Study On The Preparation And Growth Mechanism Of Aqueous Semiconductor Nanocrystals

Semiconductor nanocrystals (NCs) have attracted great fundamental and technical interests in recent years. Among various physical and chemical routes of NCs preparation, the colloidal chemistry method is the best one for synthesis NCs with high quantum yields QYs, narrow dispersion, desired shape and controllable surface functionality. Colloidal method also makes it possible to synthesize NCs alternatively in aqueous solution and non-aqueous media. Based on the investigation of non-aqueous NCs, the diffusion controlled growth mechanism of NCs is gradually revealed. In parallel with the success of non-aqueous synthesis routes, aqueous methods have also been developed to prepare II-VI NCs, especially CdTe NCs. Since the first reports of 1-thioglycerol stabilized CdTe NCs in aqueous media, various thiol ligands are used for aqueous CdTe NCs. However, less works are focus on the the growth mechanism of aqueous NCs due to the complexity of aqueous system. This has become a great obstacle for further investigation of aqueous NCs. In this paper, our aim is to build up the structure model of aqueous NCs and investigate their growth mechanism. According to such model, we also design some more complex structure of aqueous NCs, such as aqueous CdTe/CdS core-shell NCs and sub-micrometer CdS spheres.In the second chapter, we mainly investigate the relationship between electrostatic repulsion and the process of NCs nucleation and growth. According to the knowledge of colloidal chemistry, we also build up the double layer model of aqueous NCs. Namely NCs compose of core, ligand shell, adsorption layer, diffusion layer from inside to outside. The various experimental conditions, such as concentration, ions strength, pH are expected to affect the diffusion layer, thus the growth process of NCs.In the third chapter, we mainly investigate the effect of ligand shell on the growth of NCs. To study the effect of ligand in a large extent, we also develop a new sodium citrate assisted method to prepared CdTe NCs.In the forth chapter, we are focus on the adsorption layer of NCs. Such layer is especially important at room temperature. During storage at room temperature, the excess ligands or cadmium-ligands compounds in solution are preferentially adsorbed on adsorption layer before they diffuse into the ligand shell. Though the adsorption layer does not directly determine the optical property of NCs, it can affect the diffusion process of free ligands or cadmium-ligands compounds into the ligand shell. The treatments of dilution, concentration, storage, and pH adjustment affect the thickness of adsorption layer as well as the ligand diffusion into ligand shell, leading to diverse optical behavior of NCs.Based on above knowledge of NCs structure model and growth mechanism,in chapter five and six, we design some complex structure of NCs, namely the aqueous CdTe/CdS core-shell NCs and CdS sub-micrometer spheres.