Spectra And Carrier Dynamics Properties Of CdSe-based Core-Shell Semiconductor Nanocrystals

Semiconductor nanocrystals have special optical and electrical properties,such as size dependent fluorescence emission and high fluorescence quantum efficiency.Photoelectronic devices and bio-imaging based on semiconductor nanocrystals have great research and application value.However,semiconductor nanocrystals have a large surface-volume ratio,surface atoms usually lack sufficient coordination,while these surface defects have a significant impact on the optical properties and stability of nanocrystals.The usual solution is to coat the nanocrystals surface with a second semiconductor material to form core-shell structure.On the one hand,the existence of shell can passivate the surface defects of nanocrystals completely and improve the stability of nanocrystals.On the other hand,the alignment of energy levels between core and shell materials determines the exciton type of nanocrystals and produces corresponding functionality.In addition,the lattice mismatch between core and shell materials and the synthesis conditions are also important factors which affecting the surface properties of core-shell nanocrystals and the corresponding trapping states distribution,while the later have a significant impact on the carrier dynamics process and device quantum efficiency.It is important to further understand the interaction between semiconductor nanocrystals and electron/hole acceptors,the carrier dynamics process for optimizing the properties of nanocrystals and improving the performance of devices.The properties of core-shell semiconductor nanocrystals strongly depend on their structure and surface properties.The purpose of this paper is to study the optical properties of CdSe-based core-shell semiconductor nanocrystals and their interaction with carrier acceptor molecules by means of steady-state and transient spectroscopy characterization techniques.And further to analyze and understand the relationship between semiconductor nanocrystals structure and the dynamic properties of exciton recombination and charge transfer.Finally achieve the purpose to improve the performance of semiconductor nanocrystalline materials and devices.The main research works are as follows:(1)The quasi-type II core-shell structure was formed by coating CdS on the surface of CdSe nanocrystals.The shell thickness dependent optical properties of CdSe/CdS core-shell nanocrystals were studied.The experimental results show that the CdS shell can significantly improve the surface passivation and consequently the fluorescence emission ability of nanocrystals.The quenching experiments for CdSe/CdS nanocrystals with different shell thickness show that increasing the shell thickness can suppress the fluorescence quenching caused by hole quenching molecules.(2)Type I core-shell structure was formed by coating ZnS shell on the surface of CdSe nanocrystals.The effects of synthesis temperature and shell thickness on the optical properties of CdSe/ZnS nanocrystals were investigated.The experimental results show that a relatively low coating temperature is conducive to obtaining nanocrystals with higher fluorescence quantum yield and better fluorescence monodispersity.The high temperature annealing of CdSe/4ML ZnS synthesized at 200 ~oC were carried out.The results show that the annealing process results in a continuous blue shift of the spectra and the change of core-shell interface composition.The results indicate that the annealing process lead to a change of PTZ-induced fluorescence quenching behavior,which is related to the degree of cation diffusion and the change of shell quality caused by annealing treatment.(3)The ZnTe/CdSe nanocomposites was formed by coating ZnTe with CdSe shell,and the shell thickness dependent spectral and dynamic properties of ZnTe/CdSe nanocrystals were studied.The experimental results show that the CdSe shell can significantly influence the exciton recombination dynamics and fluorescence quantum efficiency of nanocrystals.The calculated results shows that the electrons and holes are localized in the shell and the core,respectively.The electron and hole quenching experiments confirms the accuracy of the calculation results.The electron and hole can be transferred to the corresponding acceptor to produce fluorescence quenching.The quenching efficiency is related to the energy level and localization of the electron/hole and the reduction/oxidation potential of the acceptor molecule.In ZnTe/CdSe,the energy level of carriers can be effectively controlled by changing the size of the core and the thickness of the shell,so as to control the dynamics of charge transfer.