Synthesis And Photoelectronic Application Of High Quality Photoluminescent Quantum Dots

In the past two decades, quantum dots (QDs) have been widely studied and applied in many fields such as light-emitting diodes (LEDs), solar cells and bio-sensor, asanalternativetoorganic dyes, because of their outstanding properties of narrow and symmetric emission peak, high stability, high photoluminescence quantum yield, and so on. Traditionally, the photoelectronic properties of QDs is tuned via the variation of the particle size, while the size-tuning route is limited in many fields. To solve this problem, we established the "structure-tuning" and "composition-tuning" method to tune the bandgap of QDs. On the other hand, with more wide application of quantum dots in health and energy field, to develop massive synthesis of high quality nano-materials isa hot issue. So far hot-injection synthetic method is commonlyused in synthesizingmostnano-materials, which cannot satisfy the requirement of scalable and reproducible production in industrial application. Moreover, high cost and strict experiment conditions also impede the industrial application of QDs. However, the one-pot non-injection synthesis, as we described, has achieved the goal of industrial application. This dissertation contains the following content:(1) CdTe/CdSe/ZnS core/shell/shell nanostructureTypically, CdO,1-tetradecylphosphonic acid, and1-octadecene were loaded in a three-neck flask clamped in a heating mantle, which was raised to290℃under an argon. At this temperature, Te precursor solution was quickly injected into the reaction flask and kept at this temperature for30min to the grow the CdTe core. Then purified CdTe core,1-tetradecylphosphonic acid, trioctylphosphine and1-octadecene were loaded in a three-neck flask clamped in a heating mantle, which was raised to150℃under an argon, an equimolar amount of the Cd precursor stock solutions, obtained by dissolving Cd(OAc)2in trioctylphosphine and1-octadecene at80℃, and Se precursor stock solutions, obtained by dissolving Se powder in trioctylphosphine and1-octadecene using sonication, was added alternately via a syringe at a30min interval for the growth of CdTe/CdSe core/shell nanocrystals (NCs). the addition of the Cd/Se precursors was stopped and the reaction temperature was lowered down to135℃for the following overgrowth of the ZnS shell. When the temperature of the reaction system stabilized at135℃, a certain amount of zinc diethyldithiocarbamate stock solution, obtained by dissolving zinc diethyldithiocarbamate in trioctylphosphine and1-octadecene (v/v,1:1) at room temperature by sonication, was added and kept at this temperature for30min, and then the temperature was raised to200℃and maintained for another30min to get CdTe/CdSe/ZnS Core/Shell/Shell QDs. The CdTe/CdSe/ZnS QDs possess photoluminescence quantum yields (PL QY) as high as94%and the emission wavelength of the obtained nanostructure can span from540to825nm. In the experiment, an effective shell-coating route was developed for the preparation of CdTe/CdSe core/shell nanostructures by selecting capping reagents with a strong coordinating capacity which is one of the factors of success. In addition, adopting a low temperature for shell deposition is also the key point. The obtained high quality of fluorescence through the kind of new structure depending on the band gap adjustment methods can still be kept when transferred into water phase. The high PL stability of the obtained CdTe/CdSe/ZnS QDs is mainly derived from the passivation effect of the outer ZnS layer with a substantially high bandgap, which effectively confines the excitons within the CdTe/CdSe interface and isolates them from the solution environment.(2) ZnCuInS/ZnS (ZCIS/ZnS) alloyed nanocrystalsThe high quality fluorescence QDs have been obtained through the "structure-tuning" method to tune the bandgap of QDs. In a typical procedure, the acetate salts of the corresponding metals, stearic acid, dodecanethiol, and octadecene were loaded in a50-mL three-neck flask clamped in a heating mantle. The mixture was heated to230℃under argon flow. Then S precursor solution, obtained by dissolving sulfur in octadecene at120℃, was injected into the reaction system and kept at this temperature for30min to allow growth of ZCIS NCs. The reaction temperature was raised to240℃for the following overgrowth of the ZnS shell. Zn stock solution (zinc acetate dissolved in oleylamine and1-octadecene at160℃) was injected into the reaction mixture in5batches with a time interval of15min to obtained ZCIS/ZnS NCs. The plain ZCIS NCs did show PL emission but with quite low PL QY (typically below3%). With the deposition of ZnS shell around the ZCIS core NCs, the PL QY increased substantially with a maximum value of56%and emission wavelength tunable from518to810nm covering most part of the visible light spectrum and near infrared spectrum. The various experimental variables, including the Zn/CuIn ratio, amount of sulfur and dodecanethiol, and reaction temperature, have a significant effect on the bandgap of the obtained alloyed NCs. The high PL emission efficiency of the ZCIS/ZnS NCs can also be preserved after phase transfer via ligand replacement. Besides the excellent optical properties, the obtained ZCIS/ZnS NCs also exhibit promising photocatalytical activity in the degradation of rhodamine B. (3) Mn:ZnS QDsOur synthetic method based on "nucleation-doping" strategy. Typically, manganese stearate, dodecanethiol, and octadecene were loaded into a three-neck flask. Then the reaction system was filled with N2, and the temperature was further raised to250℃. At this temperature, S precursor solution, obtained by dissolving sulfur powder in octadecene at120℃, was injected into the reaction system and kept at this temperature for1min to allow growth of stable and small size of MnS nanoclusters (2nm). One half Zn stock solution (zinc acetate dissolved in oleylamine and1-octadecene at160℃) was injected to the solution and kept at250℃for20min, then the temperature was set to230℃, the other half Zn stock solution was injected to the solution and kept at230℃for20min to obtained Mn:ZnS QDs. The optical properties and structure of the obtained Mn:ZnS QDs have been characterized by UV-vis, PL spectroscopy, transmission electron microscopy, and X-ray diffraction. The resulting nearly monodisperse d-dots were found to be of spherical shape with a zinc-blende crystal structure. The influences of various experimental variables, including the reaction temperature for the MnS core nanocluster and ZnS host material, the amount of S precursor solution, dodecanethiol, as well as Zn/Mn ratio have been systematically investigated. The use of dodecanethiol as capping ligand ensured the reproducible access to a stable small-sized MnS core. This paves the way for reproducibly obtaining highly luminescent doped QDs. Programmed overcoating temperature for growth of ZnS shell was employed to realize balanced diffusion of the Mn ions in the Mn:ZnS QDs.(4) Cu:ZnxCd1-xS、 Cu:Zn-In-S and Cu, Mn:Zn-In-S QDsOur synthetic method based on "single-step noninjection" synthetic approach. In a typical procedure, the acetate salts of the acetate salts of the corresponding metals, S powder, dodecanethiol, oleylamine and octadecene were loaded in a three-neck flask clamped in a heating mantle. Then the reaction system was filled with N2, and the temperature was further raised to250℃, and kept at this temperature for30min to obtained Cu:ZnxCd1-xS, Cu:Zn-In-S and Cu, Mn:Zn-In-S QDs respectively, the resulting doped QDs show composition-tunable PL emission over the entire visible spectral window and extending to the near-infrared spectral window (from450to810nm), the average dopant emission show50-80%PL QY. In addition, the doped emission can be convenient tuned by changing the ratios of host material elements, Mn or Cu ion concentration and reaction temperature. Importantly, the initial high PL QY of the obtained doped QDs in organic media can be preserved when transferred into aqueous media via ligand exchange. Furthermore, electroluminescent devices with good performance (with a maximum luminance of220cd m-, low turn-on voltages of3.6V) have been fabricated with the use of these Cd-free low toxicity Cu:Zn-In-S/ZnS QDs as an active layer in these QD-based LEDs. we explored the possibility of using Cu, Mn:Zn-In-S/ZnS QDs as colour converting materials for white light-emitting applications. The devices exhibit high colour rendering index of91, luminous efficiency of51lm/W. Overall, these materials have promising potential as less toxic NCs for applications in LEDs and biolabeling.(5) Gram-scaled synthesis of core-shell structure of QDs, CdSe multipod NCs and shape-tunable CdS NCsThe three kinds of semiconductor NCs have been obtained respectively through the "single-step noninjection" synthetic approach. The first is high quality core/shell QDs (CdS/ZnxCd1-xS, CdSe/ZnxCd1-xS, and CdTe/ZnxCd1-xS) with shell material composed of gradient alloy structure. In a typical procedure, CdO, Zn(NO3)2, chalcogenide elements, trioctylphosphine, stearic acid and octadecene were loaded in a three-neck flask clamped in a heating mantle at air. the temperature was further raised to250℃, and kept at this temperature for30min to obtained core/shell QDs (CdS/ZnxCd1-xS, CdSe/ZnxCd1-xS, and CdTe/ZnxCd1-xS). With simple variation of reaction recipe (reactants and feeding ratio), luminescence color of the resulting QDs can be conveniently tuned from violet to near-infrared (400-820nm). The emission efficiency of the as-prepared QDs can be up to80%. Moreover, the high emission efficiency can be preserved after QDs transferred into aqueous media via ligand exchange. Gram-scaled green, yellow, and red emissive core/shell QDs can be obtained in one bath reaction. And second is the CdSe multipod NCs. In a typical procedure, CdO, Se powder, trioctylphosphine, oleic acid and paraffin were loaded in a three-neck flask clamped in a heating mantle. Then the reaction system was filled with N2, the temperature was further raised to250℃, and kept at this temperature for10min to obtained CdSe multipod NCs. The influence of various experimental variables, including reaction temperature, nature and amount of surfactants, Cd-to-Se ratio, and the nature of reactants, on the morphology of the obtained CdSe NCs have been systematically investigated. After deposition of ZnS shell around the CdSe multipod NCs, the PL QY of the obtained CdSe/ZnS can be up to85%. The third is the CdS NCs with a wide variety of shapes including spheres, tetrahedrons, and branched and flower-like structures. In a typical procedure, CdO, S powder, trioctylphosphine, oleic acid and paraffin were loaded in a three-neck flask clamped in a heating mantle. Then the reaction system was filled with N2, the temperature was further raised to230℃, and kept at this temperature for30min to obtained the shape-controlled CdS NCs. The shape-controlled growth mechanism could be explained by the nuclei structure and monomer concentration. All the CdS nanocrystal samples with different morphologies exhibit good photocatalytic activity for degradation of dyes. The observed lower photocatalytic activity of the sphere-shaped CdS NCs could be ascribed to the higher PL QY relative to those with other morphologies, which results in low electron-hole separation efficiency. Overall, our reported preparation approach can satisfy the requirements of industrial production bearing the advantage of low-cost, reproducibility and scalability.