Crystallization And Growth Of Colloidal Nanocrystals PDF Download

Nanometer-sized colloidal quantum dots, or nanocrystals, are of current scientific interest based on their mesoscopic properties as an intermediate state between molecular and bulk materials. Particles sizes of semiconductor materials smaller than the Bohr exciton radius result in a quantum confinement effect that allows the optical properties to be tuned within the band energies of the bulk material such that smaller sizes lead to higher energy transitions at the band edge. The small sizes and tunable monochromatic emission properties make these nanocrystals desirable for a variety of applications, both realized and sought after. Research to further the exploitation of quantum dots includes such applications as biological fluorescence tracer tags and imaging agents, high resolution displays, light emitting diodes (LEDs), low bandwidth lasers, photovoltaic solar cells, and future "nanoelectronics." The thesis of this dissertation is that high-quality II-VI colloidal nanocrystals, CdSe and (CdSe)ZnS in particular, can be synthesized using mild precursors, exerting fine control though the proper selection of ligands. Chapters 2 and 3 show that extremely high-quality CdSe nanocrystals can be synthesized using mild precursors such as Cd(acac)2 and Cd(OH)2 while achieving higher quantum efficiencies, higher reaction yields, smaller size-distributions, and with finer control over the final size in comparison to previous methods. In addition to the batch synthesis, Chapter 3 shows the development of a microfluidic flow reactor using a mixed highboiling STP liquid solvent. It is found that the temperature, flow rate, and precursor ratios are parameters that can be used to control the size and size-distributions of the final products. The CdSe cores are overcoated with ZnS from Zn(acac)2 to form highly stable and highly luminescent (core)shell colloids in Chapter 4. Chapter 5 develops qualitative arguments based on the Drago-Wayland extension of Hard-Soft Acid-Base Theory to describe the actions of surfactant ligands in the growth solution. It is shown that crystal growth inhibitors can be used to finely control nanocrystal growth, and the precursor-to-inhibitor ratio is fundamental to the final size.