On Growth Of CdS Quantum Dots:theory And Experiment

The growth mechanisms of quantum dots are important for both crystallographic theory and synthetic chemistry of high-quality quantum dots.Plenty of growth mechanisms have been proposed and can explain some experimental phenomenons.However,none of these mechanisms can integrally describe a general growth process of quantum dots.Besides,the reproducibility and time resolution of traditional synthetic and characteristic methods of quantum dots are not high enough for the corresponding research.Using the cadmium sulfide quantum dots as research model,this thesis aims to develop a universal theoretical framework of the growth process of quantum dots and its matched synthetic and characteristic methods.Our first work is measuring the extinction coefficient of zinc-blende cadmium sulfide and cadmium selenide(CdE)quantum dots.Analysis reveals that,the deviation of previous extinction coefficient of CdE quantum dots is mainly caused by the residual cadmium-containing impurities.To solve this problem,a new three-step purification scheme is developed and its purification efficiency is strictly evaluated by the quantitive self-defined indicators,namely the R value and footprint of fatty acid ligand.The results show that our new purification scheme can completely remove all possible impurities in the reaction solution of CdE quantum dots.On this basis,the new measured working curves of extinction coefficient of zinc-blende CdE quantum dots can be consistent with the basic theoretical and experimental facts.Furthermore,we also measure the extinction coefficient of CdE unit,which can be used to directly calculate the yield of CdE quantum dots and revise the deviation from the size measurement.Based on the CdS quantum dots synthetic system,an automated micro-reactor system is developed,which is coupled with liquid-phase FTIR and UV-Vis measurements.Besides,we make two small optical path cuvettes for the UV-vis and FTIR spectrum to directly measure the original reaction solution.Through the new synthetic and characteristic system,we can prepare monodispersed CdS quantum dots and get all the key information during their growth process with high reproducibility and time resolution,including the average size and size distribution,the concentration of quantum dots,the conversion rate of precursor,the concentration of CdE unit in clusters.Except for the synthetic and characteristic methods,we also establish a universal theoretical framework for the growth process of quantum dots.Based on the mass conservation of the CdE unit from nanocrystals,clusters and precursors,this theoretical model shows that the growth process of quantum dots has three basic reaction channels,including free growth by direct incorporation of the monomers converted from the precursors,growth by dissolution of a portion of the regular nanocrystals in solution,and growth by dissolution of the clusters in solution.This theoretical model also provides the calculation methods of the absolute and relative contributions of each reaction channel.Based on above mentioned works,we use CdS quantum dots as the research model to testify the practical effect of our theoretical model.The results show that,under most reaction conditions,three basic reaction channels coexists with each other and jointly influence the growth process of CdS quantum dots.The absolute and relative contributions of each reaction channel will significantly change with different reaction conditions.The theoretical model and experimental methods developed in this thesis can completely and clearly describe the growth process of CdS quantum dots,which can provide theoretical basis and design thought for the new synthetic method.