Synthesis And Property Research Of II–VI Semiconductor Quantum Dots

Colloidal semiconductor quantum dots (QDs) are important in fundamental and application research, due to their size-dependent optical properties and excellent solution processing chemistry. In this dissertation, we mainly developed"greener"and cheaper synthesis methods for II–VI semiconductor quantum dots, discussed the nanocrystal (NC) nucleation and growth mechanism, and studied the optical and other properties of the QDs.(1) Oil-soluble CdS and CdSe QDs were prepared in multiphase systems at low temperatures. As-prepared CdS and CdSe QDs were characterized with ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) spectra, transmission electron microscopy (TEM) and X-ray diffraction (XRD). In addition, the optical properties of two magic-size cluster (MSC) species, absorbing at 359 and 379 nm respectively, were studied by using spectral analysis methods.(2) Nearly monodisperse and fluorescent CdS QDs were prepared in a 1-octadecene (ODE)/glycerol biphasic system. Compared to toluene/water, the system is environmentally friendlier, and can be used for higher temperature (generally 100~200°C) synthesis. Besides, Ag NCs can also be prepared in this system. As-prepared CdS and Ag NCs were dispersed in the ODE phase due to their hydrophobic capping layers. In a congeneric system of liquid-paraffin/glycerol, similar experimental results were obtained.(3) The properties of a CdS MSC species were studied using a UV–vis spectroscopic technique. In recent years, semiconductor MSCs have been receiving much attention due to their special structure and stability. A 323-nm-absorbing MSC species, as intermediate, developed at an early stage of the synthesis of regular CdS NCs. The colloid solution of the MSCs was unstable with respect to aggregation in spite of the extremely small size. Induced by alcohol molecules, the MSCs would transform into other two CdS species, absorbing at ~309 and ~348 nm respectively. The two species were highly unstable; they would transform into a 312-nm-absorbing MSCs after the alcohol molecules were removed from the system.(4) The NC growth in liquid-paraffin/glycerol was studied, and a MSC-mediated nucleation–growth mechanism of CdS NCs was proposed. We followed the temporal evolution of the UV–vis absorption and PL spectra, and the particle concentration of CdS NCs at different synthesis temperatures. At low synthesis temperatures, the nucleation and growth stages were overlapped, which resulted in NC ensembles with broad size-distributions. At relatively high temperatures, the nucleation stage finished in a short time, which resulted in monodisperse NCs. Taking into consideration of the properties of the cluster intermediate, we proposed a plausible mechanism for CdS NC development.(5) Alcohol-induced PL quenching of CdS QDs was studied, and a plausible quenching mechanism was proposed. The quenching effect was mainly dependent on the steric hindrance and the concentration of the alcohol. Weaker hindrance and higher concentration of the alcohol made a stronger quenching effect. The quenching effect was reversible; the PL was totally recovered after the alcohol was removed from the system. The quenching effect was significantly stronger to the band-edge emission than the trap-emission. A possible static quenching mechanism was proposed. Besides, the time-resolved PL spectra before and after alcohol addition were studied.