The Formation Mechanism Research Of CdSe Magic-Size Clusters With Absorption Doublet

The Cadmium Selenide(CdSe)nanocrystals,which exhibits absorption doublets in the UV-vis spectra,have been attracting much attentions due to their extraordinary optical properties,such as narrow emission peaks,short stocks shift.It has been pointed out that such a type of nano material is 0-dimentional magic-size clusters(MSCs),which may assemble to 2-dimentional nanoplatelets.The species exhibiting one characteristic absorption doublet at about 373/393 nm,donated as dMSC-393,is the main object investigated in this thesis.In the first part of this work,we focus on the mechanism of the transformation from dMSC-393 to another similar MSCs with absorption doublet at about xxx/xxx nm,dMSC-460.We notice that the reaction of purified dMSC-393 evolves to an absorption featureless stage upon the transformation from dMSC-393 to dMSC-460.Thus,we propose that the transformation of dMSC-393 to dMSC-460 occurs via their corresponding precursor compounds(PCs).The key step in the process is the transformation from PC-393(the counterpart PC to dMSC-393)to PC-460(the counterpart PC to dMSC-460)via an addition reaction between PC-393 and CdSe monomers.Synchrotron based small angel X-ray scattering(SAXS)provides a strong evidence that dMSC-393 is smaller than dMSC-460.Density functional theory(DFT)calculation for the transformation process suggested that the top/down side of dMSC-393 is more readily to interact with CdSe monomers via additional reactions,resulting to dMSC-460.In the second part of this work,the formation mechanism for dMSC-393 is explored.We find that a batch with a high precursor supersaturation degree facilities the production of particles with a relatively smaller size,i.e.dMSC-393.In contrast,the batch with a low precursor supersaturation prefers to produce the particles with relatively larger sizes,such as dMSC-460 and quantum dots(QDs).The evolution of purified dMSC-393 is in a great agreement with La Mer model proposed by classical nucleation theory(CNT),which is a supersaturation-dependent process.The formation mechanism of dMSC-393 provides a strong evidence for the hypothesis that the NPLs are formed by the self-assembly of dMSCs,meanwhile,it suggests an efficient method to obtain dMSCs in high quality.In the third part of this work,we design and synthesize another dMSCs,which is a core-shell material composed by a CdSe core and a CdS shell.The dMSCs obtained exhibit one absorption doublet at about 425/457 nm on UV-vis spectra,which is donated as dMSC-450.dMSC-450 is synthesized by mixing the CdSe and CdS induction period samples(IPSs)upon heating.The dMSC-450 have a high absolute fluorescence quantum yield(～85%)with a narrow full width at half maximum(～13nm),non-trap emission and blue emitting(～459 nm).Such a special dMSCs exhibits a huge potential on the applications as photoelectric devices.