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OligomerizationofcadmiumchalcogenidenanocrystalsintoCdTe-containingsuperlatticechainsw

Downloaded by Hong Kong Baptist University Library on 21 September 2011Published on 19 September 2011 on www.1mpi.com | doi:10.1039/C1CC14103A

YoshiyukiNonoguchi,*aTakuyaNakashima,aAtsushiTanaka,aKeikoMiyabayashi,bMikioMiyakebandTsuyoshiKawai*a

Received8thJuly2011,Accepted26thAugust2011DOI:10.1039/c1cc14103a

Hierarchically-assembledbinarycadmiumchalcogenide(CdX,X=S,Se,Te)oligomerswereengineered.CdTeprecursorinjectionsintoasolutioncontainingseedednanocrystalsprovidedlongeroligomerspossessingalternatingandphase-separatedCdXandCdTeconstituentsinthesamewire,likeanalternatingcopolymer.Tailoredassemblyofdistinctnanocrystalsintoisotropic(i.e.core/shell)1andanisotropicheterostructures2hasbeenstudiedextensivelyinordertoexploittheircollective,andenhancedphysicochemicalproperties.3Theheterostructuresynthesesoftenrelyonseed-mediatedepitaxialgrowth,1,4andorientedattachment.5Also,recentapproachestobottom-upnanocrystalintegrationincludemetal-,andsemiconductor-mediatedwelding(i.e.gold6andCdHgTe7).Onedimensionalsuperlatticeheterostructures,whichhavecapabilitiesforlasingmaterial8andphotovoltaicdevices,9havebeenfabricatedbygas-phasesynthesis.Veryrecently,Alivisatosetal.haveusedpartialcationexchangetofabricatesuperlatticenanorods,10demonstratingthepossibilitiesofsimplerandmoreversatilewet-chemicalcolloidalsynthesisroutes.Inthisstudy,wedemonstrateacolloidalapproachtoone-dimensional,binaryCdS–CdTeandCdSe–CdTesuperlatticeoligomersfrombothsphericalnanocrystals(NCs)andnanorods(NRs)throughahightemperaturecolloidalmethod.Itissimilartoconventionalsynthetictechniques,4butprovidesentirelydi erentconnectionmorphologiesunderprecisecontrolofreactionconditions.WesynthesizedbinaryCdXoligomersbysuccessiveCdTeprecursorinjectionsintoamixturecontainingCdSandCdSeNCs,1-hexadecylamine(HDA)asasurfactant,and1-octadeceneasasolvent,followedbyhightemperaturegrowthofoligomers(Fig.1).11Bright eldtransmissionelectronmicroscopy(BF-TEM)imagesshowedelongatednanowires(Fig.2candd)formedfromsphere-shapeCdSandCdSeNCs(Fig.2aandb,respectively).TheaveragediametersoftheCdS–CdTe

a

Fig.1Schematicsofnanowiregrowthreactionsfromsherical(upper)androd-shaped(bottom)seededparticles.

GraduateSchoolofMaterialsScience,NaraInstituteofScienceandTechnology,NAIST,8916-5Takayama,Ikoma,Nara630-0192,Japan.E-mail:tkawai@ms.naist.jp,nonoguchi@ms.naist.jpb

SchoolofMaterialsScience,JapanAdvancedInstituteofScienceandTechnology,JAIST,1-1Asahidai,Nomi,Ishikawa932-1292,Japan

wElectronicsupplementaryinformation(ESI)available:Experimentalprocedures,diameterdistributionanalyses,additionalSAEDpatterns,andXRDpatterns.SeeDOI:10.1039/c1cc14103a

nanowiresweretypicallyabout3.7Æ0.6nm,correspondingcloselytothediametersofthestartingNCs(3.5Æ0.6nm).11Thisobservationsuggeststhefollowingtwopossiblegrowthmechanismsforthenanowires:(1)seed-mediatedgrowthfromCdSandCdSeNCs,and(2)successiveattachmentsandweldingoftheNCs.Theoligomerlengthisabout50nmfor60min-growthforCdS–CdTe.High-angleannulardark- eldscanningTEM(HAADF-STEM)studiesforCdS(CdSe)–CdTenanowiresrevealedthateverynanowirecontainsseveraldiscretepartswithhighandlowcrystallinity(Fig.2eandf).Notethattheirstructuresarealmostone-dimensional,whereasthesynthesisprocedureisverysimilartothoseofsphericalcore/shellnanocrystals.4dTheonlydi erenceisthetemperaturecondition.Wehavealsocon rmedthatthereactioncouldoccurforalloleicacid,trioctyl-phosphineoxide,andHDAligand-cappedseededNCs.

ThedepositionofCdTeonCdSwascon rmedwithUV-visabsorptionspectra(Fig.3),because rst-excitonicphotonabsorptionofCdTe(500–680nm)iswellisolatedfromCdSabsorptionregions(o500nm).TheCdSNCsshowedthe rstexcitonabsorptionpeakat460nmafteroligomergrowthat1801C,whichwassuperimposedbythebroadabsorptionbandatalongerwavelengthattributabletolesscrystallineCdTeparts.TheslightblueshiftoftheCdSpeakoriginatesfromthepartialdecompositionofCdSfollowedbydepositionofCdTepartsaccompanyingsulfurdeposition.ThisresultedinthedecreaseinthesizeofCdSandpartialalloyingoftheCdTe-richpart.Post-annealingat2401Cfor10minenhancedtheabsorptionbandabove500nm,indicatingimprovedcrystallinityofCdTe-basedparts.

Thesecrystalstructurescanalsobecon rmedbyselectedareaelectrondi raction(SAED)patterns.The

lattice

Chem.Commun.

ThisjournaliscTheRoyalSocietyofChemistry2011