Structure And Properties Of CdS Nanocrystals Synthesized By Direct Thermolysis Of Molecular Clusters

CdS quantum dots (QDs) with controllable size have been synthesized by direct thermolysing a single-source precursor, (Me4N)4[S4Cd10(SPh)16]. The effect of reaction conditions on the CdS QDs growth process is explored, and the reaction mechanism is clarified. CdS nanorods (NRs) with quantum confinement effect are synthesized by using the same molecular cluster precursor in monosurfactant system. The influence of reaction conditions on the CdS nanocrystals morphology is studied, and the formation mechanism of CdS NRs is explained. Based on the method of precursor thermolysis, the Co-doped CdS QDs and NRs are produced by using two clusters, (Me4N)2[Co4(SC6H5)10] and (Me4N)4[S4Cd10(SPh)16], in virtue of the ions exchange of the molecular clusters. The effect of Co-doping on the structure, properties and growth process of CdS nanocrystals is explored.The research shows that the as-prepared CdS QDs are uniform and hexagonal structure with high crystallinity, exhibiting obvious quantum size effect and strong photoluminescence emission. In the growth process of CdS QDs, the diameter is increased with elevating reaction temperature. The increase of QDs size with temperature can be divided into two stages: the QDs size increases slightly under low temperature, demonstrating that nucleation is dominant at this stage. When the temperature is elevated above 190℃, the QDs size increases rapidly, which is indicative of a rapid growth stage for CdS QDs. Inaddition, the CdS QDs diameter is increased and the size distribution is narrowed with prolonging reaction time. The nucleation and growth are independent stages in the formation process of CdS QDs, and the two stages can be automatically separated in monosurfactant system. The method of direct thermolysing a single-source precursor is simple and controllable, and can produce CdS QDs in large-scale (>10g/L).The experimental results show that the as-produced CdS NRs are uniform and mono-dispersed with an aspect ratio of 9:1. The NRs are hexagonal structure and elongated along the c axis. Obvious quantum confinement effect and large Stokes shift are observed from the CdS NRs. The morphology of the CdS nanocrystals is influenced by reaction conditions. Precursor concentration significantly influences the morphology of the CdS nanocrystals, such as the products are QDs under low concentration and straight NRs under high concentration, further branched NRs under higher concentration. In order to obtain uniform and straight CdS NRs, the precursor concentration should be more twice than that of nanoparticles produced under the experimental conditions. Increasing reaction temperature and prolonging reaction time are favorable for the formation of CdS NRs. The uniform CdS NRs with large aspect ratio can be obtained at high temperature (>190℃) and long time (more than 5 hours). The formation of CdS NRs is oriented attachment mechanism, and the driving force for the mechanism is dipole interactions. The final morphology of CdS nanocrystals is determined by the relation between dipole interaction and steric repulsion.The Co-doped CdS QDs (CdS:Co QDs) are nearly sphere particles with uniform morphology, narrow size distribution and hexagonal structure. The CdS:Co QDs are ferromagnetism at room temperature because of the Co doping in the CdS lattice. The CdS:Co QDs exhibit obvious quantum confinement effect and photoluminescence emission which is slightly red-shift compared with CdS QDs. The Co-doped CdS NRs are successfully synthesized by increasing the molecular concentration based on the similar method. The reseach shows that the dopant of Co in the CdS is favorable for the formation of the NRs with shortening the reaction time.