| Fluorescent properties of quantum dots depend on their excitation states, which are in turn closely related to the structure and synthesis of the quantum dots. To our knowledge, synthetic chemistry of quantum dots has been mostly focused on control of size, shape, and size/shape distribution. Such mainstream synthetic efforts are evidently targeting monodisperse size and shape of the quantum dots. Monodispersity of optical properties, especially fluorescent properties,of quantum dots is barely a topic for its synthetic chemistry. This thesis aimed to explore the relationship between synthetic chemistry and optical monodipsersity of quantum dots.Our main focus was the most developed quantum dots system, i.e., CdSe/CdS core/shell quantum dots. By systematically exploring several key parameters, such as the concentration of core quantum dots, the type and composition of solvents and ligands, composition of precursors, a series of phase-pure zinc-blende CdSe/CdS core/shell quantum dots with different shell thicknesses up to16monolayers of CdS shell were synthesized. Furthermore, the crystal structure, shell thickness, and optical properties of our products were highly reproduceable. Experimental results revealed that though CdSe/CdS core/shell quantum dots synthesized using the method described here were of good uniformity in size and morphology for the entire series, their optical properties are not necessary monodisperse for all samples with different shell thiekness. Products with CdS shell thicknesses from2to10monolayers reached optical monodispersity, with photoluminescecne (PL) quantum efficiency nearly unity, PL peak width close to the intrinsic values, PL decay being single channel, and nearly non-blinking with constnat brightness at signle dot level. Experiment results further indicated that CdSe/CdS core/shell quantum dots with medium shell thickness were in favor of technical applications. For example, though the PL quantum efficiency of CdSe/CdS core/shell quantum dots with ten and four monolayers of CdS shell thickness were both near unity, efficiency of the LED device using CdSe/CdS core/shell quantum dots with ten monolayers of CdS shell thickness was about two times of that using CdSe/CdS core/shell quantum dots with four monolayers of CdS shell. In addition to optical measurements of ensemble samples, the CdSe/CdS core/shell quantum dots were studied using single molecular spectroscopy techniques, aiming to confirm their optical monodispersity at a single dot level. Experimental results revealed that PL peak postion, PL peak width, and PL decay dynamics of single dots were found to be consistent with their ensemble properties within single-exciton regime, which proved again the good optical monodispersity of these core/shell quantum dots synthesized by our method. CdSe/CdS core/shell quantum dots with shell thickness from4to16monolayers were all nonblinking, and the nonblinking volume threshold was100nm3which was far below that of wurtzite CdSe/CdS core/shell quantum dots repored in literature. This series of single quantum dots also showed a unique trend on the probability statistics of ’on’ and ’off’ states.Apart from the research on CdSe/CdS core/shell quantum dots system, we also investigated complex core/multi-shell quantum dots systems with dual energy minima in their first exciton state, such as CdSe/CdS/CdSe/CdS and CdS/ZnS/CdS/ZnS complex quantum dots. By comparing the photoluminescence decay traces of CdSe/CdS/CdSe complex quantum dots with that of CdSe/CdS/CdSe/CdS ones, we confirmed that a new decay channel with significantly longer lifetime appeared upon epitaxial growth of CdSe onto CdSe/CdS core/shell quantum dots. Furthermore, experimental results revealed that complex quantum dot systems with dual energy minima in their lowest exciton state substantially differed from each other because of the difference on the energy level alignments between semiconductors in a given quantum dot. |
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