Preparation And Optoelectronic Property Of Core-shell Semiconductor Quantum Dots

Core-shell semiconductor quantum dots(QDs) attract wide interest due to their special optical, electric and catalytic properties. We prepared ZnO and CdTe QDs, as well as ZnO/ZnS and CdTe/CdS core/shell structure QDs in aqueous solutions. On the basis of composition and morphology characterizations, we further investigated their optical and catalytic property as well as ultrafast dynamics. The main works are listed as follows:1. ZnO QDs and ZnO/ZnS QDs with different shell thickness are successfully synthesized using water bath refluxing. The samples are characterized by XPS, TEM and PL. The photocatalytic property of ZnO/ZnS QDs is studied using photodegradation of MO under ultraviolet lamp. The details include:(1) TEM and HRTEM indicate that QDs have good lattice structure. The diameters of ZnO are～4.8 nm. The shell structure further increases the dot sizes. PL peak of ZnO QDs locates at 486nm in visible range. After covered ZnS of～2.5 nm thickness, there are two PL peaks of 446nm and 496nm due to the defects of ZnS and the formation of structure ZnO/ZnS respectively.(2) The degradation of MO with ZnO/ZnS QDs as catalyst is faster and more efficient than ZnO or ZnS QDs under irradiation of 365 nm UV lamp due to the separation of electron and hole in typeⅡcore-shell structures of ZnO/ZnS QDs.2.CdTe QDs and CdTe/CdS QDs with different shell thickness are successfully synthesized using water bath refluxing. The samples are characterized by TEM and PL. Ultrafast carrier relaxation dynamics in CdTe/CdS QDs with different CdS shell thickness is studied by a pump-probe technique. The details mainly are:(1) The diameters of CdTe QDs are～3 nm. The shell structures of CdS further increase the dot sizes. Compared with CdTe core, the PL peak position of CdTe/CdS is obviously red-shifted owing to the carrier space separation between the interface core-shell structure.(2) Under 800nm excitation, CdTe and CdTe/CdS QDs appear clear two-photon fluorescence. The intensity of fluorescence and the power of excitation are quadratically dependent approximately. (3)CdTe/CdS QDs show a longer relaxation time than CdTe core structure. With the increase of CdS shell thickness,carrier relaxation process gets more slowly, which indicats the relaxation mechanism of carriers changed from surface passivation to a type-II charge separation regime.