单层石墨烯氧化物

University of Pennsylvania

ScholarlyCommons

Department of Physics Papers

Department of Physics

3-19-2009

Photoluminescence and Band Gap Modulation inGraphene Oxide

Zhengtang Luo

University of Pennsylvania, [email protected]

Patrick Vora

University of Pennsylvania, [email protected]

Eugene J. Mele

University of Pennsylvania, [email protected]

A.T. Charlie Johnson Jr.

University of Pennsylvania, [email protected]

James M. Kikkawa

University of Pennsylvania, [email protected]

Suggested Citation:

Luo, Z., P.M. Vora, E.J. Mele, A.T.C. Johnson and J.M. Kikkawa. (2009). "Photoluminescence and band gap modulation in graphene oxide."AppliedPhysics Letters.94, 111909.

Copyright 2009 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of theauthor and the American Institute of Physics. The following article appeared inApplied Physics Letters.and may be found at http://dx.doi.org/10.1063/1.3098358.

This paper is posted at ScholarlyCommons.http://repository.upenn.edu/physics_papers/64For more information, please [email protected].

APPLIEDPHYSICSLETTERS94,111909͑2009͒

Photoluminescenceandbandgapmodulationingrapheneoxide

ZhengtangLuo,PatrickM.Vora,EugeneJ.Mele,A.T.CharlieJohnson,andJamesM.Kikkawaa͒

DepartmentofPhysicsandAstronomy,TheUniversityofPennsylvania,209South33rdStreet,Philadelphia,Pennsylvania19104,USA

͑Received21January2009;accepted21February2009;publishedonline19March2009͒Wereportbroadbandvisiblephotoluminescencefromsolidgrapheneoxide,andmodiﬁcationsoftheemissionspectrumbyprogressivechemicalreduction.Thedatasuggestagappingofthetwo-dimensionalelectronicsystembyremovalof␲-electrons.Wediscusspossiblegappingmechanisms,andproposethataKekulepatternofbonddistortionsmayaccountfortheobservedbehavior.2009AmericanInstituteofPhysics.͓DOI:10.1063/1.3098358͔

Singlelayerandbilayergraphenesystemscanexhibitaremarkablediversityofphenomena,includingobservationsofaroom-temperature,unconventionalquantumHalleffect,1–3predictionsofthequantumspinHalleffect,4brokenspin5,6orpseudospin7symmetries,andﬁnitesizeeffectsthatcanbeusedtocontrolbandstructure5,6,8,9andmagnetism.5,6,9,10Takentogether,thesepropertiessuggestthatgrapheneisapromisingplatformforseamlesslyex-changinginformationbetweendifferentdegreesoffreedom.Anoutstandingchallengeinthisregardisphotonicintegra-tionandbandgapmanipulation.Severaltheoreticalworkspredictthatadirectgapinthevisiblewouldoccurforsufﬁ-cientlysmallgraphenenanoribbons,5,6,8,9butnoobservationsofthisﬁnitesizeeffecthavebeenreported.Additionally,modiﬁcationsofthegraphenesheetbyoxidationcanintro-ducedirectgapbehavior.11

Hereweshowthatgraphene,althoughintrinsicallyazero-gapsemimetal,maybeoxidizedinamannerthatpro-ducesphotoluminescence͑PL͒forsolid,drop-castsamples.Weﬁndthatdespitethehighsurfaceareaofgrapheneoxide͑GO͒andinmarkedcontrasttocarbonnanotubes,thestrengthofPLfromGOﬂakesdoesnotdiffersigniﬁcantlybetweenaqueousanddrop-castsamples.TheresilienceofPLforsolidGOsamplesisencouragingfortechnologicalapplications,implyingthatGOmaybeausefulphotonicmaterialwhenincorporatedinsolidstatedevices.Thelargeobservedgapcreatesthepossibilityforspatiallymodulatingthebandstructurewithinasinglegrapheneﬂakebylocalcontroloftheoxidationproﬁle.StudiesofprogressivechemicalreductionshowquenchingofPLforbothdrop-castandaqueoussamples,coordinatedwithchangesinabsorp-tion.Thesestudiesalsoﬁndsignaturesofbandgapmanipu-lation,albeitwithdifferentcharacterforsolidandliquidsamples.

AqueousdispersionsofsinglelayerGOwithanaverageareaofϳ100␮m2weresynthesizedfollowingaproceduredescribedelsewhere.12Solidsamples͑s-GO͒wereobtainedbydrop-castingtheconcentratedGOsolutionresultingfromthisprocedureontopolished,lowauto-ﬂuorescence,Suprasil-2substratesandthenbakingat95°Cfor30min.Liquidsamples͑l-GO͒wereheldinquartzcuvettes,dilutedahundredfoldormoreasnecessarytoadjustopticaldensity.PLforbothl-GOands-GOwascollectedat90°degreesto

a͒

theexcitation,andthereﬂection͑transmission͒geometryfors-GOcorrespondedtocollectiononthesame͑opposite͒sideoftheﬁlm.PLspectrawereexcitedbyXelamppassedthroughamonochrometer,andadditionalﬁlterswereem-ployedonexcitationandcollectiontorejectexcitationscat-ter,secondordergratingeffects,andleakageofXelampspikes.Spectrawerespectrallycorrectedfordetectorefﬁ-ciencies,andnormalizedbyexcitationpower.AllPLdatashownhere͑bothmapsandsinglespectra͒arefurthernor-malizedtoamaximumvalueofunityandtakenat300K.

Figure1͑a͒comparesPLforbothl-GOands-GOsamples.Bothpeaksinthevisiblewithalonginfraredemis-siontail.Differencesinmeasurementgeometrymakequan-titativecomparisonsofthequantumyieldimpossible,butgenerallylittledifferencewasseeninPLintensity.Aninter-estingquestioniswhetherenergyrelaxationandspectraldif-fusionarequalitativelyalteredbyaggregation.Forisolatedﬂakesinl-GO,diffusionoffreecarriersorboundexcitonsshouldbeconﬁnedtothetwodimensionalGOplane.How-ever,fors-GO,atomicforceandopticalmicroscopy,bothindicateﬁlmsoflayeredGOﬂakes,whichcouldgiverisetoadditionalinterﬂakerelaxationpathways.Ifinterlayercou-plingisstrongenough,theemissionspectrumcouldredshift.s-GOindeedshowsmorePLspectralweightintheinfrared,buttheredshiftinthePLpeakpositionisnotarobustfeatureoftheexperimentandwasinconsistentfromsampletosampleperhapsduetovariationsintheoxidationdensity.Inadditiontoexcitondiffusion,severalplausiblechangescould

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Electronicmail:[email protected].

FIG.1.͑Coloronline͒͑a͒NormalizedPLspectraexcitedat500nm͑s-GOtakenintransmission͒.͑b͒Absorption͑leftaxis,solid͒andPLintensitydetectedat752nm͑rightaxis,dashed͒,asafunctionofexcitationwavelength.

2009AmericanInstituteofPhysics

0003-6951/2009/94͑11͒/111909/3/$25.0094,111909-1

NormalizedPL(arb.units)

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111909-2

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Luoetal.Appl.Phys.Lett.94,111909͑2009͒

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FIG.2.͑Color͒NormalizedPLexcitation-emissionmapsfors-GOtakenintransmissionduringhydrazinevaporexposure.

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alsoinﬂuencethepeakposition,includingmodiﬁcationsofthedielectricenvironment,spectralreabsorption,andvaria-tionsinoxidationdensity.

TheGOabsorption͓Fig.1͑b͔͒increaseswithenergyfromthenearinfraredto3.1eV,andprovidesaninterestingcontrasttothenonmonotonicPLspectrum.WefurthernotethatPLexcitation͑PLE͒spectradonotmirrortheabsor-banceincreasesathigherenergies.Tothecontrary,asshowninFig.1͑b͒,astheexcitationenergyincreases,PLintensitydetectedataﬁxedwavelengthdecreases.Thelatterindicatesthepresenceofnonradiativeenergyrelaxationpathways,butalsocallsintoquestiontherelevanceoftheabsorbancespec-trumtotheemissiveprocess.Withthesedatainmind,onemustconsiderthepossibilitythattheelectronicstructurewithintheﬂakesisheterogeneous,andthatPLemissionoriginatesfromabsorbanceintoexcitedstateswhosetotalabsorptivecross-sectionis,nevertheless,onlyasmallcon-tributortothetotalabsorptivespectrum.Absolutevaluesfortheemissivequantumyieldwouldhelptoilluminatethisdiscussion,butaccuratemeasurementsofthisquantityareproblematicforensemblesofheterogeneousnanomaterialsandarebeyondthescopeofthiswork.Neverthelesswemayqualitativelysaythattheobservedquantumyieldsappeartobefarlessthanunity.

Tostudytherelationshipbetweenoxidationdensityandtheopticalgap,weperformedexperimentstovarytheoxi-dationdensityandwhilemonitoringchangesinthePLspec-trum.s-GOsampleswereplacedinacoveredPetridishwithacontainerofhydrazine,heatedto50°C,andtheirPLmapscharacterizedinthetransmissiongeometry.Theprocesswasrepeatedseveraltimestoprogressivelyreducethesample,resultinginamarkedredshift͑Fig.2͒.Modelingshowsthatchangesintheabsorptivespectrum͑notshown͒couldalsoproducethesespectralshiftsthroughreabsorptionoftheemittedlight.Tobettercontroltheseeffects,werepeatedthisstudyonadifferentsampleinthereﬂectiongeometry,whilealsocontinuallymonitoringchangesinabsorption.Penetra-tionoftheexcitingandemittinglightwasthenestimatedusingtherelationshipImeas͑1−T1͒ln͑T1T2͒=IPL͑1−T1T2͒lnT1,whereT1͑T2͒istheﬁlmtransmissionfortheexcitation͑emission͒wavelength,ImeasandIPLaretherawandabsorbance-correctedintensities,respectively,andmul-

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FIG.3.͑Coloronline͒NormalizedPLspectraduringhydrazineexposurefor͑a͒s-GOinareﬂectiongeometryand͑b͒l-GO,excitedat488nm.Datain͑a͒havebeencorrectedforreabsorptionasdescribedinthetext,whereas͑b͒doesnotrequireacorrection.͑Inset͒ChangesinintegratedPLintensityI͑t͒asafunctionofcumulativehydrazineexposuretforl-GO.LegendindicatesspectralregionsoverwhichI͑t͒wasintegrated.

tiplereﬂectionsareignored.IPLshowsasigniﬁcantredshiftinthePLemissionforincreasedexposuretimes͓Fig.3͑a͔͒,consistentwithanincreaseinthedisorderlengthscaleuponreduction.

Wealsostudiedchemicalreductioninl-GO,wherere-absorptioneffectscouldbebroughttoanegligiblelevel.l-GOwasreducedandheldinsuspensionfollowingthepro-cedureintroducedbyLietal.,13whichmayinvolveelectro-staticstabilization.Duringreduction,l-GOtransmissionre-mainedϾ80%relativetowaterfora1cmpathlength.Tofurtherminimizereabsorption,l-GOwasexcitedwithin1mmofthecollectionwindow.Figure3͑b͒showsthedevel-opmentofaﬂatterspectralproﬁleasafunctionofreductiontime.Currently,differencesbetweens-GOandl-GOreduc-tionstudiesarenotunderstood,butthedatadoholdcertainfeaturesincommon.Inallcases,therelativeintensityofPLintheinfraredincreases.However,theabsoluteintensityofPLwasalwaysseentodecreasewithreductioneverywhereinthespectrum,includingtheinfrared͓Fig.3͑b͒inset͔.

Atheoreticalframeworkforinterpretingthesedataisonlyjustemerging.Itcanbeexpectedthatoxidationpro-ducesadisruptionofthe␲networkandcanopenadirectelectronicbandgapforsinglesheetgrapheneinoneoftwoways.Theﬁrstisaquantumconﬁnementeffectwherebythe␲-electronwavefunctionsoccupyapotentiallandscapewithstronglyrepulsivehardwallbarriersatoxidizedsites.Intheinﬁnitepotentiallimit,adelocalized␲-electronwavefunc-tionwilldevelopnodesateachofthesesites.Thepresenceorabsenceofagapforasamplewithmanysuchoxidizedsitesthendependsonthespatialdistributionofthesenodes.Forexample,ingrapheneribbonswheretheedgesbreakthesublatticesymmetry,thelateralconﬁnementofthewave

111909-3Luoetal.Appl.Phys.Lett.94,111909͑2009͒

functionproducesabandgapatitschargeneutralitylevel.Alternatively,forthespecialedgethatpreservessublatticesymmetry͑a“zig-zag”ribbon͒oneﬁndsinsteadaresonantelectronicstateexactlyatzeroenergy.Theseresultsgeneral-izetoadisorderedpotentiallandscapewherethenodesoccurintheinteriorofthesample.Pereiraetal.14modeledtheeffectoflatticevacanciesinasinglevalleypicture,andfoundthatahardgapopensonlyinthespecialsituationwherethereiscompletesublatticeasymmetryinthevacancydistribution.Forintermediateasymmetrieswhereunequalfractionsofthedefectsresideondifferentsublattices,theyﬁndaﬁnite,butreduced,densityofstates͑asoftgap͒inanenergyintervaloforderបvFͱn,wherenisthevacancyden-sityandvFistheFermivelocity.Inthelimitoffullsublatticesymmetry,theyﬁndaresonantstateatthechargeneutralitypoint.UsuallytheobservationofPLimpliesahardgap,sinceasoftgappermitsnonradiativeenergyrelaxationun-lessapeculiarbottleneckexists.Moreover,despitealackofconsensusastothestructuralmotifinGO,15–17thereisnotyetanyobservationorcalculationsuggestingacompletesublatticeasymmetryintheoxidationproﬁle.

Asecondgappingmechanismariseswhenoneconsiders,inaddition,theeffectsofintervalleyscatteringfromtheshortrangepotentialoftheoxidizedcarbons.IntervalleyscatteringproducesacoherentsuperpositionofBlochwavesneartheKandKЈpointsoftheBrillouinzone,givingrisetoaͱ3ϫͱ3modulationofthechargedensity,whichhasbeenim-agedbyscanningtunnelmicroscope.18Forabond-centeredscatteringpotential,thisdescribesamodulation͑alternation͒ofthebondchargedensityandaself-consistentpotentialwiththisspatialsymmetry.Theamplitudeofthismodulationisdeterminedbythescatteringstrength,directandexchangeelectron-electroninteractions,andtheelectron-phononcou-pling.Thelatterislikelytoproducesomedegreeofbondalternationinthegraphiticregionsnearanoxidizedsite,whichmighteventemplatefurtheroxidationinsuchawayastoreinforcethispatternofdistortions.Inthiswaya“Kekulepattern”emergesnaturallyintheelectronicpoten-tial,andprovidesaspatiallymodulatedintervalleygappa-rameteroftheformenvisionedbyHouetal.19Notethatbondalternationinconjugatedpolymerssuchaspolyphene-lynevinylenegivesrisetogapsofasimilarenergyscale.20

Withinacontextofbonddisorderinducedenergygaps,oneregardstheGOplaneasalandscapecontainingawiderangeoflocalbandgapminima.Thisnotionisconsistentwiththeverybroadrangeofobservedemissionenergies.Ifthereductionprocedureforl-GOdoesindeedpreventaggre-gation,asclaimedinRef.13,thentheobservationofPLquenchingacrosstheentirespectrumfortheliquidsampleindicatesthatinterﬂakeorsubstratecontactisnotvitalforquenchingofPL.NeithertheoreticalframeworkdiscussedaboveaccountsforalossofquantumyieldunlessthereissigniﬁcantinhomogeneityoftheoxidationproﬁlewithintheGOplane.Ifoneimaginesthatchemicalreductionresultsinthenucleationand/orgrowthofregionswhereGOisfullyreducedtographene,thenmigrationofnonequilibriumcar-rierstothesezerogapregionscouldprovideanefﬁcientroutefornonradiativerecombination.Inthispicture,thereisnosimplerelationshipbetweentheoxidationdensityandthelengthscalebetweenoxidationsiteswithingapped,oxidizedregions.

Insummary,wedemonstratethats-GOemitsPL,andthatGOPLcanbealteredbychemicalreduction.BroadPLsuggestsadispersionofhardgaps,whichmayarisefrombondalternationwithintheGOplanegivingrisetointerval-leyscattering.Thelossofquantumyieldduringreductioninourexperimentssuggeststhatsomeregionsmightremainheavilyoxidized.Inthiscase,restorationofelectricalcon-ductivityresemblesapercolationproblemandgivesverylittleinformationaboutthegrowthoftheseregionsbelowthepercolationthreshold.Futurestudiesmightusecomplemen-tarymethodssuchasRamanscattering,21infraredLandaulevelspectroscopy,22,23ormagneticanisotropy24toquantifytheemergenceofgraphenelikeregionsinreducedGO.Op-ticalanisotropymeasurements24mayhelptoassesstheim-portanceofchargetransfertransitions,whichwehaveex-cludedfromthediscussionhereandshouldhavemarkedlydifferentopticalpolarizationanisotropieswhencomparedtotransitionsnativetothetwo-dimensionalGOplane.P.V.andJ.K.supportedbyNSFMRSECunderGrantNo.DMR05–20020,Z.L.andA.J.supportedbytheJSTODTRAandAROunderGrantNo.W911NF-06–1-0462,andE.M.supportedbyDOEunderGrantNo.DE-FG02-ER45118.

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