Research On Trap And Excited State Of Cadmium Selenide Quantum Dot

Being one member of Nano materials, quantum dots gain numerous attentions for its unique optical properties. The main factor that cause damage to the intrinsic emission of quantum dots is trap states. For now, we still leak a unified understanding of trap states. The main reason is we don't have a standard sample. This thesis aim to obtain CdSe quantum dots with "ideal" optical properties and give a clearly identify for trap states. To accomplish that we have to eliminate both interior traps and surface traps.The first part of this work focus on the eliminating of the interior traps i. e. obtaining nanocrystals with high crystallinity. Literature researches tell us the critical factor that controls the crystallinity is ligands. To gain reliable result we combine XRD simulation and high-resolution TEM together to analysis the samples. The results proved our hypothesis that ligands play the critical role in determining the crystal structure. Cadmium phosphonate ligands promoted formation of the wurtzite CdSe structure while cadmium carboxylate ligands promoted formation of the zinc blende CdSe structure. Amine is a secondary ligand and its effect is strongly depend on the reacting condition. By rational chosen the ligands in both nucleation and growth stage, wurtzite CdSe nanocrystals with high quality is successfully generated at relatively low temperature, thus accomplished the elimination of interior traps.The second part of this work focus on the eliminating of the surface traps. We develop a method to tune the stoichiometric and ligands of CdSe nanocrystals. Removing all the surface traps, we synthesis almost "ideal" CdSe quantum dots with single channel decay and 100% quantum yield, which could be the "standard sample" for CdSe quantum dots. This method could applied on CdSe nanocrystals with different sizes and crystal structures. These results tell us that:other than interior structure the trap states of CdSe quantum dots only come from surface. Though the lifetimes of CdSe nanocrystals with different crystal structure differ with each other, but all of them could gain almost "ideal" optical properties through this surface treatment. Applying another surface treatment on these almost "ideal" CdSe nanocrystals, we identify the relationship between Se-rich (Cd-rich) surface with mid-band (in-band) traps.Overall, our work prove that it is possible to prepare CdSe quantum dots with almost "ideal" optical properties, thus enrich the understand of trap states.