The Synthesis, Characterization, And Properties Of CdS Nanostructures

As a direct band gap semiconductor with E_g of 2.42 eV at room temperature, CdS has been paid more attention for its excellent optical properties, offering commercial or potential applications in light-emitting diodes, solar cell, or other optoelectronic devices. Over the past few years, many attempts have been made to synthesize inorganic colloidal nanocrystals especially for the control of their shape, size, and disperse. CdS nanomaterials, together with other compound semiconductors, become the focus of many interests owing to their novel properties that depend on their size and shape. This thesis firstly summarizes the preparation of CdS nanocrystals as well as their property studies.Based on the problems presenting in previous researches as summarized, many efforts have been made in the synthesis, characterization, and analysis of CdS nanocrystals with various shapes and sizes. The main work in this thesis is as follows:1. CdS nanoparticles were synthesized via a hydrothermal method, using poly(N-vinyl-2-pyrrolidone) (PVP) as surfactant to control the size of particles and improve the disperse and stability of nanoparticles in the solution. The results showed that, PVP is one kind of excellent surfactant and the atom of O of C=O could effectively adsorb on the surface of CdS nanoparticles, which prevented the growth of CdS nanoparticles and their congregation. The UV-Vis spectra results indicated that CdS nanoparticles exhibited novel size focusing and novel quantum size effect.2. CdS nanocystals with different rod-based shapes were directly synthesized in a mixed solvent of glycol and ethylenediamine (en) by a solvothermal route, using CdCl₂ and Na₂S.9H₂O as the reactants. The results of XRD and TEM showed that the morphologies of the products could be tuned ranging from rough spherical nanoparticles to rod-, twinrod-, and tetrapod-shaped nanoparticles, by increasing the dosage of en in the mixture. The work also indicated that en is one kind of excellent solvents which can facilitate the oriented growth of CdS. However, glycol can favor the nucleation and growth of cubic CdS for its weak interaction with reactant monomer. In the mixture with the ratio of en and glycol as about 1:1, several branches grow out along the c-axis direction because the planes of {111} in cubic CdS structure correspond to the plane of (001) in hexagonal CdS structure.3. Based on the effect of en on the oriented growth of CdS nanorods, mass CdS nanorods surface-modified by poly(N-vinyl-2-pyrrolidone) (PVP) have beensynthesized via solvothermal treatment of CdCb and thiourea. The as-prepared CdS nanorods show well crystalline with the diameter of 20nm and the length of 2um. The results of UV-Vis spectra and photoluminescence (PL) spectra indicated that the optical properties of CdS nanoparticles were improved by the modification of PVP on their side surface.4. Large-scale CdS nanowires with uniform diameter and high aspect ratios were synthesized using a simple solvothermal route that employed CdCb and S powders as starting materials, ethylenediamine (en) as the solvent. X-ray diffraction (XRD) pattern and transmission electron microscopy (TEM) images show that the products are hexagonal structure CdS nanowires with diameter of 40nm and length up to lOum. Selected area electron diffraction (SAED) and high resolution TEM (HRTEM) studies indicated the single-crystalline nature of CdS nanowires with an oriented growth along the c-axis direction. The optical absorption spectra and photoluminescence spectra indicated that CdS nanowires exhibited significantly quantum size effect and enhanced luminescence. Based on the results of contrastive experiments, it is found that the sulfur source and the solvent play significant roles in the formation of uniform nanowires. A possible formation mechanism of nanowires is discussed.5. Large-scale hierarchical CdS dendrites were synthesized by hydrothermal treatment of CdCl2 and thiourea, using PEG as a capping agent. The individual CdS dendrite is composed of a long central trunk with secondary and tertiary branches, which preferentially grew in a parallel direction with a definite angle to the trunk. The results reveal that the dendrites are single crystalline hexagonal CdS in nature. The contrast experiments demonstrate that both thiourea and the additive PEG are necessary for building hierarchically self-assembled CdS dendritic in water system. Moreover, the possible growth process has been discussed based on the analyses of detailed configuration and the effects of the surfactants in the system.