Localized Surface Plasmon Resonances in Semiconductor Nanocrystals

This dissertation examines the origins of free carrier density in semiconductor nanocrystals and relates these findings to the observed localized surface plasmon resonances (LSPRs). The first chapter introduces some of the most relevant and fundamental concepts in solid state physics, nanomaterials, plasmonics, and key analytical instrumentation used for the studies. Chapter two focuses on what is likely the most famous example of all plasmonic semiconductors, indium tin oxide (ITO) nanocrystals. ITO was one of the first semiconductors shown to exhibit a LSPR, a phenomenon previously thought to only occur in metallic systems such as gold and silver. In this chapter, the concentration of tin dopant is synthetically tuned to investigate its effect on the total number of free carriers generated and their relation to the overserved infrared LSPR. One electron chemical titrations are used to show that the Drude model is shown to be inaccurate for calculating free carrier concentrations and appropriate corrections are proposed. The third chapter examines the effect of dopant size mismatch on free carrier generation. Al3+, Ga3+, and In3+ doped ZnO nanocrystals are synthesized and shown to exhibit unique mid-infrared LSPRs that are not dependent on dopant concentration like is seen in ITO. A combination of infrared spectroscopy, powder X-ray diffraction, and chemical titrations were used to show that better dopant/host ion size matching results in higher free carrier densities. This work is further extended in chapter four, where solid state nuclear magnetic resonance spectroscopy is used as an element specific probe for Al3+ and Ga3+ doped ZnO nanocrystals. Evidence of the formation of an insulating spinel phase (AB2X4) explains the lack of tunability of the LSPR with dopant concentration.