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NANOEXPRESSOpenAccessPolycationstabilizationofgraphenesuspensionsKamranulHasan1*,MatsOSandberg2,OmerNur1andMagnusWillander1AbstractGrapheneisaleadingcontenderforthenext-generationelectronicdevices.
Wereportamethodtoproducegraphenemembranesinthesolutionphaseusingpolymericimidazoliumsaltsasatransferringmedium.
Graphenemembraneswerereducedfromgrapheneoxidesbyhydrazineinthepresenceofthepolyelectrolytewhichisfoundtobeastableandhomogeneousdispersionfortheresultinggrapheneintheaqueoussolution.
Asimpledevicewithgoldcontactsonbothsideswasfabricatedinordertoobservetheelectronicproperties.
IntroductionTheuniquephysical,electronic,andopticalpropertiesofgraphenehavebeenreportedmanytimes[1-4]andpromiseawidevarietyofapplications.
Differentmeth-odshavebeenadoptedforobtaininggraphene,e.
g.
,mechanicalexfoliationofgraphite[5],epitaxialgrowth[6],andchemicalexfoliationindifferentsolutions[3,7-9].
Averypromisingrouteforthebulkproductionofthegraphenesheetscanbechemicalreductionanddispersionofgrapheneinaqueoussolutions.
Twostepsareinvolvedinmakingwaterdispersiblegra-phene:(1)firstchemicaloxidationofgraphitetohydrophi-licgraphiteoxideand(2)exfoliatingitintographeneoxide(GO)sheetsinaqueoussolution.
GOsheetsaregraphenesheetshavingoxygenfunctionalgroups.
TheseGOsheetsarepreventedfromagglomerationbyelectrostaticrepul-sionalone[10].
TheinsulatingGOcaneasilybereducedtohighlyconductinggraphenebyhydrazinereduction.
However,thereductionofGOsoonleadstoagglomera-tion,whileastabledispersioniskeytothepossibilityoflarge-scaleprocessing.
Polymericimidazoliumsaltscanbeagoodwaytoformastabledispersionofgraphene.
Organicsaltsbasedontheimidazoliummoietyareaninterestingclassofions.
Lowmolecularweightimidazo-liumsaltscanhavealowmeltingpointandarethentermedionicliquids(ILs).
Thus,ILsaremoltensaltsattheroomtemperatureandconsistofbulkyorganiccationspairedwithorganicorinorganicanions.
Imidazoliumionicliquidshavemanyadvantageousproperties,suchasnoflammability,awideelectrochemicalwindow,highthermalstability,wideliquidrange,andverysmallvaporpressure[11].
Theyarealsoknowntointeractstronglywiththebasalplaneofgraphiteandgraphene.
Polymericimidazoliumsaltswouldthereforebeinterestingtoexploreasdispersingagentsforgraphene.
ExperimentalGrapheneoxidewaspreparedbythemodifiedHummer'smethod[12,13].
Thegraphiteflakes(PN332461,4g;SigmaAldrich,Sigma-AldrichSwedenAB,)werefirstputinH2SO4(98%,12mL)andkeptat80°Cfor5h.
Theresultingsolutionwascooleddowntoroomtemperature.
Mildsonicationwasperformedinawaterbathfor2htofurtherdelaminategraphiteintoafewmicronflakes.
Soni-cationtimeandpowerareverycriticalastheydefinethesizeoftheresultinggrapheneoxidesheets.
Excessivesoni-cationleadstoextremelysmallflakes.
Then,thesolutionwasdilutedwith0.
5Ldeionized(DI)waterandleftover-night.
ThesolutionwasfilteredbyNylonMilliporefilters(Billerica,MA01821).
TheresultingpowderwasmixedwithKMnO4andH2SO4andputinacoolingbathunderconstantstirringfor1.
5h.
ThesolutionwasdilutedwithDIwater,and20mLH2O2(30%)wasaddedtoit.
Thesupernatantwascollectedafter12handdispersedindiluteHClinordertoremovethemetalionresidueandthenrecoveredbycentrifugation[12,13].
CleanGOwasagaindispersedinwatertomakeahomogeneousdispersionandwascentrifugedat8,000rpmfor40mininordertoremovethemultilayerfragments.
Weaddedapolymericimidazoliummoltensaltintotheaqueousdis-persionofGOataconcentrationof1mgmL-1andstronglyshookthesolutionforafewminutes.
Theimida-zoliumsaltusedbyuswaspolyquaternium16(PQ-16)soldunderthetradenameLuviquatExcellencebyBASF*Correspondence:kamran.
ul.
hasan@liu.
se1DepartmentofScienceandTechnology(ITN),LinkpingUniversity,CampusNorrkping,SE-60174Norrkping,SwedenFulllistofauthorinformationisavailableattheendofthearticleulHasanetal.
NanoscaleResearchLetters2011,6:493http://www.
nanoscalereslett.
com/content/6/1/4932011Hasanetal;licenseeSpringer.
ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttributionLicense(http://creativecommons.
org/licenses/by/2.
0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.
(Ludwigshafen,Germany),acopolymerwith95%molarofimidazoliumchlorideand5%molarofvinylimidazole.
Useofthispolymericsaltforgraphenedispersionisnotfoundinliterature.
Then,thesolutionwasreducedbyhydrazinemonohydrateat90°Cfor1htoobtainastabledispersionofgrapheneinaqueoussolution.
ResultsanddiscussionThisaqueousPQ-graphenedispersionwasfoundtobestableevenafter2months,whereasthereducedGOwithouttheadditionofPQ-16formedagglomer-atessoonafterreductionwithhydrazine.
Thus,PQ-16isthemaincauseofastabledispersionofgra-phenemembranesinaqueoussolution.
Theunderly-ingmechanismhasbeenaffiliatedwithadsorptionofsomeofthepolycationsonthesurfaceofthegra-phenemembranesbynon-covalentπ-πinteractionsbetweentheimidazoliumringsofthesaltandgra-phene,soonafterreductionwithhydrazinemonohy-drate[14].
ThegraphenewasdepositedontoSi/SiO2(SiO2thicknessapproximately300nm)substratesbydip-coating.
SchematicofthewholeprocessisshowninFigure1.
ThesamplewasrinsedwithDIwateranddriedwithnitrogen.
Thedriedsampleswerefurthertreatedat400°Cfor2hinAr/H2tofurtherreducethegrapheneoxideandalsotosublimatethesolutionresidue.
Theopticalmicroscopeimagesweretakeninordertoidentifygra-phene[15].
Atomicforcemicroscopemeasurementswerecarriedouttoconfirmthepresenceofsingle-andfew-layergraphenebymeasuringstepheight[7].
Gra-pheneshowstypicalwrinkledstructurewhichisintrinsictographene[16]overrelativelylargesheetsizes.
Verylargegraphenemembraneswithsizesaround10*10μmwereidentified.
Thesizewasfoundtobedirectlyrelatedwithsonicationpowerandtime.
Exces-sivesonicationresultsinverysmallgraphenesheets,whereasinsufficientsonicationresultsinincompleteexfoliationofgraphiteoxide.
Wemeasuredtheheightprofilesofthegraphenemem-branesbyatomicforcemicroscopy(AFM)afterdropcastingthemonarelativelyflatSiO2/Sisubstrate.
TheaveragethicknessofaGOsheetwasapproximately1nm(Figure2),whichwasinagreementwiththeprecedingresearch,confirmingthatthegraphiteoxidewascomple-telyexfoliated.
Weobservedheightsfromslightlylessthan1nmtoafewnanometersthick.
Weassignedthesheetswithheightapproximately1nm,approximately1.
5nm,approximately2nm,andupto5nmtobeone-,two-,three-,andfew-layeredGOsheets,respectively.
ThiswasinagreementwiththereportedAFMresultsonfew-layergraphenesheets[5,8,17],wherethesingle-layergrapheneisalwaysapproximately1nm,probablyduetodifferentattractionforcebetweenAFMtipsandgra-pheneascomparedtoSiO2andimperfectinterfacebetweengrapheneandSiO2.
AFMimageofourchemicallyreducedGOsheetafteradditionofPQ-16,depositedonSiO2/Sisubstratebydropcasting,isshowninFigure3.
Thegraphiteinterlayerspa-cingisabout0.
34nmwhichshouldideallycorrespondtothethicknessofamonolayergraphene.
Conversely,thethicknessofsinglePQ-Gwasdeterminedtobeapproxi-mately1.
9nm.
IfweassumethatmonolayeredPQ-16cov-eredbothsidesofgraphenesheetwithoffsetface-to-faceFigure1Aqueoussolutionsofgrapheneoxideandgrapheneafterhydrazinereduction.
Inthepresenceofpolyelectrolyte,schematicofthetransfermechanism.
ulHasanetal.
NanoscaleResearchLetters2011,6:493http://www.
nanoscalereslett.
com/content/6/1/493Page2of6orientationviaπ-πinteractions(mechanismofstabiliza-tion),theestimateddistancebetweenPQandthegra-phenesheetisapproximately0.
35nm[18].
Accordingly,theaveragethicknessofthegraphenesheetinthePQ-Glayercanbederivedtobearound1.
9nm.
ThisassumptionisfurthersupportedbyFigure3b,whichshowsthestepheightfortheregionwithbilayergraphene.
Thestepheightofthegraphene-grapheneinterfacewasalsoobservedtobeapproximately1.
9nminvariousmeasurements.
Transmissionelectronmicroscopy(TEM)isalsoaveryimportanttoolforinvestigatingthequalityofexfo-liatedgraphene.
Wedroppedasmallquantityofthedis-persionontheholeycarbongridbypipetteanddriedthesamples.
Figure4ashowsbright-fieldTEMimage,Figure4bshowsthehigh-resolutiontransmissionelec-tronmicroscope(HRTEM)imageofthegraphenesur-face,andFigure4cdepictstheelectrondiffractionpatternobservedfromthesamearea.
Theanalysisofthediffractionintensityratiowasusedtoconfirmthepresenceofmonolayergraphene[19].
WeusetheBravais-Miller(hkil)indicestolabelthepeakscorre-spondingtothegraphitereflectionstakenatnormalincidence[19].
AfteranalyzingalargenumberofTEMimages,wewereabletoconcludethatourdispersioncontainsaverygoodfractionofmonolayergraphene.
Wefabricatedabottom-gatedgraphenefield-effecttran-sistor(FET)byputtingamonolayerofreducedGOFigure2TappingmodeAFMimageofGOonSiO2/Siwithstepheightprofile.
Figure3AFMimageofpolyquaternium-stabilizedgraphenemembranewithheightprofiles.
ulHasanetal.
NanoscaleResearchLetters2011,6:493http://www.
nanoscalereslett.
com/content/6/1/493Page3of6membraneinbetweenthermallyevaporatedgoldelectro-des.
Thechannellengthbetweensourceanddrainelectro-deswas5μm.
Theschematicandthescanningelectronmicroscope(SEM)imageofthedeviceareshowninFigure5.
Figure5cshowsthedraincurrent(Id)vs.
gatevoltage(Vg)curveofFETpreparedwiththisreducedmonolayergraphenemembrane.
TheFETgateoperationexhibitsholeconductionbehavior.
Puretwo-dimensionalgraphenehasazerobandgapthatlimitsitseffectiveappli-cationinelectronicdevices.
WebelievethatthisreducedGOfromPQdispersionhasakindofdopingeffectthatmakesitmorefavorableforapplicationsduetoitsimprovedelectronicproperties.
Thereweretheoreticalsimulations[20,21],whichwerelaterconfirmedexperi-mentally[22]thatthe100%hydrogenationoffreestandinggrapheneresultsinametaltoinsulatortransition.
Hydro-genationofgrapheneonasilicondioxide(SiO2)substratehasalsoledtotheenergygapopening[23].
Here,wecanattributethedeficiencyofambipolarbehaviortoholedop-ingcausedbyresidualoxygenfunctionalitiesresultinginap-typebehaviorandafield-effectresponse[2,24].
Thus,chemicalfunctionalizationisapossibleroutetomodifytheelectronicpropertiesofgraphene,whichcanbeimpor-tantforgraphene-basednanoelectronics[25],althoughthereisroomforfurtheroptimizationoftheprocessforimprovingtheproperties,inordertomakeitidealforindustriallevelapplications.
ConclusionsInsummary,wereportamethodtoproduceandfunc-tionalizegraphenemembranesinthesolutionphaseusingpolymericimidazoliummoltensaltsasatransfer-ringmedium.
Graphenemembraneswerereducedfromgrapheneoxidebyhydrazineinthepresenceofapoly-electrolytewhichwasfoundtobeaverystabledisper-sionforthegraphenemembranesintheaqueoussolution.
ThereducedGOmembranesweretransferredtoaSiO2/SisubstratebysimpledropcastingandwerefurtherreducedbyannealinginH2/Ar.
Asimpledevicewithgoldcontactsonboththesideswasfabricatedinordertoobservetheelectronicproperties.
Weconcludethatchemicalfunctionalizationisapossibleroutetomodifyandimprovetheelectronicpropertiesofgraphene.
Figure4Electronmicroscopyofgraphene.
(a)Bright-fieldTEMimagesofmonolayergraphene,(b)HRTEMimagefromthesamelocation,and(c)electrondiffractionpatternofthegraphenesheetin(a)withdiffractionspotslabeledbyMiller-Bravaisindices.
ulHasanetal.
NanoscaleResearchLetters2011,6:493http://www.
nanoscalereslett.
com/content/6/1/493Page4of6AcknowledgementsWeacknowledgethehelpofAmirKarim(AcreoKista)forhistechnicalsupportinTEMimaging.
Authordetails1DepartmentofScienceandTechnology(ITN),LinkpingUniversity,CampusNorrkping,SE-60174Norrkping,Sweden2AcreoABBredgatan34,SE-60221Norrkping,SwedenAuthors'contributionsAllauthorscontributedequally,readandapprovedthefinalmanuscript.
CompetinginterestsTheauthorsdeclarethattheyhavenocompetinginterests.
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