Crystal Control Of Core/Shell Nanocrystal:Synthesis And Relative Properties

During the last three decades, colloidal semiconductor nanocrystals were studied widely for their unique size-dependent optical properties and solution processibility. Because of quantum confinement effects, colloidal semiconductor nanocrystals are also known as quantum dots and their optical properties are strongly size dependent. Quantum dots possess some other interesting properties in addition to their continuously adjustable emission peak position, such as narrow FWHM (full width at half maximum) of fluorescence, high photoluminescence quantum yield (PL QY), good chemical and optical-chemical stability, etc. Because of these unique properties, quantum dots have attracted substantial attention in optoelectronics, display, lighting, biological fluorescent labeling, sensors, and others.Cadmium Selenide (CdSe) nanocrystals and their core-shell structures have been the mostly studied systems in the field of quantum dots. Bulk CdSe appear at room temperature in two distinguishable phases, namely wurtzite (hexagonal) and zinc-blende (face-centered-cubic). However, phase-controlled synthesis of CdSe nanocrystals has been a long-standing challenge in the field. In this work, in order to achieve a reduced reaction temperature, cadmium (zinc) diethyl dithiocarbamate was employed as the single-source precursor and primary amine was used as the activator. For CdSe/CdS core/shell nanocrystals system, zinc blende CdSe/CdS core/shell nanocrystals were successfully synthesized. They showed higher fluorescence quantum efficiency and narrower FWHM of luminescence than those of traditional wurtzite CdSe/CdS nanocrystals. Most importantly, the resulting phase-pure CdSe/CdS core/shell quantum dots in zinc-blende structure possessed single channel decay of fluorescent. Quantum dots with such unique properties should have great potential for studying blinking behavior and energy transfer phenomenon. The synthetic scheme is also comparatively simple, which should further promote the development of the field.For CdSe/ZnS core/shell nanocrystal system, only the thin layer (2ML,2monolayers) CdSe/ZnS core/shell nanocrystals were successfully synthesized as zinc-blende crystal structure. Likely because of the huge lattice mismatch (12%) between CdSe and ZnS materials, following ZnS layers were grown as Stranski-Krastanov mode (forming ZnS islands), instead of uniform growth. Single channel decay of fluoresce was only found in the samples with2ML of ZnS. The XRD patterns and TEM images revealed obvious differences between the2ML and3ML CdSe/ZnS core/shell nanocrystals, whichwere consistent with the Stranski-Krastanov growth mode.