(80b) Observation of the Nucleation and Growth of Cds Nanocrystals in a Two-Phase System

Semiconductor quantum dots have attracted widespread attention because of their size-dependent optical and electronic properties arising from the quantum-size effect different to bulk size crystals, and their band-gap can be tuned by adjusting the nanocrystal size. The size, the band-gap and the extinction coefficient of semiconductor quantum dots are correlative. Peng and colleagues have recently carried out a systematic investigation of CdTe, CdSe, CdS quantum dots between the size, the first excitonic absorption peak and molar extinction coefficient. The size and molar concentration of quantum dots can be obtained by calculating with the corresponding first excitonic absorption peak. So UV/vis absorption measurements provide a simple and useful method for monitoring the processes of the nucleation and growth of semiconductor quantum dots. A lot of work was devoted to the preparation of monodisperse nanoparticles. Synthesis of semiconductor nanocrystals has been very successful. On the other hand, there is a lack of experimental observation of the nucleation and growth kinetics of nanoparticles. The crystallization processes have not been well understood from molecular monomers to nano-size crystal. Only little work was reported on this problem. Peng and Mulvaney recently reported in-situ and ex-situ observation of the nucleation and growth of CdSe nanocrystals in a high boiling point solvent. These observations were performed at high temperatures, and the nucleation and growth take place rapidly. Moreover, in a single-phase synthesis, the fast nucleation and the slow growth (i.e. the nucleation at high temperatures and the growth at low temperatures) are overemphasized for preparing narrow size distribution nanocrystals. So it is difficult to observe the process of nucleation especially for those observations ex-situ. Some literatureds reported on how to prepare monodisperse nano-sized colloidal solution. The separation between the nucleation and growth is always emphasized. However, we have recently synthesized CdS, CdSe, TiO2, ZrO2, SnO2, Fe2O3, Mn3O4, Cr2O3 and CuO etc. nanocrystals by a two-phase approach without a clear separation between the nucleation and growth. In these cases, the nanocrystals synthesized by a two-phase approach do not exhibit a wide size distribution. So it is worth to study deeply the processes of the nucleation and growth of these nanocrytals. Moreover, the two-phase approach allows us to monitor the kinetics of the nucleation and growth due to the very slow nucleation and growth rates. In a word, we have observed the nucleation and growth kinetics of CdS nanocrystals in a two-phase system. The nucleation can occur throughout the whole synthesis process due to a very slow reaction rate. But this does not lead a poly-disperse nanocrystals. Ostwald ripening growth plays an important role in focusing of the particle size distribution. It is found that a two-phase approach could also been applied to synthesis narrow size distributions nanocrystals. The nucleation and growth kinetics of CdS nanocrystals are influenced strongly by the monomers, capping agent concentrations and non-coordinating solvent polarity. A high monomer concentration, a low capping agent concentration and low solvent polarity could lead to a higher maximum concentration of critical nuclei and a higher final CdS concentration.