纳米科技

PreparationandPeriodicEmissionofSuperlatticeCdS/CdS:SnS2Microwires

GuozhangDai,†BingsuoZou,*,†andZhonglinWang‡SchoolofMaterialsScienceandEngineering,BeijingInstituteofTechnology,Beijing100081,China,andSchoolof

MaterialsScienceandEngineering,GeorgiaInstituteofTechnology,Atlanta,Georgia30332-0245

ReceivedMay4,2010;E-mail:zoubs@bit.edu.cn

Abstract:Semiconductorsuperlatticemicro-/nanowirescouldgreatlyincreasetheversatilityandpowerofmodulatingelectronic(orexcitonic,photonic)transport,opticalproperties.Inthiscommunication,wereportgrowthofasemiconductorCdS/CdS:SnS2superlatticemicrowirethroughacoevaporationtechniquewithmicroenvironmentalcontrol.Suchanovelsuperlatticemi-crowirecanmodulatetheexcitonandphotonstoshowmultipeakemissionswithperiodsinawidespectralrange,whichariseinthe1-dphotoncrystalandconfinedexcitonemission.Thissystemcanbewidelyusedinproducingmulticoloremissions,low-thresholdlasing,studylight-matterinteraction,slowlightengi-neering,andweaknonlinearopticaldevices.

One-dimensional(1D)nanostructureswithmodulatedcompositionsandmicrostructureshavegainedatremendousamountofattentioninthepastfewyearsduetotheirfascinatingchemistryandsize-,shape-,andmaterial-dependentproperties.Among1Dsegmentedstructures,semiconductorsuperlatticenanowireswithaperiodiccompositionmodulationalongtheaxialdirectionhaverecentlybecomeofparticularinterest.Thisisdueinparttotheexcitingelectronicandopticalapplicationsofnew1Dnanophotonicbuildingblocks.1-4Forexample,the“excitoniclattice”canprovidecoherentinteractionoflocalexcitonsandmodulatelightpropagationwhichopensuppossibilitiesforstudyingnewstatesformedwhenexcitonsinteractwiththeiremittedphotons.5Todate,severalwell-developedchemicalvapordepositiontechniqueshavesuccessfullybeenusedtosynthesizethesuperlatticestructures.6-8Inthesestudies,thevapor-liquid-solid(VLS)growthmechanism9,10takeseffectinsuperlatticenanowiregrowthviaalteringthereactionatmosphereperiodically.Othersyntheticmethodssuchasthetemplate-basedelectrochemicalmethod,11-13partialcationexchangetechniques,14andatomiclayerdeposition15havealsobeenreported.Allthesetechniquescantunetheelectronicbandlocationoftheproducts,butnottomodulatelightpropagation.Therefore,developingmethodstoobtainbuildingblocksofsuperlatticenanowireswithtunableinterfaces,size,andcompositioninapredictablemannerandmodulatebothelectronsandphotonsinsituisveryimportantfortherealizationofmultifunctionalnano-andmicrometerscalephotonicdevices.

Inthiscommunication,wereportthefabricationofCdS/CdS:SnS2superlatticewiresthroughacoevaporationtechniquewithlocalenvironmentalcontrol.Suchanovelsuperlatticewirewithathicknessof400nmto5µmcanmodulateexcitonemissionandphotonpropagationwithspectralperiodicalmultipeaks,whichmaybeusedinproducingmulticoloremissionsandlow-thresholdlasing,studyinglight-matterinteraction,slowlightengineering,andweakopticalnonlinearities.

†‡BeijingGeorgiaInstituteInstituteofofTechnology.Technology.

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J.AM.CHEM.SOC.2010,132,12174–12175

1Dsuperlatticewireswereproducedbyapreciselycontrolledthermalevaporation.AmixtureofCdS(0.1g,AlfaAesar,99.995%purity)andSnO2(0.01g,99.5%purity)wasplacedinthecenterofasinglezonetubefurnacewhichwasevacuatedfor2htopurgeoxygeninthetube.AtreatedSi(100)substratewasplaced8cmawayfromthesourcepowdersandalongthedownstreamsideofflowingAr(90%)andH2(10%).Typically,thesourcesitewasheatedtoabout980°Catarateofabout120°C/min.Duringthegrowthprocess,the40sccmmixtureofcarriergaswasinjectedintothetube.Then,thesuperlatticemicrowireswithvariedsegmentlengthscanbeachievedbychangingthedepositiontime,location,andsomeotherfactors.

Figure1.(a)NormalizedX-raydiffractionpatternsand(b)SEMimage

oftheas-grownsamplepreparedbyevaporatingCdSandSnO2mixedpowders.Insetsin(b)arethemagnificationmorphologyimagesofatypicalsample,endingwithamicrosphereandahexagonalsolidwireattwosides,respectively.(candd)Opticalimagesatdifferentmagnificationsofperiodicalternatingmicrowiredispersiononacoppergrid,showingasuperlatticestructure.Insertedimagein(d)showstransmissionconfocalmode.

Theproductcoveringanapproximately10mmregionontheendoftheSisubstrate(wherethetemperaturewasabout650°Cduringreaction)wascollected,andthenitsX-raypowderdiffraction(XRD)pattern(Figure1a)wasobtained.Theresultsshowthattheas-grownproductiscomposedoftwocrystallinephases,ahexagonal(wurtzite)formofCdS(JCPDS:2-549)andhexagonalSnS2(JCPDS:23-677).Nocharacteristicpeaksfromotherimpurities,suchasCdO,SnO,andSnO2,weredetectedintheXRDspectrum.Figure1bpresentsanSEMimageofthesamplewhichshowstheirdiameterrangingfrom400nmto2µm.Mostofthewireswerecappedwithamicrosphereontheirtops(leftinsetinFigure1b),indicatingalikelyVLSprocessfortheformationofthesewires.Theinsertedtoprightimageshowsthetypicalhexagonalfacetendofawire.Far-fieldopticalimagesoftheproductsatdifferentmagnificationsareshowninFigure1cand1d,respectively.Itcanbeclearlyseenthattheproductsarelongmicrowireswithregionsofperiodicandalternatingindicesofrefraction,thatis,microwiresuperlattices.TheinsertedimageinFigure1dobtainedintransmissionmodefurtherconfirmsasuperlatticestructure.InFigure

10.1021/ja10379632010AmericanChemicalSociety

S1,thepositiondependentTEMimageandrelatedEDSprofilesindicatethelongsectionintheperiodicwireismainlycomposedofCdandS,whiletheshortsectionismainlymadeupofCd,Sn,andS.Combinedwiththeanalysisofthemicro-RamanspectruminFigureS2,itcanbeconcludedthatthelongsectionisCdSandtheshortsectionisthecompositeCdS:SnS2.PossiblegrowthprocessesforthissuperlatticewireareschematicallyshowninFiguresS3.

Figure2.Far-fieldPLimagesofanexcitedsamplewithdifferentreactiontimes:(a)20min;(b)30min.Insetsin(a)and(b)arecorrespondingopticalimages.(c)Schematicrepresentationofemissionprocessofthe1Dsuperlatticewire,λexc)488nm.(d)Micro-PLspectrumoftheperiodicCdS/CdS:SnS2superlatticewirein(a).

Asawidebandgapsemiconductor,CdSwithnear-cylindricalgeometryandlargedielectricconstantsexhibitsgoodwaveguiding.16,17However,whathappensifCdSwasseparatedbyaninactiveSnS2layerina1Dsuperlatticewire?Figure2exhibitstheemissionbehaviorofanindividualwirefromtheCdS/CdS:SnS2superlatticewireswhichwascharacterizedbyaconfocalopticalsystemusinganAr-ionlaserwithanexcitationwavelengthof488nmatroomtemperature.Figure2ashowsthemicrophotoluminescence(PL)imagewithwell-definedemissionperiodicityforawireobtainedunderconstanttemperaturegrowthfor20min.Theinsetisthecorrespondingfar-fieldopticalimage.Inadditiontoabrightfaculaontheexcitationcenter,severalperiodicgreenspotsseparatedbydarkregionscanbeseen;eachspacingbetweentwobrightspotsisabout7.3µm,whichisequaltothelengthoftheCdSsegment.Thatis,thepositionsofthepointsofluminescencearethepreciselocationsofCdS:SnS2junctions.Incontrasttothepreviousreports,whichshowsasingleemissionpeakfromthelowbandgapcomponentlocationsinaSi/Ge6andGaAs/GaP8superlatticenanowire,amultiple-peakPLspectrumisobtainedfromasuperlatticewirehere.

Inourwires,CdShasadirectbandgapof2.4eVandrefractiveindexof2.4.TheSnS2hasasmallerindirectbandgapofabout2.0eVandrefractiveindexof3-3.3.TheCdSsegmentsinthesuper-latticesmayformmanyopticalmicrocavitiesinqueuewhichconfineandtransportphotons,whileSnS2withalargerrefractionindexintheshortCdS:SnS2sectionmightworkasbothreflectionendfacesandemissioncenter.Theperiodicbrightemissionsariseexactlyfromtheinterferenceofcoherentlyscatteredlightwavesontheendfacesofthemicrocativities(FigureS6)andSnS2emission.Figure2cand2dillustratetheemissionprofileofasuperlatticewireandcorrespondingmicro-PLspectrum,whichexhibitsastrongCdSbandedgeemissionat∼509nmandmultipeakrangeof525-650nm.ThemultipeaksarenotFabry-Perotmodes,sincethecalculatedmodespacingsuggestsa2.9-3.4µmcavityandthetruedimensionis7.3µm.Themodeslikelyoriginatefromthelongitudinalphotonpropagationselectioninthecoupledopticalmicrocavities.18ImagesoftheselectedwaveguidingareshowninFigureS6.

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Asingle-microwirePLstudywasusedtoassessthedegreeofcontrolinoursuperlatticewiregrowth.Itwasfoundthatsystematicvariationsinthegrowthtimeleadtomicrowiresuperlatticeswithwell-definedchangesinperiodicity.Figure2bshowsthePLimageoftheproductwithagrowthtimeofabout30min.Itsseparationdistancebetweenbrightemissionspotsisabout15µm,whichislargerthandoublethesizeofthewireshowninFigure2a.Moreover,partiallyhollowcavitystructures,asreportedinref19,canbefoundiftheconstant-temperaturegrowthtimeislongerthan60minwithhighSnconcentration.TheconstanttemperatureheatingtimeiscrucialforformationorcontrollingthelengthofaCdSmicrocavityinthissuperlatticestructuregrowthasshowninFigureS7.Inadditiontothegrowthtimeandsourcetemperature,othermicroenvironmentalparametersincludingheatingrates,carriergasflowrates,andtheweightratioofthetwosourceprecursorsalsoinfluencethis1Dsuperlatticewiregrowth.Arelativelyhighheatingrateandstrongcarriergasflowproducemoresourcevapor,generallyleadingtomorewireswiththickerbasesandtaperedends,butnotsuperlattices.TheuniformSn-core/CdS-shellmicrowirescanformwhentheweightratioofSnO2/CdSisincreasedto1/5-3/5.

Insummary,wereportedthefabricationofaCdS/CdS:SnS2superlatticemicrowirethroughacoevaporationtechniquewithmi-croenvironmentalcontrol.Tuningthegrowthtimeandsourcetem-peraturecanmodifythedomainperiodicityofthesuperlatticemicrowires.Thenovelmultipeakemissionwithcontrollableperiodswasobservedforthefirsttime.Theemissionprofileswereproducedbythecombinedcontributionsof1-dphotoniccrystalandperiodicalexcitonconfinementwhichcouldprovideanewmaterialsplatformforawiderangeofapplicationsfromopticalmicrobarcodes,low-thresholdlasers,one-dimensionalwaveguidesto1Dphotoniccrystals,andexciton-lightinteraction,allofwhicharecurrentlybeingpursued.Acknowledgment.TheauthorsthanktheNSFCofChina(Nos.90606001,20873039)andChinaPostdoctoralScienceFoundationfoundedprojectforfinancialsupport.

SupportingInformationAvailable:ThepositiondependentTEMimageandrelatedEDS,micro-Ramanspectrum,schematicofgrowthprocess,typicalopticalimagesandmappingimages.ThismaterialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.References

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