文稿教程教案hybrid graphene nanonesh

Ligandeffectonchargetransferinhybridgraphenenanomesh-quantumdotsphotodetectorsNianzeLiu1,2,XiangLiu1,XiangbinJi1,2,andWei1DisyR&DCenter,SchoolofElectronicScienceandEngineering,SoutheastUniversity,Nanjing210096,PR.2Chien-ShiungWuHonorsCollege,SoutheastUniversity,Nanjing,WeinvestigatethedeviceperformanceofphototransistorsbasedonCVD-grownGNMandCdSeQDs.Withdefectintroducedandbandgapopenedup,theGNM-basedphototransistorsoutperformtheirpristinegraphenecounterpartintermsofON/OFFcurrentratio.Withn-typeCdSequantumdotsintroducedonp-typeGNM,theDiracpointofwholehybriddeviceistunedto~8V,makingiteasytochangethetypeofchargetransferringinGNM.BychangingdifferentligandsattachedontheQDs,weysisligandeffectonchargetransferprocess.Achargetunnelingmechanismisproposedandcomparedwithanother“chargetransport”mechanism.Asemi-classicalapproximationisthenappliedtoestimatethetunnelingrateofchargetunneling,helpfulfordesigningsuchkindofhybridphotodetectorswithhigherperformanceinthefuture.Photodetectors,ashighlydesireddevicesforvariousapplicationsfrom biologicalimagingtoremotesensing1,2,canmainlybesortedintotwotypes:photodiodesandphotoconductors1,3.Photodiodesarewell-knownforfastresponding,buthavedisadvantagessuchassmallsensingareaandlowgains(quantumefficiencies)thatarenormallylessthanunity2,3.Ontheotherhand,regardedasatypeofphotoconductorsfeaturingthelight-modulatedconductance3-6,phototransistorsareabletohaveagaingreaterthanunityduetothelongerlifetimethecarriershavewhencirculatingintheconductorbefore binewiththeoppositecharges1,3.Asameansofextendinglifetimeofcarriers,photodetectorsbasedonquantumdots(QDs)havebeenextensivelystudiedinadvantageofQDs’longfluorescencelifetime.Thegainandtheresponsivityaremeasuredbyphotocurrent????acrossthethinfilmphotoconductor,whichisgivenby????=??????????,whereeistheelectroniccharge,nisthedensityofphoto-inducedcarriersperunitarea,μisthecarriermobility,EistheappliedelectricfieldandWisthewidthofthedevice3.Whilethegainandtheresponsivityof

thephotoconductorareproportionaltothecarriermobilityμ,thecarriermobilityinQDsislowerthanthatofmaterialssuchassilicon,carbonnanotubes,grapheneandsomeorganicsemiconductors7-11,whichrestrictsQDs’applicationinthefieldofphotodetectors.Consequently,hybridmaterials,whereonecomponentwithhighcarriermobilityconductsphotocurrentandtheothercomponentdisyswell-definedfunctionalityoflightabsorbanceperformance,havebeenforefrontofscientificGraphene,withitsuniquestructureoftwo-dimensionalsheetofcarbonatomspackedinahexagonallattice,hasextremelyhighcarriermobilityupto200000cm2V-1s-1resultingfromthebehaviorofDiracFermions9.Giventhegraphene’sultrahighcarriermobilityandtheQDs’well-performedlightabsorbance,itisreasonabletoconsidercombininggraphenewithQDsinordertoimprovetheperformanceofphototransistors12,??.However,duetographene’szero,hybridgraphene-quantumdottransistorsaredifficulttohaveahighON/OFFcurrentratioandareallysaturatedcurrentathighdrainvoltage.Fortuna y,processinggraphenesheetsintonanoribbonornanomeshcanopenupabandgapthatislargeenoughforroom-temperaturetransistoroperation13-17.Sincegraphenenanomesh(GNM)field-effecttransistorscansupportcurrents~100timesgreaterthanindividualgraphenenanoribbondevices13,wechoosenanomeshashybridphototransistor’scurrentconductingcomponent.TofacilitatechargetransferacrossGNMandQDs,arobustinterfaceisessentialforhighperformanceofdevice.Therefore,inthiswork,westudytheeffectofligand,whichisencapsulatedonQDstoconnectQDswithGNM.Wefirstlyysismolecularofdifferentlengthfrompyridine(~0.5-nm-long),trioctylphophineoxide(TOPO,~1.2-nm-long),tooleicacid(OA,~2.5-nm-long),andcometotheconclusionthatchargetransferperformancefromligand-cappedQDstographeneintermsoftemporalphotocurrentresponsivitycanbedescribedbyachargetunnelingmechanism,whileasemi-classicalequationishelpfultoexinthechargetunnelingprocessandestimatethetunnelingrate.However,whenchangingtheligandtothosehavinganexcessivelylongchainsuchaspolyaniline(~0.5*xnm,x=degreeofpolymerization),thechargetransferperformanceintermsoftemporalphotocurrentresponsivityislowwhiletherise/falltimeofphotocurrentresponsedeviatefromthetunnelingmechanism.Suchananomalyisattributedtoadifferentmechanismof“chargetransport”,whichresultsfromtheinfluenceofπ?πin ctiononchargetransferprocess.Sincechargetransfermechanismwithdifferentencapsulationligandshasnotyetbeenreported,webelievethisworkwillhelpdesign

suchkindofhybridphotodetectorswithhigherperformanceinthefuture.PristinegraphenewassynthesizedbythermalCVDonCufoilsat1000°CusingCH4(40sccm)andH2(40sccm)at100Painacustom-builttubefurnace.ThepristinematerialwasthentransferredtotheSiO2/Sisubstratebywet-etchingtheunderlyingCucatalyst,usingapolymethylmethacrylate(PMMA).PMMAwasremovedbydissolutioninacetone.ThewholefabricationprocessofGNMwasillustratedinFig1(a-e).Thescanningelectronmicroscopy(SEM)imageofnanomeshisshowedinfigure1f,whichindicatesthatthesizeofasinglemeshunitinthegraphenenanomeshisabout100nm×800nm.FromtheRamanspectrumofthegraphene(Figure1g),asinglesymmetric2Dpeak(～2680cm?1),asmallG/2Dratio,andanegligibleDpeakwereobserved,indicatingthatthegrapheneisasinglelayerwithhighquality.TheCdSeQDscappedwithTOPOwassynthesizedviacolloidalsynthesisusingtheSchlenklinemethod.TheOAdecoratedCdSenanoparticlesweresynthesizedviasingle-stepsynthesisproposedbyWanKiBae18.Figure1d,e,fshowtheSEMimageoftheQDs.Theschemeofthewholedeviceisdeliveredinfigure2a.Alayer-by-layer(LBL)approachisemployedtobuildthisdevice.Electron-beamevaporatedgoldpadsworkassourceanddrain,andahighlydopedp-typesiliconsubstrateisusedtogatethegraphenenanomesh,whichservesasthesemiconductingchannel.ThechannelwasdepositedwithathinfilmofCdSecolloidalQDsfromsolutionbymeansofspincasting,withawidthof~50μm.Figure3(a)showsthedrain-sourcecurrentI-dsasthefunctionofbackgatevoltageV-gwhenthesource-drainvoltageis0.003V.Thechargeneutralitypoint(Diracpoint)V-dofGNMbeforethedepositionofQDsis~30V(blackcurve),revealingahole-dopedGNMduetothespecialstructureofGNMthatthenanomeshintroducedonthegraphenelayernotonlyopensupthebandgapofgraphenebutalsoresultsinalargerON/OFFcurrentratioof~80comparedwiththatofpristinezero-bandgapgrapheneofnomorethan1019,*.AfterthedepositionofQDsontheGNM,thelowestdraincurrentI-dsis~1.005*10-10AwhentheV-gisappliedto7.75V,reflectingalowDiracpointV-dunder10V(bluecurve).Itisworthnotingthat,afterdepositionofQDsonthep-typeGNM,theDiracpointofGNMdecreasesfrom~30Vto~8V,indicatingthatelectronsfromn-typeCdSeQDstransfertothep-typegraphenenanomeshlayer,thusformingabuilt-infieldtoequilibratetheFermilevels(Fig.3b).Consideringthedraincurrentresponsetogatevoltageunderthe

illuminationcondition(Irradiation:27μW/cm2,purplecurveinFig.3a,c),itisproventhatthetypeofchargecarrierstransferringintheGNMlayer,aswellasthesignofdraincurrentchangetoon/offillumination,canbetunedbythebackgatevoltage:forgatevoltageV-g<V-d,thechargecarrierstransferringinGNMishole,solight-inducedelectronsexcitedfromQDstransferringtohole-dopedGNMfunctionasCoulombtraps,quenchp-typecarriers,increasedeviceconductanceandleadtoalowerdraincurrentI-ds(Fig3c?curve,correspondingtoalowerpurplecurvecomparedtobluecurveinFig3a);forV-g>V-d,however,thechargeinGNMisn-typeelectron,thusalargerdraincurrentI-dscanbeobservedunderlightirradiationcomparedwiththatindarkbecause electronsofQDstransferringefficientlyintotheGNMincreasetheelectroncarrierdensityofn-typeconductivechannel19(Fig3c,?curve,correspondingtoahigherpurplecurvecomparedtobluecurve).RESULTSANDInpreviousstudies,graphenephototransistorcoatedbyQDscappedwitholeicacid(OA)asligandisreportedtoshowinsensitivitytolight?,?,?,whilecaseofthehybridphototransistorswithligandsuchaspyridine?,ethanedithiol?,orethanol?hasamanifestphoto-responsivity.ThedistinctiveresponsivityperformancebetweenphototransistorwithOAasligandandthosewithlatteronesisascribedtotheligandlength?.Itisbelievedthatchargetransferthroughlongchainligandisratherdifficult.Thus,inordertogetahigherresponsivity,itisreasonabletocaptheQDswithashorterligand,truncatinginterfacedistancebetweenQDsandgraphenesheet.Inthelightofthisidea,wetestgraphenephototransistorscoatedbyCdSeQDsencapsulatedwithdifferentligandsfrompyridine(~0.5-nm-long)trioctylphophineoxide(TOPO,~1.2-nm-long)tooleicacid(OA,~2.5-nm-long),aswellaspolyanilinethathasanextraordinarilylongmoleculechain.Thetemporalphotocurrentresponsivityinresponsetolighton/offisshowedinFig.4(a).WithligandlengthincreasesfromTOPO(~1.2nm),OA(~2.5nm),topolyaniline(~0.5*xnm,x=degreeofpolymerization),thephototransistorresponsivityis~10000,~1000,~10,~100,respectively.Inordertoexinthedifferentperformanceofphototransistorresponsivity,deviceenergylevelschemeisappliedtohelp Withback-gatevoltageappliedto20V,theGNMshowsn-type,whoseenergylevelisthusabove-4.5eVoforiginalgraphene’senergylevel.CdSeQDswiththediametersizeof~5nmisreportedtohavetheenergylevelsofHOMOandLUMOas-5.35eVand-3.32eV,respectively?.Ligandsareconsideredatvacuumenergylevelduetotheirinsulationproperty,exceptforpolyaniline,whichislongchainmolecularcapableofshowingappreciableconductivityincertainconditions??.Theconsiderationthatthepolyanilineligandmaybeconductivepromptsustofigureitsenergywhichisreportedtobe?4.75eVand?3.95eVfortheHOMOandLUMOofpolyaniline-,

respectively.Onthebasisofenergylevelabove,theenergybanddiagramofhybridinterfaceinphototransistorswithdifferentligandsisshowninFig.6(a)~(c).Forn-typeCdSeQDs,photoinducedchargecarrieriselectron.InordertotransfertoconductiveGNMlayer,itisinevitableforphotoinducedelectroninQDscappedwithinsulatingligandofTOPO(Fig6(a))orOA(Fig6(b))togetthroughapotentialbarrieratvacuumenergylevelof0eV,whichisfarhigherthanCdSeQDs’LUMO.Thustheonlypossiblemechanismforphotoinducedelectrontogetthroughistotunnelthroughthebarrier.However,forpolyanilinecappedQDs-graphenehybridphototransistor,thedistinctionintermsofenergybandthatthelowerenergylevelofpolyanilineliesbetweenHOMO/LUMOlevelsofCdSeQDsandgrapheneenergylevel,aswellasthebonddifferenceoftheπ?πinctionformedbytheencapsulationlayerofpolyanilinebetweenQDsandGNM21,enablesthepolyaniline,consideredconductive,toservenotaspotentialbarrierbutasatransportingbridgefacilitatingthechargetransferprocessbetweenthetwocomponents.Thesetwovariedmechanisms,namely“barriertunnelingmechanism”and“conductiontransportingmechanism”,accountfortheresponsivityperformanceshowedinFigure5.Incaseofbarriertunnelingmechanism,a ysisisconducivetounderstandtheresponsivitydistinctionforTOPOandOA.Becauseelectronon??????????(thebottomofconductionband)ofQDshasazerowavevector(k-vector),tunnelingelectronthatmanagestoescapefromtheQDsshouldatleastbeatthe1S-stateofQDs(??1??=??????????+???????)sothatthereisenoughenergytotunnelthroughthepotentialbarrier.Inordertoestimatetheescapetimefor1S-stateelectronsofQD,thetunnelingrate????????isreportedtohavebeenappliedinasemi-classicalapproximationas????????=ThecoefficientAisanempiricalconstantthattakesintoaccountasmallfractionofthesurfaceareaofQDwherethetunnelprocesstakesce.????????=????/2????????isthesemi-classicalfrequencyofoscillationofelectroninthe

=√2??1??isthecharacteristic

observedbetweenphototransistorwithpolyaniline-cappedQDsandthosewithQDsvelocitywhileelectronisinthe1S-state,??????istheQDsdiameter,and????????istheeffectivemassoftheelectron.????????isthesemi-classicalprobabilitytunneling,whichcanbecalculatedbyequation

cappedbyOAandTOPO,whichisbelievedtoberesultedfromdifferentmechanismsofchargetransfer.Thetypicalresponsiverise/falltimeofphototransistorwithQDscappedbyTOPOorOA(Fig5(a)(b),respectively)isnomorethan1swhenswitchingtheirradiationonandoff?2????????

(wavelength=??nm,lightintensity=????????=

?

mW/cm2,withanirradiationtimeintervalof??(??)=√2????????(??)(??1???wheretheintegralistakenoverthetunnelpath(????????)goingthroughthepotentialbarrier,??(??)istheelectronpotentialalongthetunnelingpathand????????(??)istheeffectivemassoftheelectrontakenalongthetunnelingpath.Thissimplemodelcorrespondswiththeresultsobservedinourexperiment:firstly,itagreewiththeresponsivityofdevicebasedonCdSe(TOPO)andCdSe(OA).WhenthelengthofQDs’encapsulationligandisincreasedfromTOPOtoOA,????????isincreased,resultinginrapidweakeningoftunnelingrate????????,whichreflectstheabilityofelectronescafromQDs.Thusthelongertunnelpathlength????????,thelowertunnelingrate????????,andtheworsephoto-responsivityperformance.TofurthermoreexplorethedistinctionregardingresponsivityofthreedevicesbasedonTOPO,OAandpolyaniline,aseriesoftime-resolvedphotoluminescence(PL)quenchingstudieshavebeenconductedonthehybridQDsandGNMinterface.Figure4(b)showsthePLdecaykineticsofdifferent-ligand-cappedCdSeonGNM.Anotabledistinctionintermsofrise/falltimeoftemporalphotoinducedcurrentis

for6cycles).However,incaseofphototransistorcoatedwithpolyaniline-cappedQDs,rise/falltimelargelyexceeds1s(Fig5(c)).Thisrise/falltimedistinctioniswellexinedbythetheoryof“”and“”,respectively.Forbarriertunnelingmechanism,tunnelingtimeisestimatedbyForconductiontransportingmechanism,transportingtimeissubjectedtoavarietyofBecauseelectronon??????????(thebottomofconductionband)ofQDshasazerowavevector(kvector),tunnelingelectronthatmanagestoescapefromtheQDsshouldatleastbeatthe1SstateofQDs(??1??=??????????+???????)sothatthereisenoughenergytotunnelthroughthepotentialbarrier.Inordertoestimatetheescapetimefor1SstateelectronsofQD,thetunnelingrate????????isreportedtohavebeenappliedinasemiclassicalapproximationas??????????=ThecoefficientAisanempiricalconstantthattakesintoaccountasmallfractionofthesurfaceareaofQDwherethetunnelprocesstakesce.????????=????/2????????isthesemiclassicalfrequencyofoscillationofelectronintheQDs,??=√2??1??isthecharacteristic velocitywhileelectronisinthe1Sstate,??????istheQDsdiameter,and????????istheeffectivemassoftheelectron.????????isthesemiclassicalprobabilitytunneling,whichcanbecalculatedbyanequationas?2??????????????=??? ??(??)=√2????????(??)(??1???wheretheintegralistakenoverthetunnelpath(????????)goingthroughthepotentialbarrier,??(??)istheelectronpotentialalongthetunnelingpathand????????(??)istheeffectivemassoftheelectrontakenalongthetunnelingpath.Thissimplemodelcorrespondswiththeresultsobservedinourexperiment:firstly,itagreewiththeresponsivityofdevicebasedonCdSe(TOPO),CdSe(OA).WhenthelengthofQDs’encapsulationligandisincreasedfromTOPOtoOA,????????isincreased,resultinginrapidweakeningoftunnelingrate????????,whichreflectstheabilityofelectronescafromQDs.Thusthelongertunnelpathlength????????,thelowertunnelingrate????????,andtheworsephotoresponsivityperformance.However,forpolyanilinecappedQDsgraphenehybridphototransistor,whichislongerthanOA,itsresponsivityislargerthanthatofOA.Thisiscausedbyanotherchargetransfermechanism:sincethelowerenergylevelofpolyanilineliesbetweenHOMOLUMOofCdSeQDsandgrapheneenergylevels,aswellastheπ?πin ctionformedbytheencapsulationlayerofpolyanilinebetweenQDsandGNM21,thepolyanilineligandisconsideredconductive,servingnotaspotentialbarrierbutasatransportingbridgefacilitatingthechargetransferprocessbetweenthetwocomponents.Thedifferenceoftwodifferentmechanismsisobvious:electronstunnelingthroughtheligandusuallytransferquicklywithupandfalltimelessthan1s,whilecarriersspendmuchmoretime(normallymorethan20sintermsofupandfalltime)transferringthroughtheconductiveπ?πinctionwithinpolyanilinelayer.Table1showsthemechanismsofchargetransferthroughdifferentligandsencapsulatingonPbSorCdSeQDs12,21,22.Insummary,weinvestigatedthemechanismofphotoinducedelectrontransferbetweenCdSeQDsandGNM.Byintroducingspecialstructureofnanomesh,thehybriddeviceshowsbetteron/offratioperformance.Thankstothecombinationofn-typeQDswithoriginallyp-typeGNM,thehybridtransistorhasarelativelylowchargeneutrality

point,thusmakingiteasytotunethetypeofchargecarriersinGNMconductivelayerfromholestoelectronsbyapplyingalowvoltage.Bychangingtheribbonwidthofnanomeshongrapheneaswellasthesizeofp-typeQDs,theextenttowhichthep-typeGNMandn-typeQDsaredopedcanbecontrolled,thusmakingitpossibletofinetunetheoriginDiracpointV-dtoanywherearound0V,suggestingagoodmethodforbuildingenergy-savingphotoelectricdevicesinthefuture.Withdifferentlinkingligand,thedistanceandtheelectronpotentialalongthetunnelingpatharedifferent,resultingindifferentchargetransfermechanisms.Itis mendedtousethetunnelingmechanismtoproducefastphototransistorswithhighresponsivity.Asemi-classicalequationisproposedtoevaluatethetunnelingrateofchargetransferprocess,suggestingthatshorterligandshowsbetterperformanceintermsofresponsivetimeandresponsivity.WebelievethattheseresultsprovideadeeperunderstandingofchargetransferprocessbetweeninterfaceofQDsandGNM,andofferinspirationforbuildingfutureenergy-saving,fastandultra-sensitivedevicesforapplicationssuchaslightdetection,sensing,imaging,opticalcommunication,andmemoryThisworkwassupportedbyDisyResearch&DevelopmentCenter,SchoolofElectronicScienceandEngineering,SoutheastUniversity,Nanjing210096,P.R..1G.Konstantatos,E.H.Sargent,Nat.Nanotechnol.2010,5,2A.Rogalski,J.Antoszewski,L.Faraone,J.Appl.Phys.2009,105,091101.3S.M.Sze,PhysicsofSem