobtaingiantsingle-layergraphenesheetswithnecessarysurfacefunctionalizationtorealizethep-typedopinglevelandtoopentheDiracconeforfuturegraphene-basedmicro/nanodevices.
MethodsThefew-layergraphenesheetsusedinthecurrentstudywereproducedbymechanicalexfoliationfromhighly*Correspondence:gshuang@fudan.
edu.
cn;yingchun.
cheng@kaust.
edu.
sa;yfm@fudan.
edu.
cn1DepartmentofMaterialsScience,FudanUniversity,Shanghai200433,People'sRepublicofChina2PSEDivision,KingAbdullahUniversityofScienceandTechnology(KAUST),Thuwal23955-6900,KingdomofSaudiArabiaFulllistofauthorinformationisavailableattheendofthearticle2012Shenetal.
;licenseeSpringer.
ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttributionLicense(http://creativecommons.
org/licenses/by/2.
0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.
Shenetal.
NanoscaleResearchLetters2012,7:268http://www.
nanoscalereslett.
com/7/1/268orderedpyrolyticgraphiteandtransferredonasiliconsubstratecoveredwith300-nmSiO2.
Thesampleswerethencleanedbyultrasonicationinacetonefor30store-movetheresidualofthescotchtapeandunattachedgraphitepieces.
TheCF4plasmatreatmentwascarriedoutinthereactionchamberofaJupiterIIIreactive-ionetchingsetup.
Thesampleswereexposedto0.
8-TorrCF4underradio-frequencyplasma(20W)withdifferenttimesrangingfrom1to5swithastepof1s.
Inourex-periment,thenumberoflayersinthefew-layergraphenesheetscanbeestimatedbyacombinationofopticalmi-croscopyandRamanspectroscopy.
RamanspectraweretakenonaRenishawinViamicro-Ramanspectrometerwiththe514-nmlineofanAr+laseraslightsource(Shanghai,China).
Allthemeasurementswerecarriedoutatroomtemperature.
ResultsanddiscussionAsweknow,morphologyobtainedbyopticalmicroscopyisadirectandconvenientwaytoidentifythefew-layergraphenesheets.
OpticalimagesinFigure1demonstratethesurfacemorphologiesofthesamplesbeforeandafterCF4plasmatreatmentofdifferenttimes.
Forashortplasmatreatment(e.
g.
,2s,seeFigure1a,b),noobviouschangecanbenoticedatleastintheopticalimages.
However,withprolongedplasmatreatmentofupto5s,athinningeffectisobviousasthecolorcontrastofthesamplesurfacechangessignificantly.
OnecanseeinFigure1c,dthatdarkpurpleregionsappearlighteraftertreatmentandsomethinsheetscanhardlyberecordedbythecamera(althoughtheycanbeobservedthrougheyepiecelens).
Duringtheprocessofplasmatreatment,ionsproducedbyanelectromagneticfieldattackedthesamplesurfaceandreactedwithit.
Therefore,thetoplayerofthesheetcanberemovedinlongertreatment,whichsuggestsanaccumulationeffectoftheionetch-ingforsuchalowplasmapowerof20W.
However,astructuralmodificationorsurfacefunctio-nalizationmayexistduringCF4plasmatreatment[14,16,17],eventhoughtheycannotbedetectedbynor-malopticalmicroscopy.
Previousresearchesrevealedthatthestructuralmodificationmayinduceashiftofthephononfrequenciesingraphenesheetswhichcanbeop-ticallyprobedbyRamanspectroscopy[10,21,22].
Inordertoguaranteethereproducibilityofourexperiment,theRamansignalwascollectedfromthesamespotwiththeassistanceofopticalmicroscopy.
Figure2adisplaystheRamanspectraofagraphenesheetbeforeandafter2sofCF4plasmatreatment,and,aspreviouslydis-cussed,theopticalmicroscopyshowsnochange.
ThelowerpanelinFigure2aistheRamanspectrumofthesamplebeforeplasmatreatment,wheretwostrongpeaksarenoticeable.
TheGbandlocatedatapproximately1,580cm1iscorrespondingtoopticalE2gphononsattheBrillouinzonecenter.
Thesharp2Dband,whichistheovertoneoftheDpeakandthesumoftwophononswithoppositemomentum,appearsatapproximately2,680cm1[23].
Theintensityratiobetweenthe2DandGbands(I2D/IG=3.
2)andthesmallfullwidthathalfmaximum(FWHM)ofthe2Dband(26cm1)indicateFigure1OpticalimagesofsamplesbeforeandafterCF4plasmatreatment.
(a,c)Opticalimagesdemonstratingthemorphologiesofas-preparedsamplesbeforeCF4plasmatreatment.
(b)Opticalimageofthesamplein(a)after3sofCF4plasmatreatment.
(d)Opticalimageofthesamplein(c)after5sofCF4plasmatreatment.
Shenetal.
NanoscaleResearchLetters2012,7:268Page2of8http://www.
nanoscalereslett.
com/7/1/268thatthegraphenesheetmeasuredcontainsonlyonelayer(i.
e.
,single-layergraphene)[14,23].
Besides,aweakDbandisdetectedatapproximately1,350cm1andisconnectedtotransverseopticalphononsneartheKpointwhichrequiredefectsorlatticedisorder(e.
g.
,non-sp2composition)fortheiractivationviaaninter-valleydouble-resonanceRamanprocess[24].
Therefore,theweakDbandinthesamplebeforeplasmatreatmentsuggeststhatthesingle-layergraphenesheetinourex-perimentisofhighquality.
ItisinterestingtonotethatobviouschangesexistintheRamanspectraofthesam-pleafter2sofplasmatreatment(asshownintheupperpanelofFigure2a):(1)TheDpeakatapproximately1,350cm1isremarkablyintensified,suggestinganintroductionoflatticedisorderinthegraphenesheet[24];(2)theGpeakisbroadenedandashoulderD'bandarises,whichoriginatesfromintra-valleyresonantRamanscattering[24,25];(3)anewbandoccursatap-proximately2,941cm1whichisbelievedtobeacom-binationofDandD'bands[17]orGandDbands[26,27].
ThesefeaturesintheRamanspectraarereportedinthegraphenesheettreatedwithoxygenplasma[14]andarethesymbolsofsurfacefunctionaliza-tion[14,16].
However,iftheplasmatreatmentiselon-gated,thefew-layergraphenesheetcanbethinnedandtheevolutionofRamanspectrabecomescomplicated.
Inourexperiment,wefoundthatsingle-layergraphenecannotsurvivethe5sofCF4plasmatreatment.
Thus,theRamanmeasurementwascarriedoutonafew-layergraphenesheetwhichcontainsmorethanonelayerbe-foreplasmatreatment.
Theobtainedspectrumwasplot-tedinthelowerpanelofFigure2b.
TheFWHMofthe2Dpeakisbroadenedto53cm1andtheratioofI2D/IGis1.
1,elucidatingthatthissheetconsistsofafewlayers.
After5sofplasmatreatment,theRamanspectrumintheupperpanelofFigure2bshowsthattheFWHMofthe2Dbandisreducedto32cm1andtheratioofI2D/IGisincreasedto2.
5.
Thespectralevolutionisdifferentfromthatin2sofplasmatreatmentandindicatesthatthefew-layergraphenelayeristhinnedtooneortwolayers[14,23,28],ascanbeobservedbyopticalmicroscopy.
TheevolutionoftheDbandisthemostimportantconcernofmanyresearches.
Theincreaseofitsintensityisgenerallyconsideredtobeevidencethatlatticedis-orderexistsinthegraphenesheet[14].
AninterestingphenomenonariseswhenwelookclosetotheintensityoftheDbandinafew-layergraphenesheetsubjectedtoCF4plasmafordifferenttimes.
TherelativeintensityoftheDband(ID/IG)inthesampletreatedfor5sissmal-lerthanthatfor2s,implyingthatthedisorderisevenremarkableforshortertreatment.
Tounderstandthispeculiarbehavior,wemustfirstmakeclearthemechan-ismofdisorderproductionduringplasmatreatment.
ThedisorderandthecorrespondingemergenceofDandD'bandsinoursamplesmayarisefromtwoFigure2RamanspectraofgraphenesheetsbeforeandafterCF4plasmatreatment.
(a)Ramanspectraofasingle-layergraphenesheetbefore(lowerpanel)andafter(upperpanel)2sofCF4plasmatreatment.
(b)Ramanspectraofafew-layergraphenesheetbefore(lowerpanel)andafter(upperpanel)5sofCF4plasmatreatment.
Athinningeffectisobvious.
Shenetal.
NanoscaleResearchLetters2012,7:268Page3of8http://www.
nanoscalereslett.
com/7/1/268processes:(1)physically,theorderofpristineCatomsingrapheneisdisruptedbytheCF4plasmaandsomeoftheCatomsmaybesputteredout,whichisnamedasionbombardmenteffect[29,30];and(2)chemically,co-valentbondsoffluorine-relatedspeciestothegraphenelatticeformduringplasmatreatment,leadingtocorre-spondingsurfacemodificationandfunctionalization.
Bothprocessescontributetotheincreaseofthedis-order.
Asforthechemicalprocess,thebondenergiesneedtobeconsidered.
ItwasdisclosedthattheC-Cbondingrapheneownshigherbondenergy(607kJ/mol)thantheC-Fbond(485kJ/mol).
Thus,theC-Cbondcanbehardlybroken,andhenceCFn(n=1to3)orFspecieswereonlyadsorbedonthetopgraphenelayerbyformationofcovalentbonds,whichwillbecomesatu-ratedbecausethenumberofavailableactiveCatomsdecreaseswithtime.
Itisworthnotingthattheforma-tionofcovalentbondscanbeevidencedbytheupshiftof2Dpeakfrom2,685to2,691cm1[31].
Onemayinferthatthefew-layergraphenesheetsubjectedtoCF4plasmatreatmentthereforepossessesdisorderfeaturesfromboththephysicalandchemicalinteractions.
Thechemicalinteractiontakesplaceonlyonthetopmostlayer,whilethephysicalinteractionaccumulatedwithtimeandalongtreatmentcanremovethetoplayer(i.
e.
,thinningeffect,asisreflectedintheopticalFigure3Ramanspectraofplasma-thinnedfew-layergraphenesheetswithdifferentthicknesses.
Thethreespectrawerecollectedfromthreedifferentspotsrespectively.
Thepositionsofthethreespotsarelabeledintheinsetopticalimageofasamplesubjectedto5sofCF4plasmatreatment.
Scalebar,5μm.
Shenetal.
NanoscaleResearchLetters2012,7:268Page4of8http://www.
nanoscalereslett.
com/7/1/268microscopyandRamanspectroscopy),exposingthebe-neathlayer.
TheemergenceandintensificationoftheDandD'bandsinthesamplesubjectedto5softreatmentthusoriginatesfromthedisordercreatedinthisnewtoplayerandshouldincreasegradually.
Consequently,thedis-orderprobedbyRamanspectroscopyisevensmallerinthesampletreatedbyCF4plasmafor5sthaninthesampletreatedfor2s(seeFigure2a,b).
SincetheintensityoftheDbandreflectsthedisorderinthetoplayerofthefew-layergraphenesheet,therela-tiveintensityoftheDbandmaygiveusaclueaboutthethicknessofthesample.
Toprovethis,wemeasuredRamanspectraofplasma-thinnedfew-layergraphenesheetswithdifferentthicknesses.
ThethreespectrainFigure3wererespectivelycollectedfromthreespotsla-beledintheinsetopticalimage.
ThecolorcontrastamongthethreespotsindicatesthattheirthicknessesareratherdifferentandincreasefromAtoC,whilenoneofthemconsistasinglelayer.
OnecanseethattheID/IGisobviouslyrelevanttothethicknessoffew-layergraphenesheet.
Forathickersheet,theweightofthetopmostlayerwithremarkabledisorderissmaller,andthusID/IGshouldbecorrespondinglysmaller.
Therefore,therationofID/IGprovestobeaneffectiveRamanfactortocom-parethethicknessesofplasma-treatedfew-layergra-phenesheets.
Therefore,theID/IGprovestobeanothereffectivefactortoestimatethethicknessesofplasma-treatedfew-layergraphenesheetsotherthanI2D/IG.
Althoughadetailedexperimentalinvestigationissofardifficult,wemanagedtosimulatetheionadsorptiondur-ingtheplasmatreatmentbyemployingdensityfunc-tionaltheoryandthegeneralizedgradientapproximationoftheexchangecorrelationfunctionalwithultrasoftpseudopotentials[32,33]torevealthedisorderproducedbyfluorinefunctionalizationandtheunderneathmech-anism.
Ahighcutoffenergyof800eVandak-pointsam-plingwithan8*8*1meshareemployedtoachievehighaccuracyinthecalculations.
Structuraloptimizationiscarriedoutonallsystemsuntiltheresidualforcesareconvergedto0.
003eV/.
Inordertoavoidanydrawbackoftheperiodicboundaryconditions,anover20--thickvacuumlayerisincluded.
Weestimatethatintheplasma,thereareF,CF,CF2,andCF3ions.
Theenergybarrieriscalculatedbytheclimbing-imagenudgedelas-ticbandmethod[34],whichenablesustofindthemini-mumenergypathbetweenthegiveninitialandfinalstatesofatransition.
Bycalculatingthestructuresofionsonthegraphenesheet(seeFigure4),wefindthatCFandCF2canhardlybeadsorbed,formingcovalentbonds,whileFandCF3ionscanbeadsorbed.
Thecorrespond-ingadsorptionenergiesare2.
2and0.
4eV,respectively.
Thedifferentabsorptionbehaviorscanbeunderstoodasfollows:Becauseofthesp3hybridizationnatureforCatomsinCF,CF2,andCF3,therearetwoun-bondedelectronsinCF2andonlyoneinCF3,whiletheCatomsingraphenearesp2hybridizedandthereisonlyoneun-bondedelectronforeachCatom.
Thus,itiseasierforCF3tobeadsorbedontopoftheCatomviaacovalentbond(seeFigure4d).
However,itisdifficultforCF2toformtwocovalentbondswithtwonearbyCatomsinthegraphenesheet.
Wecheckeddifferentabsorptionconfig-urationsbystructurerelaxationandfindthatforallcases,theCF2willleaveawayfromthegraphene,asshowninFigure4c.
Fordifferentartificialabsorptionconfigurations,therewouldbelargestrain/stressaroundtheabsorptionsite,indicatingthatthedifficultyofform-ingtwocovalentbondsbetweenCF2andgrapheneismainlyduetothelargestrain/stress.
AsimilarsituationisalsonotedintheCFcase.
Inaddition,thecalculatedenergybarrierfortheCF3adsorbedonthegraphenesur-faceisonly0.
04eV,whichdemonstratesthattheCF3ad-sorptionisenergeticallyfavorable.
DuetotheexistenceofcovalentbondsbetweenF/CF3andConthegraphenesheet,thereisdistortionaroundtheadsorptionposition,Figure4Atomicstructuresof(a)F,(b)CF,(c)CF2,and(d)CF3ionsongraphenesheets.
CandFatomsareindicatedbyyellowandgrayballs,respectively.
TheoreticalsimulationindicatesthatCFandCF2canhardlybeadsorbed.
Shenetal.
NanoscaleResearchLetters2012,7:268Page5of8http://www.
nanoscalereslett.
com/7/1/268whichgivesbirthtotheappearanceofDandD'bandsintheRamanspectraoftheplasma-treatedsamples.
Here,wewouldliketobrieflydiscussthepotentialapplicationsofthisfluorinefunctionalization.
Wecalcu-latedthebandstructuresanddensitiesofstatesofgra-phenesheetsfluorinatedwithFandCF3withcoverageof5.
6%,asshowninFigure5.
DuetotheBrillouinzonefolding[20],theDiracpointislocatedattheΓpointintheelectronicbandstructure.
Itisworthnotingthatduetothesymmetrybreaking,theDiracconesplitswithasmallgap(approximately0.
1eV).
Moreover,theDiracconeshiftsto0.
5/0.
1eVabovetheFermilevelafterF/CF3functionalizationduetoelectrontransferfromtheCatomsinthegraphenesheettoF/CF3,indicatingthatthegraphenesheetscanbep-typedopedbyfluorinatingwithbothFandCF3.
However,itcanbefoundthatFismuchmoreefficienttointroducep-typedopingingraphenesheetsthanCF3.
Inaddition,theadsorptionoftheF/CF3ongraphenesheetsalsointroducespin-ningstatesaroundtheFermilevels,asdemonstratedinFigure5.
Detailedexperimentalverificationisrequiredinthefuture,whilethepresentedcalculationhasalreadyproventheapplicationpotentialsofgraphenesheetsfunctionalizedbyCF4plasmainmicro/nanoelectronics.
Figure5BandstructuresanddensitiesofstatesofF-andCF3-functionalizedgraphenesheets.
(a)Bandstructureand(b)densityofstatesofaF-functionalizedgraphenesheet.
(c)Bandstructureand(d)densityofstatesofaCF3-functionalizedgraphenesheet.
TheFermilevelissetat0eV.
Shenetal.
NanoscaleResearchLetters2012,7:268Page6of8http://www.
nanoscalereslett.
com/7/1/268ConclusionInconclusion,few-layergraphenesheetswerepreparedbymechanicalexfoliationandthestructuralevolutionduringCF4plasmatreatmentwasstudiedindetailbyop-ticalmicroscopyandRamanspectroscopy.
Theexperimen-talresultsindicateathicknessreductionunderprolongedplasmatreatmentwhileshorttreatmentleadsonlytofluorinefunctionalizationonthesurfacelayer.
Thecom-binationofbothphysicalandchemicalreactionsintheplasmatreatmentleadstostructuralmodificationswhichcanbewellprobedinRamanspectra.
Theoreticalsimula-tionsuggestsaF/CF3functionalizationbyformationofcovalentbondsandalsopredictsacorrespondingp-typedopingandDiracconeopeningaftertheF/CF3adsorp-tion.
Althoughfurthercharacterizationsareneededtoevaluatetheelectronicpropertiesoftreatedsamples,thecurrentworkofthinningandfunctionalizingfew-layergraphenesheetsbyCF4plasmaundercontrolrepresentsanintegrativepathwaytoindustrialfabricationofgra-phene-basedmicro/nanodevices.
CompetinginterestsTheauthorsdeclarethattheyhavenocompetinginterests.
Authors'contributionsCS,GH,FD,andYMdesignedthestudy.
CSperformedtheexperimentswithhelpfromRC.
YCandUScarriedoutthetheoreticalstudy.
CS,GH,YC,andYMcontributedindraftingthemanuscript.
Alltheauthorstookpartinthediscussionoftheresults,andeditedandapprovedthemanuscript.
AcknowledgmentsThisworkissupportedbytheNaturalScienceFoundationofChina(Nos.
61008029and51102049),ProgramforNewCenturyExcellentTalentsinUniversity(No.
NCET-10-0345),andShanghaiPujiangProgram(No.
11PJ1400900).
Authordetails1DepartmentofMaterialsScience,FudanUniversity,Shanghai200433,People'sRepublicofChina.
2PSEDivision,KingAbdullahUniversityofScienceandTechnology(KAUST),Thuwal23955-6900,KingdomofSaudiArabia.
3IBMResearchZürich,Sumerstrasse4,RüschlikonCH-8803,Switzerland.
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