Synthesis, Assembly And Application Of Metal Chalcogenide Semiconductor Nanocrystals

Metal chalcogenides are important semiconductors which show promising applications in optical devices, biological labeling, solar cells and sensors. In order to obtain materials with particular compositions, sizes and shapes, it is essential to develop simple, controllable, general, and environment friendly synthetic methods, and study the growth mechanism of nanocrystals. Assembling the nanocrystals into advanced functional structures also paves the way for the practical applications. In this dissertation, valuable explorations have been carried out on the novel synthetic method for chalcogenides nanocrystals as well as their assembly. The growth mechanism of the nanocrystals and the driven force for the assembling has been investigated. The as-obtained nanocrystals have been used for fabricating solar cells and gas sensors, aiming at understanding the relationship between performance and structure.A water/oil two phase system has been developed to synthesize Ag₂S and Cu₂S nanocrystals. The interface of the water and oil can also work as template to induce the nanocrystals to assemble into superlattices. The Ag₂S nanocrystals can assemble into supercrystal with a tetrahedral profile. The Cu₂S nanocrystals assemble into monolayer or multilayer superlattice, in which the dipole-dipole interaction plays important role. Afterwards, a dodecanethiol assistant method has also been developed for synthesizing metal sulfide nanocrystals. Dodecanethiol is used as both sulfur source and surfactant. Metal ions firstly react with dodecanethiol to generate metal thiolates intermediate compounds, and then these intermediate compounds melt to form a homogeneous reaction system. Finally, the thiolates decompose to metal sulfides. In this method, common inorganic metal salts are employed as the raw materials, and it is a heating-up process without precursor injection. It is suitable for large-scale synthesis which can be adoped in industry.In order to fit the biological applications, highly water soluble ultrathin CdS nanorods have been direly synthesized with the assistance of polyethyleneimine (PEI). PEI plays as the passivator to control and induce the growth of the CdS nanorods and also makes the as-obtained nanorods dispersible in water. The as-obtained CdS nanorods are quite thin thus showing the quantum effect in the fluorescence. By adjusting the alkalinity of the solution, CdS nanorods can be controlled to precipitate or redisperse in water. Thus it is a smart material that can respond to alkalinity of the solution, which is important in biological separations and sensors. With the assistance of polyvinylpyrrolidone (PVP), water-soluble CdS and ZnS colloidal spheres have also been directly synthesized.Nanodevices have been fabricated by the as-obtained nanocrystals and their performances have been investigated. CdS quantum-dot sensitized solar cell has been prepared by the in-situ generated CdS from cadmium thiolates. Ethanol sensor has been succefully made of In₂O₃ nanocrystals and the detection limit of which is lower than 1 ppm. The relationship between the performance and the phase and shape of the materials has also been discussed, which is helpful to improve the performance of the nanocrystals-based device.