Ultraviolet photodetector arrays assembled by dielectrophoresis of ZnO nanoparticles
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IOPPUBLISHING
Nanotechnology21(2010)115501(7pp)
NANOTECHNOLOGY
doi:10.1088/0957-4484/21/11/115501
Ultravioletphotodetectorarrays
assembledbydielectrophoresisofZnOnanoparticles
WenjingYan1,2,NormanMechau1,HorstHahn1,3andRalphKrupke1,3
Institutf¨urNanotechnologie,KarlsruheInstituteofTechnology,D-76021Karlsruhe,Germany2
SchoolofElectronicandInformationEngineering,Xi’anJiaotongUniversity,ShannXi710049,People’sRepublicofChina3
DFGCenterforFunctionalNanostructures(CFN),D-76028Karlsruhe,GermanyE-mail:ralph.krupke@kit.edu
1
Received7August2009,infinalform2January2010Published22February2010
Onlineatstacks.iop.org/Nano/21/115501
Abstract
SensitiveandfastultravioletsensorarrayshavebeenproducedbydielectrophoreticassemblingofZnOnanoparticles.Thesub-microndevicedimensionsinducelowoperatingvoltageandlowpowerconsumption.Thedevicesarelong-termstableandoperateinair,oxygenandnitrogen.Wehavedeterminedtheabsorptionanddesorptiondynamicsfromthetime-resolved
photoresponseandconcludethatoxygenorcarbondioxidearethephotodesorbedspecies.Wecouldderivethechargecarrierconcentrationandmobilityofthedevicefrommeasurementsofthelow-biasandhigh-biasphotocurrent.ThepresenceofdefectsisdiscussedbycomparingelectroluminescencespectrafrombiaseddeviceswithphotoluminescencespectralmapsofZnOdispersions.
SOnlinesupplementarydataavailablefromstacks.iop.org/Nano/21/115501/mmedia(Somefiguresinthisarticleareincolouronlyintheelectronicversion)
1.Introduction
Thereliablefabricationofnanoscaledevicesis,formanysystems,stillachallenge.Oftenitrequireseitherlithographicpatterningofamaterialwithnanoscaleprecisionorprecisesite-selectivedepositionofsynthesizednanoscalematerial.Inrecentyearssite-selectivedepositionschemeshavebeendeveloped,suchastheselectiveadsorptiononchemicallyfunctionalizedtemplates[1]ordielectrophoresisonthesub-micronscale[2].Therebydielectrophoresis(DEP)hasproventobeaversatilemethodforassemblingfunctionaldevicesfromarangeofbuildingblockssuchascarbonnanotubes[3],inorganicnanowires[4]orDNA[5].InthisworkwestudythedielectrophoreticassemblingofZnOnanoparticles(ZnO-NP)toformfunctionalsub-micronZnO-NPclusterarraysforUVsensing.High-densityUVsensorarraysbasedonDEPassemblingofZnO-NPhavenotbeenreportedbefore;however,ourapproachisrelatedtopreviouswork
0957-4484/10/115501+07$30.00
onmeasurementsonsol–gel-processedZnOnanowires[6]andspin-coatedcolloidalZnO-NPs[7].OurintentionistoexploretherathersimpleandscalabledielectrophoreticdepositionschemeincombinationwithcommercialZnO-NPs.SincetheUVsensingmechanisminZnOismediatedbysurfacechemistry,weanticipatesuperiorsensorperformanceforournanoscaleZnO-NPdevices.Indeedwewillshowthattheresponsetimeofourdevicesisshortandthatthesensitivityinvariousatmospheresishigh.Moreover,weobservethathighsensitivityisobtainedatsignificantlylowerpowerconsumptionascomparedtopreviousZnO-basedsensors.Tounderstandtheunderlyingphysicswehavemeasuredthelow-biasandhigh-biasphotocurrenttoderivethecarrierconcentrationandmobility,measuredthetime-resolvedphotoresponsetodeterminetheabsorptionanddesorptiondynamics,andstudiedelectroluminescence(EL)andphotoluminescence(PL)fromtheZnO-NPdevicestoobtaininformationaboutthedefectsintheZnO-NPs.The
1
©2010IOPPublishingLtdPrintedintheUK
Nanotechnology21(2010)115501WYanetal
dataiscomplementedbycharacterizationofthesamplemorphologywithscanningelectronmicroscopy(SEM)andatomicforcemicroscopy(AFM).
ThegeneralpropertiesofZnOhavebeenoutlinedbefore[8].ZnOisan-typesemiconductorwithnegligibleholeconduction,duetoexcesszincatomsoninterstitiallatticesites.MeasurementsoftheopticaltransmissionshowthatZnOisopaquebelow385nmandthattheexcitonbindingenergyis∼60meV[9].PhotoconductivityonZnOhasbeenreportedbyMollwoandStockmann[10],whohaveshownthatamaximumsensitivityoccursforilluminationinthevicinityoftheabsorptionedge.ThefirstsystematicstudyontheroleofoxygenonthephotoconductivityanditsinfluenceontheresponsetimehasbeenreportedbyMelnick[11].HisstudiessupportthemodelofMorrison[12]thatphysicallyabsorbedoxygenmoleculesactasanacceptortypeimpurity,removingfromtheconductionbandtheelectronswhichareprovidedbytheinterstitialzincatoms.ThedarkconductanceofZnOtherebydecreasessignificantlyifthesurfaceacceptorconcentrationexceedsthebulkdonorconcentration,whichisthecaseinsampleswithahighsurface-to-volumeratio,asinourZnO-NPclusters.
Melnickextendedthismodeltoexplaintheeffectoflight:illuminationintheultraviolet(UV)createselectron–holepairs,whichreadilydissociate.Aholeisattractedtothechargedoxygenmoleculeonthesurface.Asaresult,theoxygenlosesitschargeandbecomesphysicallyabsorbed.Thermalfluctuationswillcauserapiddesorptionandthephysicallyabsorbedmoleculeswilleventuallyreachequilibriumwiththegasmoleculesinthesurroundingatmosphere.Theelectronremainsintheconductionbandtoincreasethelocalconductivity.
MorrisonandMelnickhavedevelopedtheirmodelstoexplainmeasurementsonsinteredZnOpowdersamples.WewillseethatthecombinedmodelappliesalsototheresponseofDEP-assembledZnO-NPclusters.Inthefollowingwedescribetheexperimentalset-upandtheDEP-assistedassembling,discussthesteady-stateandtime-resolvedphotocurrentresponse,andpresentphotoluminescenceandelectroluminescencedata.
Figure1.(a)Schematicstructure(top)ofaZnOnanoparticledeviceassemblyonasilicon(blue)/SiO2(green)substrate.ZnO
nanoparticles(purple)areassembledbydielectrophoresisfromdispersionbetweentheelectrodes(yellow).(b)ScanningelectronmicrographoffivedeviceswithZnOnanoparticles(white)bridgingeachelectrodegap.(c)AtomicforcemicrographofthecentralregionofoneZnOnanoparticledevice.Theinsetshowsacross-sectionalheightprofile.
2.Experimentaldetails
Figure1(a)exhibitsaschematicoftheZnO-NPdevicearraystructure.Thedeviceswerepreparedonap-dopedsiliconsubstrate(<0.001cmsq−1)withan800nmthickthermalSiO2surface.Thesourceanddrainelectrodesweredefinedbyelectronbeamlithographyandsputteringofa5nmTiadhesionlayerand40nmPd,withatypicalgapsizeof800nm.Thesampleswereexposedfor5mintoanoxygenplasma(200Wpower,50mTorrpressure,30sccmflow)tomakethesubstratesurfacehydrophilic.
ZnOnanoparticlesproducedbyEvonikDegussaweresuspendedinwaterataconcentrationof10wt%withthecopolymericstabilizerTegoDispers752Watafixedconcentrationof10wt%,andcharacterizedbyUV–visabsorptionmeasurement(figure6(c))andphotoluminescencespectroscopy(figure6(b)).TheprimarysizesofZnO
2
nanoparticlesrangefrom20to100nm,determinedby
scanningelectronmicroscopy,asshowninfigureS1(availableatstacks.iop.org/Nano/21/115501/mmedia).Thedispersionwasdilutedbyafactorof1:20000withdouble-distilledwaterandsonicatedfor15mintobreakupagglomerates.AdropoftheZnOnanoparticlesuspension(∼5μl)wasplacedonthechipusingamicropipette.Then,anacelectricfieldwasappliedacrosstheelectrodesofthesample.Afunctiongeneratorwasoperatedatafrequencyfof300kHzandapeak-to-peakvoltageVPPof10V.After3min,thesamplewasrinsedwithdouble-distilledwater,gentlyblowndrybyastreamofnitrogengasandthepowerwasswitchedoff.Thesamplewasthenbond-wiredandmountedintoaceramicpackageandcharacterizedbyscanningelectronmicroscopy(SEM),atomicforcemicroscopy(AFM)andelectrontransportmeasurements.
DevicecharacteristicsweremeasuredwithanAgilent4155CSemiconductorParameterAnalyzerandaKeithley2400SourceMeterinastainless-steelchamber(40.2cm3)underaflowofoxygen,drynitrogen,wetnitrogenorair.Weusedaflowrateof3lpmat1barpres-sure.Wetnitrogenwasproducedbybubblingdryni-
Nanotechnology21(2010)115501WYanetal
trogenthroughawater-filledflask(figureS2availableatstacks.iop.org/Nano/21/115501/mmedia).Aquartzglassviewportprovidesaccesstoilluminatethesamplewithultraviolet(UV)lightfromafiber-coupledMikropackDH-2000-BALdeuteriumlightsource.Thelightintensityis∼20μWcm−2nm−1at370nm(figureS3availableatstacks.iop.org/Nano/21/115501/mmedia),thecorrespondingwavelengthtotheopticalgapofZnO.Thelightwasfocusedbyacollimatortoaspotsizeof∼2mmradius,yieldinganintegratedintensityof∼200μWcm−2nm−1atthesamplesurface.BytakingintoaccounttheZnO-NPdevicedimen-sions,thistranslatesintoanirradiationlevelof1pWnm−1perdevice.Thelong-termphotocurrentmeasurementshavebeenmadeunderilluminationwithaNichiaNSHU590BUVLEDwhichemits1.5mWat368nm(figureS4availableatstacks.iop.org/Nano/21/115501/mmedia).Thetime-resolveddatawasrecordedwithasamplingrateof2s−1.Inaddition,wemeasuredelectroluminescenceundervacuumbydetectingtheemittedlightwithanopticalmicroscopecoupledtoanActon2150ispectrometerandaPixis256ECCDcamera.Wepreparedintotaltendeviceswithcomparablemorphologyanddevicecharacteristics.Thedeviceswerestableunderambientconditionsforaboutfivemonthswithoutsignificantdegradationofdeviceperformance.
Figure2.Source–draincurrentIversussource–drainvoltageVSDofatypicalZnOnanoparticledeviceunderUVirradiationinN2,O2andambientair.Thesolidredlinesarepower-lawfits.
3.Resultsanddiscussions
3.1.Morphology
Figure1(b)showsanSEMmicrographoffiveadjacentdeviceswithtypicalmorphology.TheZnOnanoparticleshaveassembledintotheelectrodegapregionsandformsemi-spheroidalclustersof∼1μmdiameter,∼1μmlengthand0.5μmheight(figure1(c)).ThedepositionpatternofZnOnanoparticlesismarkedlydifferentfromthedepositionofconductingmetallicnanoparticles,wherepearl-chainformationisobservedduetoelectricfieldreconfiguration.Here,thedepositionpatternshowsthattheZnOparticlesdonotnotablychangetheelectricfieldduringassembling.ThiscanbeunderstoodiftheZnOnanoparticlesareinsulatingundertheassemblingconditions.IndeedthetypicalresistanceofourdeviceswithoutUVirradiationislargerthan1011andshowsatransient-current-inducedhysteresis(figureS5availableatstacks.iop.org/Nano/21/115501/mmedia).Therefore,theZnOclusterdimensionsaredefinedbythedielectrophoreticforcefield,whichdependsontheelectrodedimensions,theelectrodegapsizeandtheoxidethickness.Asaresult,thevariationsinthemorphologyfromdevicetodevicearerathersmallandyieldonlysmallvariationsinthedevicecharacteristics.3.2.Steady-statephotocurrent
Wewillnowdiscusstheelectrontransportmeasurements.AlldevicesshowagreatlyreducedresistanceuponilluminationwithUVlight,albeitwithsignificantdifferencesdependingonthegasatmosphere.Figure2showsinalog–logplotthesource–draincurrentIversussource–drainvoltageVSDcharacteristics(IV)ofatypicalZnOnanoparticledeviceindifferentgasatmospheresandunderidenticalUVillumination.
3
TheIVsareverysimilarformeasurementsinO2andin
α
ambientair.InbothcasesIobeysapowerlawI∝VSD,forVSD>0.8V,withα=2.3andα=2.8forO2andair,respectively.lnN2weobservetwopower-lawregimeswithα=1forVSD<0.3Vandα=1.8for0.3V>VSD>3V.
ThedatacanbeexplainedbytheMorrison–Melnickmodel:inZnO-NPswithalargesurface-to-volumeratio,chemisorptionatthesurfacecansignificantlyinfluencetheelectrontransportandthephotoconductivity.OurtransportdatasuggeststhatadsorptionofoxygendepletesthecarrierconcentrationintheZnOnanoparticledevices.SincechargecarriersmustpropagatethroughthesurfacelayeratthecontactpointsofZnOnanoparticles,weessentiallyprobethesurfacecarrierconcentrationanditsdependenceonadsorbatesandillumination.UVilluminationleadstoapartialdesorptionoftheadsorbatefromtheZnOnanoparticlesandtheresultingcurrentleveldependsonthesteady-stateadsorbateconcentration,whichinturndependsonthedynamicequilibriumbetweendesorptionrateandadsorptionrate,aswewillalsoseeinthetime-resolvedmeasurementslater.Innitrogen,wheretheoxygenadsorptionrateislow,nosurfacetrapstatesarepresentunderUVilluminationandweobserveanohmicbehavioratlowbias.Thisregimeisdominatedbythefreecarrierdensity.ForVSD>0.2V,wecanfittheslopeoftheI–V(∼1.9×10−8AV−2)toMott’sspace–charge-limitedcurrentmodel(SCLC)[13],whichinitssimplestformisgivenby
j=9εrε0μV2/8L3,(1)wherejisthecurrentdensity,μthedriftmobilityofthe
chargecarriers,εrtherelativedielectricconstantandLtheelectrodeseparation.SCLCisoftenobservedinwidebandgapsemiconductorswhenthematerialresistancegreatlyexceedsthecontactresistanceandthechargeinjectedbytheelectrodesexceedsthefreecarrierdensityinthematerial[14].Althoughstrictlyvalidonlyforaplane-paralleldielectric,weuseequation(1)toestimatethemobilityμ.Aslowerandupperboundsforthecurrentcarryingcrosssectionwetaketheelectrodecrosssection,200nm×45nm,andthe
Nanotechnology21(2010)115501WYanetal
clustercrosssection,π(0.5μm)2/2,respectively.Weobtain3cm2V−1s−1<μ<130cm2V−1s−1withεr=8.7andL=800nm,nottakingintoaccounttheclusterporosityandtherebyunderestimatingμ.Interestingly,theestimatedcarriermobilityinourZnOnanoparticledevicesunderUVilluminationislargerthaninZnOnanoparticledevicespreparedbyspin-casting[15–17]butiscomparabletovaluesreportedforepitaxialZnOthinfilms[18]andZnOnanowires[19].Wecanusethedriftmobilityμtoestimatethefreecarrierconcentrationnfromthelow-biasconductanceusingtheDrudeconductivityσ=qnμ.Weassumeμtobevoltage-independentandcalculaten≈1014cm−3.
Theconditionsaresomewhatdifferentinoxygen-containingenvironments,wheretheoxygenadsorptionrateishighandsurfacetrapstatesarepresentevenunderUVillumination.Thosestatesdepletethesurfacefreecarrierconcentrationandthesampleisinsulatingatlowbias(VSD<0.5V).ItisknownthattheenergydistributionofthetrapstatesdeterminestheI–Vcharacteristics.RosehasshownthatIdependsexponentiallyonVforauniformdistributionoftrapsinenergyE,whereasapower-lawdependenceisobtainedforatrapdensitythatdecreaseswiththedistancefromtheconductionband[14].Inourcase,weobserveathighbiasI∝Vαwithα≈2.8,andthereforeapproximateanexponentialdistributionntoftraps:
nt∝exp(−E/kTC),(2)
withthecharacteristictemperatureTC,andEbeingmeasuredfromthebottomoftheconductionband.Thisdistributionleadstothetrap-modulatedspace–charge-limitedcurrent
I∝V(TC/T)+1.
Fromα≈2.8wederiveTC=540KorEC=45meV.3.3.Time-resolvedphotocurrent
(3)
Sofarwehavereportedontheconductionundersteady-stateconditions.Inthefollowingwepresenttime-resolvedphotoconductivitymeasurementsinresponsetoUVirradiationforthepurposeofderivingabsorptionanddesorptionratesofthecarrierconcentrationquenchingspecies.
Figure3showsthetime-resolvedresponseofthecurrentIatVSD=2VtorepeatedUVlightswitchingwithcyclingtimestCbetween100and1000s.Theinitialsteady-statedarkcurrentIOFFis∼1–10pA,independentofthegasatmosphere.WhenUVirradiationisswitchedon,thecurrentincreasesbyseveralordersofmagnitudeandIreachesthesteady-statelevelION,withinanorderofmagnitude,afteracharacteristictimeτON.InN2,IONislargeandreachesupto100nA.InO2andinambientair,IONreachesonly1nAand10nA,respectively.WhenUVirradiationisswitchedoff,thecurrentdecreasesandIOFFreachestheinitialsteady-statecurrentifthecharacteristictimetOFFissmallerthanthecyclingtimetC.ThisisthecaseformeasurementsinO2.InN2,however,tOFF>tCandIOFFdoesnotreachtheinitialsteady-statelevel,evenafter20h.Asaresult,thereisadifferenceintheION/IOFFratiobetweenthe
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Figure3.Time-resolvedphotocurrentmeasurementsinN2(a),inambientair(b)andinO2.OnandoffswitchingofUVilluminationisindicatedbythearrows.In(a)ambientairisinsertedafterthefourthcycle.
firstcycleandsubsequentcycles:TheinitialION/IOFFratiois104,3×103and103inN2,airandO2,respectively.InthesubsequentcyclesION/IOFFisreducedtoapprox.101and102inN2andair.However,underO2conditions,tOFF Nanotechnology21(2010)115501WYanetal Table1.CharacteristictimeconstantsτON,τOFF,1andτOFF,2fromthefittingofthetime-resolvedphotocurrentdataoffigure3withequations(4)and(5). τON N2O2Air 0.97s±1.05s0.67s±0.25s1.51s±1.49s τOFF,1 197s±96s8.8s±0.6s7.0s±1.1s τOFF,2 578s±362s22.4s±2.4s29.7s±7.0s thefollowingtype: I=I0+Aexp(−t/τOFF,1)+Bexp(−t/τOFF,2),(4) withtwocharacteristictimeconstantsτOFF,1andτOFF,2.ForIONwefoundgoodagreementwithalogarithmictimedependenceofthetype I=I0+Alog(t/τON+1),(5) withthecharacteristictimeconstantτON.ThetimeconstantsτON,τOFF,1andτOFF,1aregivenintable1.Atypicalfittingforonecycleisshowninfigure4(a).Melnickhasalreadysuggestedalogarithmicratelawforthedesorptionprocess[11],whichappearstobeuniversalformanysystemsasexplainedbyLandsberg[20].InMelnick’smodeltheincreaseinconductivityisproportionaltothedecreaseofthedensityofabsorbedgasonthesurfacenC.WecanthereforeinferfromthechangeinIthechangeinnCusingδnC=|δI|.Thebi-exponentialdecayofthecurrenthasalsobeenobservedbefore[21],andrecentlythetwoprocessesforcarrierlosshavebeenproposedtobeduetotrappingbythesurfacestatesandrecombinationatdeepdefectstates[22]. Thetime-resolveddataclearlyshowsthatthesteady-statecurrentandtheresponseofthecurrenttoUVlightstronglydependsonthepresenceofoxygen.Theentiredatasetcanbeunderstoodbyoxygenadsorptionanddesorptionprocesses:theZnOnanoparticlesintheas-prepareddeviceareinitiallycoveredwithoxygenandwatermolecules,duetoprocessingin,anddepositionfrom,aqueousdispersion.Theabsorbedoxygen,whichquenchesthesurfacecarrierconcentration,appearstobestableevenunderaflowofN2,anddeterminestheinitialvalueofIOFF. WhenweswitchtheUVlighton,presumablyoxygenstartstodesorb.Thedatashowsthatthedesorptionprocessisfastandindependentfromthegasatmosphereandfollowsequation(5)withτON≈1s.Asthedesorptionratedecreaseswithtime,thesteadystateofnCandIONisreachedwhenthedesorptionrateisequaltotheadsorptionrate.WeassignfluctuationsinIONduetothisdynamicequilibrium.InO2andairtheadsorptionratesarelargeandnCisreducedbytwoordersofmagnitude(insubsequentcycles).InN2theabsorptionrateisverylowandnCisreducedbyfourordersofmagnitude,withintC. WhenweswitchtheUVlightoff,thedesorptionrateiszeroandtheabsorptionratecausesnCtoincrease.InO2andairwithlargeadsorptionrates,nCincreasesrapidlyandthenewsteadystateIOFFisreachedfollowingequation(4)withτOFF,1≈8sandτOFF,2≈25s.InN2,wheretheoxygenabsorptionrateisverylowandonlyduetoresidualoxygen, 5 Figure4.(a)Zoomintofigure3(c).Thetime-resolvedphotocurrentisfittedbyabi-exponentialfunctionforthecurrentdecay(UVoff)andalogarithmicfunctionforthecurrentrise(UVon).τOFF,1,τOFF,2andτONarethecorrespondingcharacteristictimeconstants.(b)PhotocurrentresponseoninsertionofwetN2anddryN2at constantUVillumination.(c)Long-termphotocurrentmeasurementunderconstantUVilluminationindryO2. nCincreasesonlyveryslowlyandreachesthenewsteadystatefollowingequation(4)withτOFF,1≈200sandτOFF,2≈580s.Attheendofthelastcycleinfigure3(a)IOFFdropssharplyafterinsertionofambientair. AtthispointwewouldliketoemphasizethattheDEP-assembledZnO-NPclustersensorsarecharacterizedbyfastswitching(oftheorderofseconds),highsensitivity(ofthe Nanotechnology21(2010)115501WYanetal orderoftwodecadesofcurrentper∼1pWnm−1at370nmexcitation),lowoperatingvoltage(∼0.3V)andlowpowerconsumption(∼1nW),ThisisauniquecombinationascomparedtootherZnO-baseddevices[6,7,23].3.4.Theroleofwater WehaveshownthatDEP-assembledZnOclusterdevicesaresensitiveandfastUVdetectorsinoxygenandair.Toascertaintheinfluenceofwateronthephotocurrent,wemeasuredthetime-resolvedphotoresponseindryandwetnitrogenunderUVlight.Infigure4(b)weobserveacontinuousslightincreaseinthephotocurrentwithtime,atrendlinewithapositiveslope,whichweattributetotheongoingphotodesorptionofoxygen.Whenweinsertwater-saturatedN2,thistrendisinterruptedandthephotocurrentdecreasessignificantly,mostlikelyduetoadsorptionofwater.WhenweswitchbacktodryN2,thephotocurrentrecovers.Interestingly,thephotocurrentapproachesasymptoticallytheprevioustrendline.Thiscanbeunderstoodiftheoxygendesorptionisnotinfluencedbytheadsorptionanddesorptionofwater.WealsonotethattheresponsetowetN2andtothesubsequentexposuretodryN2isslowconsideringthetimescaleinvolved.WethereforeconcludethatwaterplaysonlyaminorroleforthephotoresponseofourZnO-NPdevicesinthepresenceofoxygen.Thisexplainswhythecharacteristictimescalesforadsorptionanddesorptioninoxygenandairareidenticalwithintheexperimentalerror.Itmeansthatthedeviceperformanceunderambientconditionsisnotsensitivetohumidity,whichisimportantforpracticalapplications.3.5.Theroleofcarbon TheZnO-NPclustershavebeenassembledfromadispersionstabilizedbyacopolymer,andweexpectthesurfaceoftheZnO-NPstobecontaminatedwithcarbon.Thequestionariseswhethercarbonimpuritiesinfluencethephotoresponse.Interestingly,ShapiraandLichtmanhaveshownbymass-spectrometricanalysisofphotodesorptionfromZnOthatCO2istheonlyphotodesorbedspeciesfromsingle-crystalandpowdersamples[24].TheirconclusionisthatoxygendoesnotchemisorbonZnOsurfacesbutonlyonimpuritycarbonsites.InthisprocessoxygenchemisorptiontakesplaceuponelectroncapturefromtheZnOconductionbandandchemisorbedCO−2moleculesareproduced.Underilluminationphoto-generatedholesrecombinewithCO−2toformphysisorbedCO2species,whichcanthenbethermallydesorbed.Theauthorscouldalsoshowthatcarbon-freesurfacesareinerttooxygenandthatsurfaceswithcarbonimpuritiesbecomeinertunderextendedUVillumination. WehavestudiedthephotocurrentofourZnO-NPclusterdevicesunderextendedUVilluminationindryO2.Themeasurementisshowninfigure4(c)andshowsasmallbutcontinuousdecreaseofthephotocurrentwithtime,whichcouldbeduetoacontinuousdepletionofcarbonimpurities.However,inthemorerecentliteraturetheinfluenceofcarbonimpuritiesonthephotoconductanceisnotmentioned.InsteaddesorptionofwaterfromtheZnOsurfaceisoften 6 Figure5.ElectroluminescenceintensityversusemissionwavelengthfromabiasedZnOnanoparticledevice(VSD=5V).Thebroadpeakcenteredat710nm(redcurve)hasbeenderivedfromthedata(blackcurve)bysubtractingaPlanckspectrum(bluecurve).Inset:photographofalight-emittingcontact. discussed[6,25].Itwouldrequirecouplingofourexperimentwithamassspectrometertoobtaininformationaboutthedesorbedspecies. 3.6.Electroluminescenceandphotoluminescence Wewillnowextendthediscussiononelectrontransporttowardstheroleofdefects.ThemobilityinourdevicesismostlikelylimitedbysurfacescatteringattheNPinterfaces.Unfortunately,itisdifficulttoobtainaninsightintothenatureofthescatteringmechanismatlowbias.Weuseelectroluminescence(EL)todetectinelasticscatteringprocessesatlargebias.Thedatahasbeenacquiredonasamplewithamodifiedelectrodeconfiguration(Insetinfigure5).Figure5showsaspectrumobtainedatVSD=5V.Thespectrumshowsanincreasingintensitytowardslongerwavelengthplusabroadpeak.WehavefittedthemonotonicallyincreasingbackgroundwithaPlanckspectrumandderivedadevicetemperatureof1140K.Thedeconvolutedpeakhasitsmaximumat710nmandawidthof∼100nm.Electroluminescenceatasimilarwavelengthhasbeenreportedrecentlyandassignedtoanoxygen-richZnOphase[26].WewantedtoinvestigatewhethertheelectroluminescenceisduetoinelasticscatteringatthesurfaceorinthebulkoftheZnO-NPs.WeperformedphotoluminescencemeasurementsondispersedZnO-NPsoverawiderangeofexcitationwavelengthsλexc.Figure6showsthedataintheformofatwo-dimensionalphotoluminescencecontourmap(PLmap).Firstofall,wenoteamarkeddifferenceintheoverallemissionintensitybelowandaboveλexc=370nm.Weemphasizethatthisisexactlythewavelengthatwhichtheopticalabsorptionsetsin(rightinsetinfigure6).ThereforewecandividethePLmapintotworegions:PLfromλexc>370nm,whichmustbeduetoinelasticsurfacescattering,andPLfromλexc<370nm,whichisprimarilyduetoinelasticscatteringinthebulk.Interestingly,theelasticRayleighscatteringintensityatλexc>370nmismuchlargerthanatλexc<370nm,whichweassigntotheincreaseof Nanotechnology21(2010)115501WYanetal indicatesthepresenceofanoxygen-richphase.ThedatahasbeencomparedtophotoluminescencespectralmapsofZnOdispersions,whichindicatesthepresenceofanoxygen-deficientbulkphase.Analysisofthephotoluminescencemapatlowexcitationenergieswheresurfacescatteringdominatesisongoing. Wenotethat,intheprocessoffinalizingthedocument,Kumarandco-workersreportedDEPtrappingofZnOnanoparticlesintonanogaps[29].Theauthorsannouncedtheywouldpublishdetailedphotoconductivitycharacteristicsinaforthcomingpublication.Itwillbeinterestingtocomparetheirresultswithourwork. Acknowledgments TheauthorsthankSDehmforassistancewithelectronbeamlithography,MEngelforAFMmeasurement,SEssigforelectroluminescencemeasurements,andCWMarquardt,AVijayaraghavanandSLebedkinforhelpfuldiscussions.ThefinancialsupportbytheInitiativeandNetworkingFundoftheHelmholtz-GemeinschaftDeutscherForschungszentren(HGF)andanequipmentgrantbyAgilentTechnologiesaregratefullyacknowledged.WealsothanktheEvonikGmbHforsupplyingtheZnOnanoparticlesandthestabilizer. Figure6.(a)Two-dimensionalphotoluminescencecontourmap(photoluminescenceintensityversusexcitationandemissionwavelengths)ofdispersedZnOnanoparticles.First-andsecond-orderRayleighscatteringindicatedbyblackdashes,respectively.(?)marksanunidentifiedfeature. (b)Photoluminescencespectraat350nmexcitation.Peakpositionsaregivenandsecond-orderRayleighscattering(∗)andfluorescencepeak(∗∗)indicated.(c)Correspondingabsorptionspectra. thenumbersofscatteringcentersandthelargesurface-to-volumeratiooftheZnO-NPs.Inthebulk-relatedregimeatλexc<370nmwehaveidentifiednear-band-edgeemissionatλem=381nmandabroaddefect-relatedgreenfluorescenceemissionwithanmaximumatλem≈500nm(topinsetinfigure6).Thegreenfluorescenceemissionisoftenattributedtodefectstatesofoxygenvacancies[27,28].Bothemissions,thenear-band-edgeemissionandthegreenemission,appearasverticallinesforexcitationinthebulkatλexc<370nm.Inthesurfacescatteringregime,atλexc>370nm,theintensityofemissionincreasesovertheholespectralrange.Furthermore,theintensityoftheexcitedlightincreasesduetolowabsorptionandanincreaseofthenumbersofscatteringcentersforelasticRayleighscattering.At700nmweobservespectralfeatureswhichfittotheELdata.Thisfeaturecancorrelatetodefectstates,butunfortunatelywecannotexcludeanexperimentalartifactinducedbyscatteringeffects. 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