Control The Trap States Of Quantum Dots With Complex Structure

The field of quantum dots is moving from simple systems, such as plain core with intrinsic crystal and electronic structures, to complex systems. At present, it is clear that for most real life applications complex quantum dots are necessary. Control of electronic traps of complex quantum dots would be magnitudes more challenging but it is absolutely necessary for complex quantum dots as emissive and optoelectronic materials. As fluorescence emitters based on their band-edge emission, eliminating entire and surface traps would be essential. For developing phosphorescence emitters, some impurities such as transition metal ions should be introduced intentionally in a controllable manner.We firstly developed a new selenium precursor, namely a suspension of selenium powder in octadecene (Se-SUS). Se-SUS showed extremely high reactivity, necessary flexibility, and high conversion yield. After clarifying the reaction mechanism, Se-SUS was proven applicable for synthesis of almost all kinds of metal selenide quantum dots. This new type of precursor solved a series of longstanding challenges in synthetic chemistry of quantum dots, such as size control in a desirable size range, avoiding alloy during the synthesis of core/shell quantum dots, and co-nucleation doping for doped quantum dots.CdSe/CdS core/shell quantum dots were applied as model systems for studying electronic traps of fluorescence emitters based on complex quantum dots. The electron traps-presumably excess or unpassivated Cd surface sites-are identified as shallow ones and could be readily isolated from the electron wavefunctions of the excitons if-2 monolayers of CdS shell was in place. Two types of deep hole traps were identified within the bandgap of the QDs. The very deep one was likely associated with the surface adsorbed H2S and could be removed by either degassing processes or photochemical decomposition without damaging the QDs. The other type of hole traps seemed to be unpassivated surface S sites, which could be removed by surface treatment with cadmium carboxylates. Understanding of surface traps enabled establishment of new synthetic schemes for CdSe/CdS core/shell QDs with nearly unique photoluminescence quantum yield and mono-exponential photoluminescence decay dynamics with 1-10 monolayers of CdS shell. These new phosphine-free schemes worked for either single-precursor or successive-ion- layer-adsorption-and-reaction approach. These results further confirmed that, for common core/shell systems, there is unlikely internal electron and hole traps as suggested in literature.Mn ions deoped ZnSe (Mn:ZnSe) quantum dos were studied for developing phosphorescence emitters based on quantum dots through intentional introduction of internal emissive traps. Key parameters of the synthesis were studied systematically, including the chain length of metal carboxylate, free fatty acids, amount of elemental selenium in Se-SUS, reaction temperature, reaction temperature, etc. This results in a successful new doping strategy-co-nucleation doping-for obtaining Mn:ZnSe quantum dots with an extremely large concentration range of Mn ions (1% to 40%). These new phosphorescence emitters based on quantum dots were found to possess outstanding phosphorescence properties, with single-exponential yet tunable phosphorescence decay lifetime from 1000 to 50 us.To further promote the phosphorescence emitters based on doped quantum dots to real life applications, a new epitaxy strategy was developed for growth of ZnS shell onto the Mn:ZnSe quantum dots with decent thickness by anion activation. Under mild reaction temperatures (-140 ?), highly emissive and stable Mn:ZnSe/ZnS core/shell quantum dots was synthesized with near single-exponential phosphorescence decay. These outstanding characteristics made Mn:ZnSe/ZnS core/shell desirable emitting materials for photoluminescence lifetime multiplexing.