Core/shell Semiconductor Nanocrystals Based On CdSe: Structure And Photoluminescence

Semiconductor nanocrystals (NCs) are usually referred as quantum dots because of their distinct exhibition of the famous small scale effect in nanomaterials, that the color of their luminescence changes with the particles size. As a whole new type of inorganic emitter, semiconductor nanocrystals have been considered highly potential in applications of biology, photovoltaic device, and displays, etc.In this thesis, core/shell NCs were studied from the perspective of crystal structure. For more than a decade, core/shell NCs based on wurtzite CdSe have been the focus of quantum dots research and are widely used in biomedical applications; while studies on core/shell NCs based on the newly developed zinc-blende CdSe NCs are relatively rare. In this thesis, zinc-blende CdSe is found to be more suitable for preparation of core/shell NCs. NCs with a zinc-blende core have higher luminescence efficiency and are easier to prepare, which can be more advantageous in industrial production and in applications such as fluorescent marker.Also, as a representative core/shell NCs, CdSe/ZnS has been the most famous one due to its outrageous light-emitting properties and stabilities, but reports on its shape variation are still rare. Meanwhile, for many other types of hetero-structured nanocrystals, they have been manufactured into nearly all kinds of shapes, and these variants are thought to be highly promising in future applications. In this thesis, the conventional "dot" shaped CdSe/ZnS NCs are developed into different shapes. Typical tetrapod CdSe/ZnS NCs are obtained at 240℃, and yet it is found that higher temperature can be destructive to the tetrapod structure due to interface diffusion between CdSe and ZnS, which also leads to obvious blue shift in the luminescence. Also, the growth of ZnS in CdSe/ZnS tetrapod is found to be not straight forward; ZnS first elongates and then splits because of the accumulation of crystal defects. The luminescence efficiency also declines when the splitting starts.Finally, CdZnSeS NCs were studied. They showed rare blue to green luminescence which is hard for CdSe to reach, and besides, the preparation process was quite simple. In the experiment,12-dodecanethiol was adopted as a precursor of sulfur because it decomposes at high temperature. Meanwhile, research also found it was potent in controlling the particle size of final NCs. With this effect of 12-dodecanethiol and together with the adjustment of Cd to Zn ratio, the luminescence of obtained NCs can easily reach the wavelength range of blue to green, and moreover, the luminescence color can be tuned by the reaction design. These blue-green-emitting NCs can be useful in many applications such as displays and LEDs.