Nanocrystals（Nano Lett, 2001）

AlternativeRoutestowardHighQualityCdSeNanocrystalsLianhuaQu,Z.AdamPeng,andXiaogangPeng*DepartmentofChemistryandBiochemistry,UniVersityofArkansas,FayetteVille,Arkansas72701ReceivedMay15,2001(RevisedManuscriptReceivedMay23,2001)ABSTRACTDifferentkindsofsafe,common,andlow-costcompoundswereproventobegoodsolvents/precursorsforthesynthesisofhighqualityCdSenanocrystals.Thesize,shape,andcrystalstructureofCdSenanocrystalssynthesizedbythesealternativeroutescanbevariedinacontrollablemannerinaverybroadsizerange,fromabout1.5nmtoabove25nm.Withoutanysizesorting,thesizedistributionofthewurtzitenanocrystalsinthesizerangesmallerthanabout15nmcanbereadilycontrolledasnarrowaswith5−10%relativestandarddeviation.Thehighestphotoluminescencequantumefficiencyofthenanocrystalswasupto20−30%.Cd(Ac)2andfattyacidswerefoundtobethemostversatilecadmiumprecursorandsolvent/ligand,respectively.ThesynthesisofhighqualityCdSenanocrystalsinthecurrentsystemsisnotfeasiblewhenthesystemcontainsanyanionsfromastrongacid,eitherintheformofacadmiumprecursororasanaddedcadmiumligand.Theexperimentalresultsindicatethatthesynthesisofhighqualitycolloidalnanocrystalsisfarlessdelicatethanithasbeenthoughttobe.Synthesisofhighqualitysemiconductornanocrystalshasbeenanimportanttopicinthefieldofmaterialschemistryinrecentyears1-10becauseofthetechnicalandfundamentalimportanceofthesenovelmaterials.11-15Intheearly90s,highqualityCdSenanocrystalsbecameavailablebyusingCd(CH3)2asthecadmiumprecursorandtechnical-gradetrioctylphosphineoxide(TechTOPO)asthereactionsol-vent.9,16ThesynthesisofCdSenanocrystalsthroughthisorganometallicroutehasledthesynthesisofhighqualitysemiconductornanocrystalsforabout10years.3,5However,somekeychemicalsusedinthistraditionalrouteareextremelytoxic,pyrophoric,explosive,and/orexpensive.Thesynthesisishardtocontrolandisnotveryreproducible.Thismotivatesustodevelopalternativesyntheticmethodsforthiswell-studiedmodelsystem.Wehopeknowledgegainedbystudyingthisimportantmodelsystemwillhelpustoeventuallydevelopgreenchemicalapproachesforthesynthesisofotherkindsofhighqualitynanocrystals.TheresultstobedescribedbelowwilldemonstratethatthesynthesisofhighqualityCdSenanocrystalsisnotasdelicateasithasbeenthoughttobe.Byaone-potapproach,CdSenanocrystalscanbeobtaineddirectlyinmorethan10differentsolventsystems,suchasfattyacids,amines,phosphineoxides(TechTOPOorpureTOPO),phosphonicacids(onlyinthemixturewithTOPO),andthemixtureofthesecommonchemicalswithcertaincompositions(seeexamplesinFigure1).Ourveryrecentwork1describedthatCdOisamoredesirableprecursorforthesynthesisofhighqualityCdTe,CdSe,andCdSnanocrystalsincomparisontoCd(CH3)2.Inadditiontothisdiscovery,wewilldemon-stratebelowthatcadmiumsaltswithananionofaweak*Correspondingauthor.Phone:501-575-4612.Fax:501-575-4049.Email:xpeng@comp.uark.edu.Figure1.UV-visandphotoluminescence(PL)spectraofdifferentsizedwurtziteCdSenanocrystals(as-prepared).TDPA:tetra-decylphosphonicacid.NANOLETTERS2001Vol.1,No.6333-33710.1021/nl0155532CCC:$20.00©2001AmericanChemicalSocietyPublishedonWeb06/13/2001acidareallpresumablyexcellentcadmiumprecursorsforthesynthesisofhighqualityCdSenanocrystals.Amongalltestedcompounds,Cd(Ac)2(Figure3)andfattyacids(Figures2and4)wereproventobethemostversatilecadmiumprecursorandsolvent,respectively.Theexistenceofanyanionofastrongacid,eitherintheformofcadmiumprecursororasanaddedcadmiumligand,madeitimpossibletoformhighqualityCdSenanocrystalsinthecurrentsystems.Thiols,whichareverystrongligandstocadmium,werefoundtoinhibitthenucleationprocess.Wewilldemonstratethat,bythemethodsdiscussedbelow,thecrystalstructureofCdSenanocrystalscanbecontrollablyvariedbetweenzincblendeandwurtzitephases.TheachievablesizerangeofnearlymonodisperseCdSedots,fromabout1.5nmtoabove25nm,ismuchbroaderthanthatachievedbytheoriginalorganometallicmethod.Experimentally,achosensolventandacadmiumprecursorwereheatedtoabout250-360°CunderArflowandresultedinanopticallyclearsolution.Consequently,aseleniumsolution(Sepowderdissolvedintrioctylphosphineortri-butylphosphine)wasinjectedintothereactionvessel.Thegrowthtemperaturewassetbetween200and320°C.Ifamines,suchasdodecylamine(DA),areinvolved,theinjection/growthtemperaturesweretypicallysetat220/200°C.ThereactionsweremonitoredbyrecordingtheUV-visspectraofthealiquotstakenfromthereactionvessel.Ingeneral,theadditionofTOPOasacosolventmakestakingaliquotsmucheasierbecauseofitsrelativelyslowsolidifica-tionrate.TheinitialCd:Semolarratiowasvariedbetween2:1to1:5.ItshouldbepointedoutthatthepowderX-raydiffractionpatternsverifiedthatthecrystalstructureofCdSenanocrystalskeptunchangedbyvaryingtheCd:Seratio.Thecadmiumconcentrationwasvariedbetween0.015mol/kgand0.30mol/kg.Allofthesamplesforthemeasurementswereas-preparedandwithoutanyfurthersizing.Inatypicalsynthesisusingastearicacid/TOPO/CdCO3system,0.2mmolCdCO3wasmixedwith2gstearicacidandheatedtoabout130°CunderArflowuntilallofthemixturebecameopticallyclear.Thesystemwasallowedtocooltoroomtemperature,and2gof99%TOPOwasadded.Theflaskwasresealedandreheatedupto360°CunderArflow.Aseleniumsolution(2g)with0.5mmolpowderseleniumand0.2goftoluenedissolvedintrioctylphosphinewasquicklyinjectedintothereactionflask.Thetemperatureofthereactionmixturedecreasedto300°Cbytheinjectionandmaintainedatthistemperatureforthegrowthofthenano-crystals.ThereactionwasmonitoredbyUV-visbytakingaliquotsfromtheflask(seeFigure4).Afterthereactionwascompleted,thereactionflaskwasremovedfromtheheatingmantleandallowedtocoolto20-50°C.Acetonewasaddedtoprecipitatenanocrystals.Thenanocrystalsfurtherisolatedbycentrifugationanddecantation.Forlargesizednano-crystals,itisadvisabletostoreatamuddystateinsteadofadrypowderform.Theas-preparednanocrystalsweresolubleintypicalnonpolarsolvents,suchaschloroformandtoluene.Amongallthesystemstested,fattyacidsweredeterminedasthemostversatileligand/solventforthesynthesisofwurtziteCdSenanocrystals.Usingstearicacid(SA)asanexample,withoutanysecondaryinjection,thissolventsystemyieldsCdSenanocrystalsinaverybroadsizerangewithoutanysecondaryinjection,fromabout2nmtoover25nm(allsamplesshowninFigure2).Thissolventsystemisidealforsynthesizinglarge-sizedCdSenanocrystalswithadiameteraboveabout4nm.TheshapeofCdSenanocrystalswithadiameteruptoabove25nmcanbepurposelycontrolledtodot-shape(Figure2).Oncontrast,large-sizedCdSenanocrystalssynthesizedbythetraditionaldimethylcadmium/TOPOmethodusingmultipleinjectionsweregenerallylimitedtoaround11nmandoftenwithasignificantaspectratio(seemoredetailsbelow).3,5,9,16Theverybignanocrystalsmaybeusedforstudyingmechanicalpropertiesofnanocrystals,althoughtheyaretoobigtoexhibitquantum-confinementeffects.15ThesizedistributionofCdSedotsinthesizerangesmallerthan12nmcanbereproduciblycontrolled,with5-10%relativestandarddeviation(Figure2).Forverylargenanocrystals,thesizedistributionissomewhatbroader(Figure2).Fortheentiresizerange,nanocrystalssynthesizedinthissolventsystemluminescequitewell,withaquantumefficiencyfromafewpercentto20-30%.Thephotoluminescencequantumefficiencyshowedatendencytodropwiththeincreaseoftheparticlesize.Figure2.TransmissionelectronmicrographsofdifferentsizedwurtziteCdSenanocrystals(as-prepared)synthesizedintheSArelatedsolvents.ThepowderX-raydiffractionpatternofasample(about6nminsize)demonstratesthehighlycrystallinefeatureofthesenanocrystals.334NanoLett.,Vol.1,No.6,2001Althoughallresultingnanocrystalsemit,thecontroloverluminescencequantumyieldwasnotasreproducibleaswasthecontrolofthenanocrystalsizes.Ingeneral,thetemporalevolutionofthesizeandsizedistributionofCdSenanocrystalsinfattyacidsystemswerequitereproducible,althoughthereactionrateswerefast.WeconsiderthisistobedueprobablytothemorecontrollablenucleationstepinitiatedbycadmiumcarboxylatesinsteadoftheextremelyactiveCd(CH3)2usedinthetraditionalorganometallicapproach.Thisphenomenonimpliesthatthecontrolofthenucleationprocessmaybethekeysteptowardafullycontrollablesynthesis.Inpractice,thefattyacidsystemsarenotrecommendedtosynthesizesmallnano-crystalsbecauseofthefastgrowthratesofthenanocrystals.Thefattyacidsolventsystemiscompatiblewithallofthecadmiumprecursorstested(seeexamplesinFigure4).Growthofnanocrystalsinfattyacidswithdifferentchainlengths,suchaslauricacid(LA),speedsupasthechainlengthofthefattyaciddecreases.Withhighcadmiumconcentrations,aboveabout0.1mol/kg,thefattyacidsystemsmayshowsomedegreeofdecompositionathightemperatures.Thecolorofthemixturebeforetheinjectionsmaychangetoyellowishtobrownish.Theshorterthechainwas,themorepronouncedwasthedecompositionthatoccurred.Forinstance,theCd(Ac)2/TOPOsystemwithhighconcentrationswouldnotstabilizeattemperaturesoverabout300°C.Uptopresent,noinfluenceoverthegrowthofnanocrystalswasobservedaslongasnoprecipitationwasformedinthesystembeforeinjections.Comparedtophosphonicacids,whichweretheessentialligandsaddedinpureTOPOforthepreviouslyreportedCdOscheme,1fattyacidsaremuchlessexpensiveandmoreenvironmentallyfriendly.EncouragedbytheoutstandingsyntheticsuccessforCdSenanocrystalsusingCd(CH3)2astheprecursor,9,16thetradi-tionalTOPOsolventsystemcomposedofeitherTechTOPOorpureTOPOplustherelatedimpuritiesinthetechnicalgradereagenthasbeenthemostpopularchoiceforthesyntheticdevelopmentofavarietyofcolloidalnanocrystalsinnonaqueoussolutionsinthepast10years,1,3,4,7,17-19althoughthefinalresultsmayhaverevealedthatitdidnotworkatall.6,20ForthesynthesisofCdSenanocrystalsusingthenewprecursors,thistraditionalsolventsystemworkedwellinthestrongquantum-confinementsizeregime(roughlyspeaking,withthefirstexcitonabsorptionpeakbelow600nmandadiametersmallerthanabout4.5nm).Thesynthesisintherelativelylargesizeregimeoftenresultedinelongatednanocrystals(notveryuniformly),withlittleornophoto-luminescence.Weobservedthat,usingCd(Ac)2astheprecursor,pureTOPOactuallyworkedwellforsynthesizingnearlymono-disperseCdSenanocrystalswithasizeuptoabout6-8nm(Figure3),althoughtheemissionofthenanocrystalswasweakerthanthatofthosenanocrystalssynthesizedinfattyFigure3.GrowthkineticsofCdSenanocrystalsindifferentsolventsystems.Allofthereactionswereperformedunderthesamereactionconditions,exceptthattheinjection/growthtempearaturesinDA/TOPOwerelower(seetheExperimentalSection).Theinitialcadmiumconcentrationwas0.33mol/kg.TheCd:Semolarratiowas1:5.Forthereactionsthattookplaceinabicomponentsolvent,theratiobetweenthetwocomponentswas1:1,exceptintheHPA/TOPOcase.FortheHPA/TOPOsystem,HPAwassetat0.66mol/kg.NanoLett.,Vol.1,No.6,2001335acids.TheCd(Ac)2/pure-TOPOsystemwasmuchmorereproduciblethantheCd(CH3)2/pure-TOPOsystem.21Itshouldbepointedoutthat,basedonthepresentresults,itwasimpossibletojudgetheroleofacetateionsforthegrowthofnanocrystalsintheCd(Ac)2/TOPOsystem.Likely,bothTOPOandacetateionsmayhaveplayedasignificantroletostabilizethenanocrystals.UsingCd(Ac)2asthecadmiumprecursor,highlycrystal-lineCdSenanocrystalsweresynthesizedinamines(Figure3),althoughthesizedistributionofthosenanocrystalswasonlymoderatelygood.Thisisprobablybecauseoftherelativelylowinjection/growthtemperatures(170-220°C).ThesystemtestedcouldnotwithstandhightemperaturesbecauseCd(Ac)2decomposedrapidlyinthepresenceofaminesathighertemperatures.Interestingly,underthesamereactionconditions,thenanocrystalsformedinaminesseemedtobezincblendecrystals,insteadofthecommonwurtzitestructureformedintheothersolventsystems(Figure2).Thisphenomenonisunderactiveinvestigationanddetailswillbereportedlater.AlcoholscouldnotbeusedasthesolventfortheformationofhighqualityCdSenanocrystalsunderthetestedconditions,becausealltestedprecursorswereinsolubleinalcoholsatelevatedtemperatures.Thesynthesisusingsulfonicacidasligandsresultedininsolublelargecrystals.Similarly,nano-crystalswithcontrolledsizesweredifficulttoforminthiolsunderthereactionconditionstested.Cd(Ac)2(Figure3)andothertypesofcadmiumcarboxy-latewerefoundtobeveryversatilecadmiumprecursors.BothCdCO3andCdOworkedonlyinthepresenceofeitherfattyacidsorphosphonicacids.ThegrowthkineticsofCdSenanocrystalsusingdifferentprecursorsdidshowsomevariation(Figure4).Atthismoment,wedonotknowwhy.AlthoughthesolubilityofCdCl2orCdSO4intestedsolventswasusuallygood,therelatedone-potsynthesisgeneratedonlybulk-sizedCdSe.ThisisprobablyaresultoftherelativelyhighstabilityunderacidicconditionsofthesaltformedbycadmiumandastrongacidincomparisontothatofCdSe.1Wenoticedthatsometypesofcadmiumsaltswithstrongacidswereusedforwater-basedsynthesisofcadmiumchalcogenidesnanocrystals.8Inaqueoussolution,thesesaltsexistmoreorlessintheformofionsinsteadofverystablecompoundsinthepresentsystems.Inconclusion,manysafe,lowcost,andcommoncom-poundswereproventobegoodprecursors/solvents/ligandsforreproduciblysynthesizinghighqualityCdSenanocrystalsinaverybroadsizerange.Theresultsreportedhereprovideastartingpointtoestablishaprecursor/solvent/liganddatabasefordesigningoptimizedgreen-chemicalsyntheticapproachesforhighqualitynanocrystals.Amongallofthesolventsystemstested,fattyacidsareexcellentcandidatesforsynthesizingrelativelylarge-sizedCdSenanocrystals(4-25nm),althoughtheformationofCdSenanocrystalsbetween2and4nmwasalsoobserved.Incomparison,phosphonicacids/TOPO,TechTOPO,orpureTOPOaregoodsolventsforsynthesizingCdSenanocrystalsinthestrongconfinementsizeregime.PreliminaryresultssuggestedthatCdSenanocrystalswithzincblendestructuremaybeprepared.AllanionsofstrongacidsandthiolatesshouldbeavoidedinthesynthesisofhighqualityCdSenanocrystalsinnonaqueoussolutionsbecausetherelatedcadmiumsalts/Figure4.GrowthkineticsofCdSenanocrystalsusingdifferentcadmiumprecursorsunderthesamereactionconditions(seetheExperimentalSection).336NanoLett.,Vol.1,No.6,2001complexesaretoostabletoproperlyinitiatethenucleationprocess.Acknowledgment.ThisworkissupportedbytheNSFCareerAward(AwardId:0094248),bytheUniversityofArkansas,andbytheYoungInvestigatorAwardpresentedbyQuantumDotCorporation,Hayward,California.References(1)Peng,Z.A.;Peng,X.J.Am.Chem.Soc.2001,123,183-184.(2)Peng,Z.A.;Peng,X.J.Am.Chem.Soc.2001,123,1389-1395.(3)Peng,X.G.;Manna,L.;Yang,W.D.;Wickham,J.;Scher,E.;Kadavanich,A.;Alivisatos,A.P.Nature2000,404,59-61.(4)Manna,L.;Scher,E.C.;Alivisatos,A.P.J.Am.Chem.Soc.2000,122,12700-12706.(5)Peng,X.G.;Wickham,J.;Alivisatos,A.P.J.Am.Chem.Soc.1998,120,5343-5344.(6)Hines,M.A.;GuyotSionnest,P.J.Phys.Chem.1998,102,3655-3657.(7)Nozik,A.J.;Micic,O.I.MRSBull.1998,23,24-30.(8)Vossmeyer,T.;Katsikas,L.;Giersig,M.;Popovic,I.G.;Diesner,K.;Chemseddine,A.;Eychmuller,A.;Weller,H.J.Phys.Chem.1994,98,7665-7673.(9)Murray,C.B.;Norris,D.J.;Bawendi,M.G.J.Am.Chem.Soc.1993,115,8706-8715.(10)Wilson,W.L.;Szajowski,P.F.;Brus,L.E.Science1993,262,1242-1244.(11)Heath,J.R.,Ed.;SpecialIssueforNanostructures;Acc.Chem.Res.1999.(12)Bruchez,M.;Moronne,M.;Gin,P.;Weiss,S.;Alivisatos,A.P.Science1998,281,2013-2016.(13)Chan,W.C.W.;Nie,S.M.Science1998,281,2016-2018.(14)Colvin,V.L.;Schlamp,M.C.;Alivisatos,A.P.Nature1994,370,354-357.(15)Alivisatos,A.P.Science1996,271,933-937.(16)Brennan,J.G.;Siegrist,T.;Carroll,P.J.;Stuczynski,S.M.;Reynders,P.;Brus,L.E.;Steigerwald,M.L.Chem.Mater.1990,2,403.(17)Rockenberger,J.;Scher,E.C.;Alivisatos,A.P.J.Am.Chem.Soc.1999,121,11595-11596.(18)Trentler,T.J.;Denler,T.E.;Bertone,J.F.;Agrawal,A.;Colvin,V.L.J.Am.Chem.Soc.1999,121,1613-1614.(19)Micic,O.I.;Curtis,C.J.;Jones,K.M.;Sprague,J.R.;Nozik,A.J.J.Phys.Ch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