Preparation And Surface Modification Of Ⅱ-Ⅵ Semiconductor Quantum Dots

In the past two decades, the II-VI family semiconductor quantum dots as an important class of materials with homogeneous or core-shell structure (such as CdSe, CdS, CdTe, CdSe/ CdS, etc.), have been widely applied in the field of nano-materials science and technology. Particularly in the functional nano-devices,Ⅱ-Ⅵsemiconductor quantum dots play an important role. Compared with organic dye molecules,Ⅱ-Ⅵsemiconductor quantum dots have more advantages, outstanding performance, such as adjusting its optical properties and electrical properties by only changing its particle size, and with higher color purity of emission light. Owing to the quantum confinement effect, we can use the same excitation wavelength to excite a variety of semiconductor quantum dots (QDs), and get a variety of emission light. These emission lights can not cross each other and they can be clearly distinguished.Recently, the researchers have done a lot of widely and deeply work in II-VI semiconductor quantum dots, especially in synthetic methods of high-quality QDs. However, the traditional synthetic conditions for high-qualityⅡ-Ⅵsemiconductor are usually harshly and its route is not friendly to the environment. Thus, the researchers have been trying to improve the current synthesis methods and to explore moderate and green approaches to high-quality semiconductor QDs. In addition, cadmium, mercury, lead-related of high-quality II-VI semiconductor quantum dots contain highly toxic heavy metal elements in their own components, and its thermal stability and chemical stability is sensitive to the external environment. These factors severely limit its further applications. Therefore, many works focus on how to reduce the drawback of semiconductor QDs. Our aim of this study concentrates on solving these two problems. The main results are described in the following.Firstly, through the two-phase method, UV-visible absorption spectra are applied to monitor the kinetic process of nucleation and growth of CdSe quantum dots in the two-phase system. We found that nucleation and growth kinetics include three stages:1) Nucleation stage:it is very sensitive to the experimental conditions and it isn't easy to detect. By adjusting the reaction conditions, we observed the critical nucleus successfully at this stage in UV-visible absorption spectra at 331 nm, whose position doesn't move with the reaction time.2) The transition stage from nucleation to growth:the UV-visible absorption peak locates between 360-370 nm and exhibits a red-shift with reaction time. This stage is very short and usually is difficult to be observed clearly.3) Growth stage:this stage is very long. We can clearly observe each slight change in the whole growth stage. For example, as the CdSe quantum dots grow gradually, the luminescence from defects is significantly reduced meanwhile the photoluminescence quantum yield has been remarkably improved. Further growth leads to Ostwald ripening stage, the full width at half-maximum (FWHM) becomes widely and the size distribution begins to broaden. Definitely, the reaction temperature, surface ligand concentration, precursor concentration, solvent polarity and the ratio between the precursors etc influence the nucleation and growth kinetics. The above results are worthful to the synthesis of high-quality CdSe quantum dots and it also can guide on the synthesis of other quantum dots.Secondly, in the two-phase system at 40℃-70℃under stirring, a series of high-quality CdSe/CdS core-shell structure QDs with higher quantum yield and controllable emission are available. N-heptane is exploited to replace toluene, and low toxic CdMA as cadmium precursors and OA as surfactant are applied in the system. This method is relatively green and the conditions are mild. In addition, we studied the photoluminescence quantum yield (QY) and fluorescence lifetime of as-prepared CdSe/CdS core-shell structured quantum-dot with changing both the core size of CdSe and shell thickness of CdS. Fixing the thickness of CdS shell and increasing the core size of CdSe nanocrystals makes the QY of CdSe/CdS quantum dots lower and fluorescence lifetime of them shorter gradually. Fixing the core size of CdSe and increasing the shell thickness of CdS leads to the QY increased at first and then reduced, and fluorescence lifetime shorter gradually.Thirdly, we have successfully prepared fluorescent CdSe/CdS/SiO2 composite nanospheres. Through TEOS hydrolysis under basic conditions, the SiO2 layer is effectively coated on the outer shell of oil-soluble CdSe/CdS core-shell quantum dots. By adjusting the emission of quantum dots and reaction conditions, a series of of fluorescent CdSe/CdS/SiO2 composite nanospheres with different colors are available, such as green, orange, and yellow. Definitely, quantum dots and ammonia amount, reaction temperature and time, solvents ratio all influence the size of nanospheres and their QY. Under an optimal condition, the QY of CdSe/CdS/SiO2 composite nanospheres can reach 8% and the size is 33.0±2.0 nm. We found that SiO2 layer can effectively reduce the fluorescence decay of quantum dots. Thus, the fluorescence lifetime of CdSe/CdS/SiO2 nanospheres is much longer than the corresponding CdSe/CdS quantum dots.Fourthly, the fluorescent CdTe/SiO2 composite nanospheres are synthesized. Through TEOS hydrolysis under basic conditions, the SiO2 layer is effectively coated on the outer shell of water-soluble CdTe quantum dots. Definitely, the quantum dots and ammonia amount, reaction temperature and time all influence the resulting nanosphere size and fluorescent intensity. The CdTe/SiO2 composite nanospheres with tunable size can be obtained through adjusting the reaction conditions.