Green Syntheses And Characteristics Of Semiconductor Nanocrystals CdSe And ZnSe

Colloidal semiconductor nanocrystals (quantum dots) show quantum confinement effects due to the size of them on the order of the Bohr radius and hence have attracted broad attention in recent years for usage in a variety of applications such as solar cell, lasers, biological labels and light-emitting diodes. Among the colloidal semiconductor nanocrystals, II-VI CdSe has shown almost full range visible light emission within a reasonable size range and attracted a great deal of interest. It is important to develop safe, low-cost, green, fast, and mass-production routes for the synthesis of CdSe NCs with high quality. Here, we present a much cheaper and greener non- tri-n-octylphosphine (TOP)-based route for the syntheses of CdSe and ZnSe nanocrystals. The optical properties, structures and appearances of CdSe nanocrystals were charactered. The main contents are listed below:Based on oleic acid - paraffin liquid route, CdSe nanocrystals were prepared in open air. The CdSe nanocrystals were charactered by UV-vis absorption and photoluminescence (PL) spectroscopy, powder X-ray diffraction (P-XRD) and transmission electron microscopy (TEM). The data obtained in air were compared with those obtained under N2. The feasibility to synthesize CdSe nanocrystals in air was discussed.Using stearic acid as ligand, a new, green, non-TOP system -was proposed. CdSe nanocrystals were synthesized under N2 and in air respectively. The quality of CdSe nanocrystals prepared at different conditions and temperatures, and the effects of different testing solvents were investigated. The results show that CdSe nanocrystals are suitable to be synthesized at lower growth temperature in stearic acid - paraffin liquid system and the quality of CdSe nanocrystals prepared in air is as better as that obtained under N2. Choosing oleic acid and stearic acid as ligands, the data obtained at 230℃growth temperatures in air were compared. The results show that the CdSe nanocrystals prepared by stearic acid ligands have smaller Stokes shift, narrower full widths at half-maximum, higher photoluminescence quantum efficiency and crystallinity. The results also show CdSe nanocrystals in the less common zinc blende crystal structure in TOPO (tri-n-octylphosphine oxide)-TOP routes have been obtained in the systems with oleic acid or stearic acid ligands at lower growth temperatures (220℃-260℃).The effects of surfactant n-hexadecylamine (HDA) on the synthesis of CdSe nanocrystals and the reproducibility of the synthesis in liquid paraffin system were investigated. The results show that the nucleation rates are faster and the growth rates are slower in presence of hexadecylamine and the use of hexadecylamine generally can produce highly homogeneous CdSe NCs with higher photoluminescence quantum efficiency and smaller sizes and the system is reproducible. The study on the effects of injection ways of hexadecylamine shows that higher quality CdSe NCs can be obtained by adding HDA to Cd precursor rather than to Se precursor.A green route using dodecanoic acid to synthesize ZnSe nanocrystals who will replace toxic, biological labeling CdSe nanocrystals in liquid paraffin matrix is proposed. Highly homogeneous and size-tunable ZnSe nanocrystals were finally obtained by comparing different methods to separate ZnSe nanocrystals from the reaction mixture. Although reproducibility of experiment is not well enough, it provides a new approach to the green synthesis of ZnSe nanocrystals.