Synthesis Of Semiconductor Nanocrystals With Controllable Morphology And Tunable Emission Via High Boiling Point Organic Solvents

Nanoscience and nanotechnology have emerged to become one of the most exciting areas of research today and have attracted the imagination of a large number of researchers. The study on the preparation and properties of nanomaterials is the most active and most important part of applicable nanotechnology and it also constitutes the basic base of nanoscience. Nanoparticles are basic research elements for nanotechnology and nanocrystals occupy a special place amongst nanomaterials because the have enabled a proper study of size-dependent properties. Nanoparticles constitute the building blocks for nanotechnology and thus for numerous potential applications in fields such as energy and power, health and biomedicine, electronics and environmental applications, new engineering materials.The synthesis and preparation of nanomaterials are the primary foundation for researches of their applications, and the choose of reaction parameters such as solvents, reaction temperature, reaction time and the use of surfactants has important and direct effects on the results of reactions. The understanding of the reaction mechanism and some key factors in the progress of nuclea an growth will greatly help us to develop the new synthetic concept towards efficiently controllable scale-up preparation of nanocrystals through the elaborately design of synthesis procedures,. The dissertation focuses on the study of high temperature synthetic route and the effects of experimental parameters such as temperature, surfactants and precursors on the morphology, composition and properties of as-synthesized nanocrystals, correspondingly. The mechanism for the formation nanocrystals were discussed as well. Some representative nanocrystals systems including visible and infra-red emission semiconductor nanocrystals, metal chalcogenides nanocrystals and related nanocomposites, and metal oxides nanocrystals were used as the research subjects by meanse of the high boiling-point organic solvent procedure. In comparison with organic dyes and fluorescent proteins, quantum dots have unique optical and electronic properties including narrow size-tunable light emission, improved signal brightness, resistance against photobleaching, and simultaneous excitation of multiple fluorescence colors. In chapter 2, based on previous work of phosphine-route synthesis of ZnSe nanocrystals, ZnSe/CdSe core-shell nanocrystals were successfully synthesized by utilizing Se (Se-TBP) in pre-prepared ZnSe precursors by the introduction of Cd²⁺. The crystal structures, shapes and optical properties of as-synthesized nanocrystals were characterized by X-ray diffraction, transmission electron microscopy, UV-vis absorption and photoluminescence spectroscopies. The results indicate that the epitaxial growth of CdSe shell onto ZnSe nanocrystals led to the formation of ZnSe/CdSe core-shell structures with well-crystallized and the tunable PL emission peak from 500 nm to 620 nm by tuning the size of cores or the thickness of shells. A band-gap offset structure which was feathered by either reverse Type-I or Type-II was proposed to be responsible for the red-shift of PL emission. For the synthesis of high quality ZnSe nanocrystals, we developed a so-called"phosphine-free"approach where Se-ODE and zinc stearate were adopted as precursors for the preparation of ZnSe nanocrystals with various sizes. Susequently, the new PL windows covering from 480 to 520 nm was obtained by diffusing the Cu into as-synthesized ZnSe nanocrystals with different sizes. The PL efficiency was improved by coating the ZnS shell onto the ZnSe:Cu/ZnSe nanocrystals ,In chapter 3, we start from the morphology controllable synthesis of CdTe nanocrystals with multi-pods were obtained during the early stage of reaction. With increasing reaction time, the system entered into the Ostwald ripening regime and the tetrapod CdTe nanocrystals were then transformed into dot shaped particles. Besides high initial concentration of cadmium precursor and high ratio (up to 10) of Cd²⁺ to Te-TOP, ODA was found to be an activator and play a key role for forming zinc-blende CdTe tetrapods right after the injection due to activating cadmium-oleic acid precursors The PL efficiency of the nanocrystals was very low in the early stage of the reaction. The fast growth of the nanocrystals could cause many defects on the surface sites. The ripening process could modify the surface of the nanocrystals and generate a perfect surface so that the nanocrystals emitted strong PL. On the basis of tetrapod and particle CdTe nanocrystals synthesis, we adopted a simple cation exchange process to obtain infra-red emission HgCdTe nanocrystals by facile treatment on as-synthesized CdTe nanocrystals under room temperature. Through the comparation of cation exchange results, we found that the morphologies of CdTe precursors were well preserved as well as the good crystallinity. The introduction of Hg²⁺ ions into CdTe system resulted in a red-shift of PL to longer wavelength of about 827 nm for the maximum. In addation, we also tried the direct synthesis of fluorescent nanocrystals which were feathered with infrared characteristics. Monodisperse zinc blende HgTe nanocrystals were successfully synthesized at room temperature in noncoordinate organic solvent of ODE. Thiol was applied to control the reaction at a suitable nucleation and growth speed. In the early stage of the reaction, HgTe nanocrystals formed aggregates, and then became individual dot-shaped nanocrystals with stronger photoluminescence emission.In chapter 4, we prepared Cu, Cu₂S and CdS/Cu₂S core/shell nanocrystals in organic solvents and characterized as-prepared nanocrystals by TEM, XRD etc. For the preparation of copper nanoparticles, copper oleate was used as precursor and copper nanoparticles were obtained through the thermolysis of precursor at elevated temperature. The influences of applied reaction temperature on the final product were also tested. Monodisperse Cu₂S nanocrystals were also prepared with copper stearate and dodecanthiol act as copper and sulfur sources, respectively. The as-synthesized Cu₂S nanocrystals exhibit some kind of self-assemble characteristic under specific conditions. The composite of CdS/Cu₂S in form of hybrid film is a traditional solar cell material. At last, based on the synthesis of Cu₂S nanocrystals, we also tried the preparation of CdS/Cu₂S core/shell composite nanocrystals, and the red shift of its PL compared with CdS was explained by the bandgap offset between CdS and Cu₂S.The major work of the last chapter deals with the preparation of metal oxide nanocrystals in noncoordinating solvent of paraffin oil with metal acetylacetonate as precursors and OA, OAm and dodecanol as composite ligands, respectively. The general growth model of nanocrystals involves two steps, which ideally should occur separately: the nucleation of the nanocrystals and the actual process of growth were realized by injection of additional paraffin oil during the initial stage of reaction. The size of as-synthesized CoO and MnO nanocrystals can be easily adjusted by regulating the reaction temperature and reaction time. Novel structure of CoO such as flower-like morphology can be achieved by slowering the injection speed of paraffin oil, which was used to lower the temperature and steer the reaction into the second stage for nanocrystals growth. In the case of manganese oxide synthesis, a serial of experiments with different reaction temperatures were conducted to investigate the role of different surfactants during the thermal decomposition of metal acetylacetonates. The results indicate that the reaction temperature and the methods used to adjust the temperature and had an important effect on the formation of products. Especially the use of ligands is the guarantee for the preparation of monodisperse nanocrystals.