The Study Of Surface Modification And Optical Properties Of Ⅱ-Ⅵ Composite Nanocrystals

Ⅱ-Ⅵ semiconductor nanocrystals （NCs） have shown great potential in the field of biomedicalscience, owing to their novel optoelectronic properties resulting from the size quantization effect,color-tunable, narrow emission, wide absorption peaks and high chemical modification of the surface. Allthis time, Synthesis of high quality NCs was the most investigated object among inorganic semiconductorNCs and their optical properties only get little attention, such as photoluminescence quantum yields （PLQY） and the stability of the emission. When the shells were not thick enough, the quantum yields （QYs）might be well improved, but the PL stability of the core/multishell NCs was not greatly improved.Moreover, we have successfully prepared multicolored aqueous NCs by a versatile phase transfer method,in which amphiphilic oligomers （polymaleic acid n-hexadecanol ester, PMAH） were used as surfacecoating agents. This article aims to identify the optimized shell thickness, in which core/multishell NCscan have not only high QYs but also high stablity, by explaining the impact of shell thickness on theoptical properties in hydrophobic and hydrophilic core/shell NCs. The study of optical properties had agreat significance for the promotion of the synthesis and practical application of Ⅱ-Ⅵ semiconductorcore/multishell NCs.This thesis includes the following main aspects:In chapter2, the focused primarily on the impact of shell thickness on the optical properties intwo types of core/multishell NCs: CdSe/CdS/Zn^0.5Cd0.5S/ZnS （type A） and CdSe/CdS/ZnxCd1-xS/ZnS （typeB） NCs. In order to obtain quantitative information on the optimized shell thickness, the PL QY andstability in the process of purification, phase transfer, PBS bufer and continuous UV radiation were plottedas a function of shell thickness.The optimized shell thickness was found, in which core/multishell NCs can have not only high QYs but also high stablity.In chapter3, we aim to examine the result by explaining the stability of Zn_1-xCd_xSe/ZnScore/multishell NCs. We prepared Zn_1-xCd_xSe/ZnSe/ZnSe_xS_1-x/ZnS core/multishell NCs whose PL QY andstability in the process of purification, phase transfer, PBS bufer was tested. The multicolorZn_1-xCd_xSe/ZnS core/multishell NCs can be transferred into water successfully using an amphiphilicoligomer （PMAH） as a surface coating agent, when the shell thickness is the optimized shell. Thesesuggested that the conclusion about optimized shell thickness also fits to Zn_1-xCd_xSe/ZnS core/multishellNCs.