Formation of High-Quality CdTe, CdSe, and CdS Nano

FormationofHigh-QualityCdTe,CdSe,andCdSNanocrystalsUsingCdOasPrecursorZ.AdamPengandXiaogangPeng*DepartmentofChemistryandBiochemistryUniVersityofArkansas,FayetteVille,Arkansas72701ReceiVedOctober10,2000High-qualitycolloidalsemiconductornanocrystalsarenanom-eter-sized,singlecrystallinefragmentsofthecorrespondingbulkcrystals,whichhavewell-controlledsizeandsizedistributionandaredispersibleindesiredsolvents/media.Recently,semiconductornanocrystalsareofgreatinterestforbothfundamentalresearchandtechnicalapplications,1-8duetotheirstrongsizedependentpropertiesandexcellentchemicalprocessibility.Synthesisofhigh-qualitysemiconductornanocrystalshasbeenplayingacriticalroleinthisveryactivefield.1,9-15Asthemostdevelopedsystemintermsofsynthesis,1,9,10,15high-qualityCdSenanocrystalswithnearlymonodispersesizeandshapeareinactiveindustrialdevelopmentforbiologicallabelingreagents.5,6SinceMurrayetal.15reportedthesynthesisofhighqualitycadmiumchalcogenidesnanocrystalsusingdimethylcadmium(Cd(CH3)2)asthecadmiumprecursor,thesynthesisofCdSenanocrystalsusingthisprecursorhasbeenwelldeveloped.1,9,10Incomparison,thesynthesisofCdTeandCdS15,16arenotasadvanced.Forinstance,thereisnomethodtocontrollablyvarytheshapeofCdTeandCdSnanocrystals.Cd(CH3)2isextremelytoxic,pyrophoric,expensive,unstableatroomtemperature,andexplosiveatelevatedtemperaturesbyreleasinglargeamountofgas.Duetothesereasons,theCd(CH3)2-relatedschemesrequireveryrestrictedequipmentsandconditionsandarenotsuitedforlarge-scalesynthesis.Inthispaper,wewillprovethatCd(CH3)2canbereplacedbyCdO.Surprisingly,thisnewsyntheticschemeworkssignificantlybetterthantheCd-(CH3)2-relatedones.Withoutanysize-sorting,thequalityofquantum-confineddotsandrods(quantumdotsandquantumrods)ofallcadmiumchalcognidesformedbythenewmethodiscomparabletothatofthebestCdSenanocrystalsreportedintheliterature.Thenewschemeisreproducibleandsimpleandthuscanbereadilyscaledupforindustrialproduction.Recently,weidentifiedthatCd(CH3)2decomposesinhottrioctylphosphineoxide(TOPO)andgeneratesinsolublemetallicprecipitate.9Withastrongligand,eitherhexylphosphonicacid(HPA)ortetradecylphosphonicacid(TDPA),Cd(CH3)2isim-mediatelyconvertedintocadmiumHPA/TDPAcomplex(Cd-HPA/Cd-TDPA)ifthecadmiumtoHPA/TDPAratioislowerthan1.Aftertheformationofthecomplex,aninjectionofSedissolvedintributylphosphine(TBP)generateshigh-qualityCdSenanocrystals.ThisresultimpliesthatCd(CH3)2maynotbenecessary,ifwecangeneratethecomplexbyothermeans.WefirstsynthesizedandpurifiedCd-HPAfromCdCl2orCd(CH3)2.High-qualityCdSenanocrystalswereindeedyieldedfromthiscomplex.Thissuccessencouragedustodevelopaone-potsyn-thesiswhichdoesnotrequireseparatedpreparationofcadmiumcomplex.Wefailedtomakehigh-qualityCdSenanocrystalsusingCdCl2bytheone-potapproachalthoughCdCl2canbedissolvedinthereactionmixtureatelevatedtemperatures.Incontrast,CdOworksverywellfortheone-potapproach.WethinkthisisduetothelowstabilityofCdOrelativetophosphonicacids,comparedtothatofCdCl2.Experimentally,CdO,TOPO,andHPA/TDPAwereloadedinathree-neckflask.Atabout300°C,reddishCdOpowderwasdissolvedandgeneratedacolorlesshomogeneoussolution.Introducingtellurium,selenium,andsulfurstocksolutionsyieldshighqualitynanocrystals.17Thesamplesforallofthemeasure-mentsshowninthispaperaredirectlyfromsynthesiswithoutanysizeseparation.ThegrowthkineticsofnanocrystalsgrownbythenewapproachpossessesapatternsimilartothatofthebestCdSenanocrystalsformedbytheCd(CH3)2approach(Figure1).10Figure1andFigure2furtherrevealthatthesizeofallthreekindsofnanocrystalscanbeclosetomonodisperse,representedbythesharpabsorptionpeaksifthegrowthstopsinthe“focusingofsizedistribution”regime.10Transmissionelectronmicroscopy(TEM)measurementsindicatethatthesenanocrystalshaveverynarrowdistribution.TherelativestandarddeviationofthesizeofthenanocrystalsshowninFigure3(top)isabout10%.ThehighcrystallinityofthesewurtzitenanocrystalswasconfirmedbyX-raypowderdiffraction.ForthisCdOapproach,thesizeofrelativelymonodisperseCdSenanocrystalscanbecontinuouslytuneddowntothesizeswiththefirstabsorptionpeakat440nm(seethefirstabsorptionspectruminFigure1).RelativelymonodisperseCdSenanocrystalswiththefirstexcitonabsorptionpeakbelow480nmaredifficulttosynthesizedirectlywiththeexistingCd(CH3)2-relatedap-proach.10,18(1)Peng,X.G.;Manna,L.;Yang,W.D.;Wickham,J.;Scher,E.;Kadavanich,A.;Alivisatos,A.P.Nature2000,404,59-61.(2)Heath,J.R.(editor).Acc.fChem.Res.1999.(3)Alivisatos,A.P.Science1996,271,933-937.(4)Huynh,W.;Peng,X.;Alivisatos,A.P.AdV.Mater.1999,11,923-927.(5)Bruchez,M.;Moronne,M.;Gin,P.;Weiss,S.;Alivisatos,A.P.Science1998,281,2013-2016.(6)Chan,W.C.W.;Nie,S.M.Science1998,281,2016-2018.(7)Schlamp,M.C.;Peng,X.G.;Alivisatos,A.P.J.Appl.Phys.1997,82,5837-5842.(8)Mattoussi,H.;Radzilowski,L.H.;Dabbousi,B.O.;Thomas,E.L.;Bawendi,M.G.;Rubner,M.F.J.Appl.Phys.1998,83,7965-7974.(9)Peng,Z.A.;Peng,X.G.J.Am.Chem.Soc.,inrevision.(10)Peng,X.G.;Wickham,J.;Alivisatos,A.P.J.Am.Chem.Soc.1998,120,5343-5344.(11)Murray,C.B.;Norris,D.J.;Bawendi,M.G.J.Am.Chem.Soc.1993,115,8706-8715.(12)Nozik,A.J.;Micic,O.I.MRSBull.1998,23,24-30.(13)