Facile preparation and upconversion luminescence of graphene dot

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Facilepreparationandupconversionluminescenceofgraphenequantumdotsw

JianhuaShen,YihuaZhu,*ChengChen,XiaolingYangandChunzhongLi*

Downloaded by Charles Darwin University on 27 May 2011Published on 21 December 2010 on http://pubs.rsc.org | doi:10.1039/C0CC04812GReceived5thNovember2010,Accepted1stDecember2010DOI:10.1039/c0cc04812g

Afacilehydrazinehydratereductionofgrapheneoxide(GO)withsurface-passivatedbyapolyethyleneglycol(PEG)methodforthefabricationofgraphenequantumdots(GQDs)withfrequencyupconvertedemissionispresented.Andwespeculateontheupconversionluminescenceduetotheanti-Stokesphotoluminescence(ASPL),wherethedEbetweenthepandrorbitalsisnear1.1eV.

Recently,carbonquantumdots(CQDs)orcarbondots(C-Dots)havereceivedmuchattention,astheymaygraduallyreplacetraditionalsemiconductorquantumdotsduetosuperiorityinchemicalinertness,biocompatibilityandlowtoxicity.1–9CQDsareusuallysurface-passivatedbypolymers,suchasPEG,andexhibitstrongphotoluminescence(PL).2,10,11Thesurfacepassivationismosteffectivefollowingfunctionalizationwithbiomoleculesinbioimaging,diseasedetectionanddrugdelivery.ButthedisadvantageisthatCQDspossesssizeeffects,thediameteroftheCQDswiththevisiblelightemissionislessthan10nm.10,12,13Inaddition,theCQDswereusuallypreparedbylaserablationofgraphite,electrochemicaloxidationofgraphite,electrochemicalsoakingofcarbonnanotubes,thermaloxidationofcarbonprecursors,vapordepositionofsoot,proton-beamirradiationofnano-diamonds,microwavesynthesis,andbottom-upmethods.14–21Wehavereportedasimplebottom-upsynthesismethodfortheC-Dotsbyusingamesoporoussilicamicrospherestemplate.22Thesecomplexprocessesaredifficultformassproduction.

Veryrecently,Panandcolleagues23havedevelopedasimplehydrothermalrouteforcuttingpreoxidizedmicrometre-sizedrippledgraphenesheetsintoultrafineGQDswithdiametersmainlydistributedinthe5–13nmrange.TheseGQDswerefoundtoexhibitbrightbluePL(quantumyieldca.6.9%),whichhasneverbeenpreviouslyobservedamongtheCQDsbecauseoftheirlargelateraldimensions.However,theproducedGQDsemitstronglyunderalkalineconditionsbut

KeyLaboratoryforUltrafineMaterialsofMinistryofEducation,SchoolofMaterialsScienceandEngineering,EastChinaUniversityofScienceandTechnology,Shanghai200237,China.E-mail:yhzhu@ecust.edu.cn,czli@ecust.edu.cn;Fax:+8621250624;Tel:+8621252022

wElectronicSupplementaryInformation(ESI)available:Experimentaldetailsandinstrumentation,quantumyieldmeasurements.SeeDOI:10.1039/c0cc04812g/

arealmostcompletelyquenchedinacidicmedia.Herein,wepresentanewfacilemethodtoprepareGQDssurface-passivatedbyPEG.TheyexhibitbrightPLinawatersolutionofneutralpH.Mostinterestingly,theyalsopossessupconversionPLproperties.

Inthiswork,wereportGQDspreparedbyhydrazinehydratereductionofGOwiththeirsurfacepassivatedbyPEG.Firstly,GOwasfurtheroxidisedbyHNO3,andcutintosmallGOsheets.Then,thepreparationofaGQDsprecursoroftreatedGOwithanoligomericPEGdiamine(PEG1500N)asthesurfacepassivationagent(Fig.1)wasbasedonthepreviouslyreportedprocedure.2,3,10TheprecursorwasfinallyreducedbyhydrazinehydrationtofabricateGQDs.StrongbluePLwasclearlyshownunder365nmandthegreenfluorescencewasobservedundera980nmlaser.ThePLquantumyieldmeasuredusingrhodamineBasareferenceis7.4%(TableS1inESIw),comparablewiththoseofreportedluminescentcarbonnanoparticles.10Interestingly,thefluorescenceresultsofGQDsresembledthoseofband-gaptransitionswhiletheupconvertedPLpropertiesofGQDsweresimilartotheASPL,andthereisaconstantenergydifferencebetweentheexcitationandemissionlight.24Fig.2showsatransmissionelectronmicroscope(TEM)imageofGQDs.Theirdiametersaremainlydistributedintherangeof5–19nm(13.3nmaveragediameter,similartothatreportedbyPanetal.),23suggestingthattheas-preparedmonodisperseGQDsareuniformlyarranged.Bythepresentmethod,GQDsconsistedofamixtureofdifferentsizedgraphenesheets.Fig.2bshowsahigh-resolutionTEM(HRTEM)imageofanindividualGQD.Tofurtherexploretheopticalpropertiesofas-synthesizedGQDs,adetailedPLstudywascarriedoutbyusingdifferentexcitationwave-lengths.Fig.3showstheUV-visabsorptionandnormalPLspectraofGQDs.FortheUV-visabsorption,therewasno

Fig.1RepresentationofGQDscontaininganoligomericPEGdiaminosurfacepassivatingagent.

2580Chem.Commun.,2011,47,2580–2582ThisjournaliscTheRoyalSocietyofChemistry2011View Online

Downloaded by Charles Darwin University on 27 May 2011Published on 21 December 2010 on http://pubs.rsc.org | doi:10.1039/C0CC04812GFig.2(a)TEMimageoftheGQDs;(b)high-resolutionimageofanindividualGQD;(c)diameterdistributionoftheGQDs.

Fig.3UV-visabsorption(Abs)andPLspectraoftheGQDsatdifferentexcitationwavelengths.Inset:photographoftheGQDaqueoussolutiontakenundervisiblelightand365nmUVlight,fromlefttoright,respectively.

Fig.4(a)PLspectrumexcitedat980nmlaser.Inset:photographoftheGQDaqueoussolutiontakenundera980nmlaser;(b)UpconvertedPLpropertiesofGQDs,insetistheenergyoftheexcitationlightasafunctionoftheemission,andthefunctionofthefitlineisEm=1.00Ex+dE(R2=0.9983)withdE=1.1eV.

clearabsorptionpeak,butalongabsorptionedge.ThePLspectraaregenerallybroadanddependentonexcitationwavelength,thePLpeaksshiftedtolongerwavelengthswithamaximumintensityastheexcitationwavelengthwaschangedfrom300to470nm,andthestrongestpeakexcitedat360nm,consistentwithpreviousfluorescenceanalyses.12,23ThepH-dependentPLspectra(Fig.S1inESIw)showsthattheGQDswiththeirsurfacepassivatedbyPEG1500NexhibitedbrightPLinawatersolutionofneutralpH.AndtheintensityofPLpeaksdecreasedbyabout25%underbothacidicandalkalineconditions,whichwasmorestablethanthatreportedunderacidicconditions.23ThesynthesizedGQDswereshowntopossesstheupconversionPLproperties.Fig.4ashowsthePLspectrumofGQDsexcitedbya980nmlaserwiththeupconvertedemissionslocatedatabout525nm,whichisconsistentwithourobservation.TheseresultsrevealthattheGQDshavetheupconversionproperty,soweconductedfurtherstudiesontheupconversionperformance.AsshowninFig.4b,theexcitationwavelengthchangedfrom600to800nm,theupconvertedemissionspeaksshiftedfrom390to468nm,respectively.Remarkably,theshiftingbetweentheenergyofupconvertedemissionlight(Em)andexcitationlight(Ex)wasalmostunchanged,about1.1eV.Theinsetshowsthelinearrelation-shipbetweenEmandEx,andthefunctionofthefitlineisEm=1.00Ex+dE(R2=0.9983)withdE=1.1eV.ManyreportsindicatedthattheupconvertedPLpropertyofCQDsshouldbeattributedtothemultiphotonactiveprocess.2,12However,webelievethatthisexplanationisnotsufficient.

ThisjournaliscAllthesechangesshowthatthesurfacepassivationbyPEG1500Nexertsastronginfluenceontheformation,micro-structure,andopticalpropertiesoftheGQDs.Tofurtherconfirmandexplainhowthesechangescomefromthequantum-sizedGQDs,weestablishedanenergylevelstructuralmodeloftheGQDstoinvestigatethefluorescenceproperties(Fig.3)andtheupconversionPLproperties(Fig.4).Fig.5showsaschematicillustrationofvarioustypicalelectronictransitionsprocessesofGQDs.ThePLspectrumcanbeconsideredasatransitionfromthelowestunoccupiedmole-cularorbital(LUMO)tothehighestoccupiedmolecularorbital(HOMO),asdemonstratedinFig.5(a)and(b).Theenergygapdependsonthesizeofthegrapheneandthebiggestgapwas4.9eV.25SincethegapdecreasesgraduallyasthesizeofGQDsincreases,thesamplemixtureofdifferentparticle-sizedGQDshavedifferentexcitationandemissionspectra,whichisinagreementwithpreviousreports.12,25,26TheupconvertedPLspectrumcanberegardedasananti-Stokes

Fig.5Aschematicillustrationofvarioustypicalelectronictransi-tionsprocessesofGQDs.NormalPLmechanismsinGQDsforsmallsize(a)andlargesize(b);UpconvertedPLmechanismsinGQDsforlargesize(c)andsmallsize(d).

TheRoyalSocietyofChemistry2011Chem.Commun.,2011,47,2580–25822581View Online

transitionasdemonstratedinFig.5(c)and(d).Theenergylevelsofpandsorbitalswereprovidedbythecarbeneground-statemultiplicity.23,27–29Thecarbeneground-statemultiplicityisrelatedtotheenergydifference(dE)betweenthepandsorbitals.HoffmanndetermineddEshouldbebelow1.5eV.30,31Intheworkhere,theenergybetweentheexcitationlightandtheemissionlightintheupconversionprocesswascloseto1.1eV(o1.5eV).MostscholarsindicatedtheupconvertedPLpropertyofCQDsshouldbeattributedtothemultiphotonactiveprocess.2,12ButwespeculatethattheGQDsweremoreliketheASPL.24,32Whenabunchoflow-energyphotonsexcitetheelectronsoftheporbital,thepelectronswouldtransitiontoahigh-energystatesuchastheLUMO,andthentheelectronstransitionbacktoalow-energystate.Thus,anupconvertedPLisemittedwhentheelectronstransitionbacktothesorbital.Althoughtheelectronsofthesorbitalcanalsobetransitioned,itonlycanemitnormalPL(Fig.5).Thisalsoexplainswhytheupconversionexcitationandemissionlightisaconstantenergydifference.24,32,33Inconclusion,wehavedevelopedhydrazinehydratereductionGOwithsurfacepassivationbyPEG1500NintoultrafineGQDswithstrongblueemissionandhighupconvertedPL.ThesurfacepassivationcanproduceGQDswithhigherfluorescenceperformanceandupconversionproperties.TheenergylevelstructuralmodelsofGQDsexplainedtheprocessoftheformationoffluorescenceandupconverted.GQDsmayprovideanewtypeoffluorescenceandupconversionmaterialforapplicationsinbioscienceandenergytechnology,andtheymayexpandtheapplicationofgraphene-basedmaterialstootherfields.

ThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(20925621,209760),theKeyProjectofScienceandTechnologyforMinistryofEducation(107045),theInnovationProgramofShanghaiMunicipalEducationCommission(09ZZ58),theProgramofShanghaiSubjectChiefScientist(08XD1401500),theShuguangScholar-TrackingFoundationofShanghai(08GG09),theFundamentalResearchFundsfortheCentralUniversities,andtheProgramforChangjiangScholarsandInnovativeResearchTeaminUniversity(IRT0825).

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Downloaded by Charles Darwin University on 27 May 2011Published on 21 December 2010 on http://pubs.rsc.org | doi:10.1039/C0CC04812G2582Chem.Commun.,2011,47,2580–2582ThisjournaliscTheRoyalSocietyofChemistry2011