Abstract
For CdSe nanocrystals, the widening of band gap appears and the fluorescence emission wavelength is red shifted as the particle size increases. The study of CdSe nanocrystals is a very active field of research because of the wide range of emission wavelength and the high quantum yield. The quantum yield can be improved by the formation of a core/shell structure. In this study, CdSe and core/shell CdSe/ZnSe nanocrystals were synthesized in a lipophilic phase at high temperature. The structure and optical properties of CdSe and core/shell CdSe/ZnSe were studied. The mean particle sizes and crystal structures of CdSe and CdSe/ZnSe nanocrystals were determined by XRD, and the morphology and component were analyzed by the TEM micrographs and diffraction patterns. The information of surfactants attached to the surface of CdSe nanocrystals was revealed by NMR. The absorption band gap was examined by UV-visible absorption spectroscopy. The identification of core/shell structure was made by XRD and TEM. The peak shift in the X-ray diffraction patterns of CdSe/ZnSe increases with increasing the ratio of ZnSe to CdSe. TEM diffraction patterns demonstrated the coexistence of CdSe and ZnSe in a small area. The core/shell structure of CdSe/ZnSe was examined by etching with TPPO, and a ZnSe shell on the surface of CdSe nanocrystal core was evidenced by the etching. Two parts of fluorescence experiment were conducted. In the first part, the fluorescence emission wavelength and quantum yield of CdSe nanocrystals with different sizes were studied. In the second part, the optical properties of core/shell CdSe/ZnSe were studied. Formation of a core/shell structure improved the quantum yield of the nanocrystals, evidenced by the higher quantum yields of CdSe/ZnSe. A more amount of ZnSe resulted in a higher quantum yield. A slight peak shift in the fluorescence emission spectra was observed as the length of reaction time was increased, implying that the structure of CdSe/ZnSe was not a perfect core/shell structure.