Study On The Semiconductor Quantum Dots:Magic-Sized, Multi-Mode Emission And Different Structure Quantum Dots

Colloidal QDs have attracted great interest for their potential use in biological imaging, photovoltaics, lighting, lasers, photocatalysts, new generation electronics, and thermoelectrics. Among the semiconductor materials studied, CdSe has emerged as a model material with numerous 1st such as 1st colloidal QDs synthesised with size distributions less than 10%,1st colloidal QDs covalently coupled to biomolecules for use in ultrasensitive biological detection, and 1st detailed comparison of experimental spectra with theoretical predictions. Ternary alloy QDs have more complicated components and structures which allow independent tuning of photoelectrical properties without changing particle size facilitating application. In this thesis, researches mainly focus on alloy and core-shell structured QDs based on CdSe QDs, with the aim to explore new strategies for synthesizing of series alloy and core-shell structured QDs with high PL QY. Via a noninjection route, high QY QDs and multimodal emitting QDs were sythesised. Via a high-temperature thermolysis method high quality quantum dots with various morphology and structure were synthesized. Growth mechanism and PL theory were intensively studied. New methods were explored for phase transfer of QDs from organic to aqueous phase. Then we carry out tentative exploration for using of QD fluorescent probe in bio-labeling.In chapter 1, we first briefly introduced the properties, structure and luminous mechanism of semiconductor QDs and devoted a lot of paragraphs to the synthetic stragies. Then we reviewed the latest progress in theretical and experimental progress in QD study. Finally, the significance, research ideas and the outline of this thesis were summarized.In chapter 2, we discussed the synthesis of CdSe QDs via a noninjection approach using ODE as the solvent, Se powder and Cd(SA)2 as the raw materials. Under optimized conditions, magic-sized CdSe QDs were observed, which showed an uncommon continuous growth at low temperatures. The growth kinetics of the CdSe QDs were studied using UV-vis absorption and photoluminescence spectroscopy. The results indicated that the dissolution of Se powder limited the starting point and the rate of nanocrystal nucleation. Comparative studies validated that the growth of QDs in the noninjection synthesis approach showed distinctly different kinetic features from those synthesized by the traditional hot-injection route. In addition, the changes of the Cd/Se feed molar ratio primarily influence the nucleation moment of the QDs, thus they control the particle size. The as prepared CdSe QDs were of high quality with a QY of 34%. After passivating with ZnS, the QY increased to 63%. Moreover, with an additional ZnSe transition layer between the ZnS shell and CdSe core, the QY reached as high as 74%, which was comparable with the QY of QDs obtained by the hot-injection route. Via in-situ injection of S precursor and further ZnS encapsulation multimodal emitting CdSe/CdS/ZnS QDs were synthesized.In chapter 3, wide emission-tunable and different morphological alloyed CdTeSe QDs, CdTeSe/ZnS and CdTeSe/ZnSe/ZnS core-shell QDs were successfully synthesized via an injection controlled process. The effect of injection procedure and reaction temperature were systematically discussed and the growth mechanism was proposed. Most efficient PL wavelength was correlated with reaction time and temperature. The 3-D PL spectra of spherical bare CdTeSe and core-shell QDs with different passivation showed different excitation wavelength dependency. The PL intensity of CdTeSe/ZnSe/ZnS core-shell QDs increased greatly in comparison with that of CdTeSe and CdTeSe/ZnSe QDs. ZnSe transition layer played an important role in improving the PL intensity by providing a smoothened interface and gradient band offsets. The core—shell QDs were transferred into aqueous phase and successfully conjugated with E coli 0-157 and YAC-1. The proposed phase-transfer and bio-labeling strategy may be applicable to various QDs with different compositions.In chapter 4, high QY yellow-red emitting alloy and core-shell QDs based on CdSe were synthesized in industrial solvent N-235 via a high-temperature thermolysis method. The most efficient QDs have a QY as high as 94%. System study was carried out to research effects of QD structure and shell species on QD fluorescence properties. Magic-sized QDs were synthesized via changing surfactant concentration and reaction temperature. Then core-shell QDs were trasfered to aqueous phase by SiO2 encapsulation. The resulting QDs can be well dispersed in water, pH 6.8 PBS buffer, pH 7.4 Tris buffer, pH 9.4 borate buffer, methanol, and ethanol. The QDs show long-term stability when dispersed in alcohols.In chapter 5, we summarized our work, and discussed the problems remained to be solved. Finally, we made a plan and looked forward to the future work.In summary, high QY green emitting QDs were synthesized via a non-injection approach, yellow-red emitting QDs were synthesized via a high-temperature thermolysis routes. The most efficient QY can be as high as 94%. By modulating of heating rates, precursor concentration, reacation temperature and surfactant dosage, magic sized QDs can be acquired either by low temperature noninjection approach or high-temperature thermolysis routes. Through adjusting reaction temperature and silane concentration monodispersed SiO2 QDs can be prepared. The study has profound sinifacance to deepen our understanding of dynamica process in QDs growing and enlarge the application domain of QDs.