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.

Semiconductor nanostructures are ideal candidates for non-metallic plasmonic materials that operate in the near- to mid-infrared range. In contrast to metal nanostructures, semiconductor nanomaterials have the advantage of possessing tunable carrier concentrations. However, unlike metal nanoparticles which are already widely exploited in plasmonics, little is known about the shape-dependent localized surface plasmon resonances (LSPRs) and near-field electromagnetic behavior of semiconductor nanocrystals. Moreover, a major challenge in the fabrication of plasmonic semiconductor nanomaterials is the ability to control LSPRs by independently varying the size, shape, and carrier density of the nanocrystal. In this dissertation, I describe colloidal synthetic methods for fabricating shaped Cu2-xS nanocrystals in which the morphology and stoichiometry of Cu2-xS can be modulated. These shaped Cu2-xS nanocrystals are used to observe the plasmon response for specific LSPR modes. Specifically, I discuss the plasmon response of Cu2-xS nanodisks as a model nanocrystal system. I demonstrate that LSPR wavelength can be tuned by independently varying the aspect-ratio of the disk and the overall carrier density of the nanocrystal. Increased carrier density in Cu2-xS occurs with oxidation and the formation of copper vacancies, an effect which can be suppressed by carrying out synthesis under an inert atmosphere. Using post-synthetic oxidation, Cu2-xS nanodisks achieve a critical carrier density beyond which the nanocrystals undergo an irreversible phase change, which limits tuning capability. To circumvent this, I use a solvothermal process to generate nanodisks with different crystal phases that enable carrier densities beyond this critical limit. This dissertation also explores the differences in near-field coupling between Cu2-xS nanodisks. These experiments were carried out on self-assembled two-dimensional nanodisk arrays. Varying nanodisk orientation produces a dramatic change in the magnitude and polarization direction of the local field generated by LSPR excitation. Moreover, plasmonic coupling is only observed for Cu2-xS phases that possess carrier densities above a critical value. Overall, this dissertation provides new methods for tuning the plasmonic response of semiconductor nanocrystals by controlling size, shape, and carrier density. It also demonstrates new strategies for designing electromagnetic junctions or coupled plasmonic architectures that operate in the infrared using nanocrystals as building blocks.

Quantum confinement of electronic wavefunctions in semiconductor quantum dots (QDs) yields discrete atom-like and tunable electronic levels, thereby allowing the engineering of excitation and emission spectra. Metal nanoparticles, on the other hand, display strong resonant interactions with light from localized surface plasmon resonance (LSPR) oscillations of free carriers, resulting in enhanced and geometrically tunable absorption and scattering resonances. The complementary attributes of these nanostructures lends strong interest toward integration into hybrid nanostructures to explore enhanced properties or the emergence of unique attributes arising from their interaction. However, the physicochemical interface between the two components can be limiting for energy transfer and synergistic coupling within such a hybrid nanostructure. Therefore, it is advantageous to realize both attributes, i.e., LSPRs and quantum confinement within the same nanostructure. Here, we describe well-defined LSPRs arising from p-type carriers in vacancy-doped semiconductor quantum dots. This opens up possibilities for light harvesting, non-linear optics, optical sensing and manipulation of solid-state processes in single nanocrystals.

This unique collection of knowledge represents a comprehensive treatment of the fundamental and practical consequences of size reduction in silicon crystals. This clearly structured reference introduces readers to the optical, electrical and thermal properties of silicon nanocrystals that arise from their greatly reduced dimensions. It covers their synthesis and characterization from both chemical and physical viewpoints, including ion implantation, colloidal synthesis and vapor deposition methods. A major part of the text is devoted to applications in microelectronics as well as photonics and nanobiotechnology, making this of great interest to the high-tech industry.

Explore this comprehensive discussion of the foundational and advanced topics in plasmonic catalysis from two leaders in the field Plasmonic Catalysis: From Fundamentals to Applications delivers a thorough treatment of plasmonic catalysis, from its theoretical foundations to myriad applications in industry and academia. In addition to the fundamentals, the book covers the theory, properties, synthesis, and various reaction types of plasmonic catalysis. It also covers its applications in reactions including oxidation, reduction, nitrogen fixation, CO2 reduction, and more. The book characterizes plasmonic catalytic systems and describes their properties, tackling the integration of conventional methods as well as new methods able to unravel the optical, electronic, and chemical properties of these systems. It also describes the fundamentals of controlled synthesis of metal nanoparticles relevant to plasmonic catalysis, as well as practical examples thereof. Plasmonic Catalysis covers a wide variety of other practical topics in the field, including hydrogenation reactions and the harvesting of LSPR-excited charge carriers. Readers will also benefit from the inclusion of: A thorough introduction to plasmonic catalysis, a theory of plasmons for catalysis and mechanisms, as well as optical properties of plasmonic-catalytic nanostructures An exploration of the synthesis of plasmonic nanoparticles for photo and electro catalysis, as well as plasmonic catalysis towards oxidation reactions and hydrogenation reactions Discussions of plasmonic catalysis for multi-electron processes and artificial photosynthesis and N2 fixation An examination of control over reaction selectivity in plasmonic catalysis Perfect for catalytic chemists, materials scientists, photochemists, and physical chemists, Plasmonic Catalysis: From Fundamentals to Applications will also earn a place in the libraries of physicists who seek a one-stop resource to enhance their understanding of applications in plasmonic catalysis.

Abstract Optical Characterization of Localized Surface Plasmon Resonances in Doped Metal Oxide Nanocrystals by Robert Walker Johns Doctor of Philosophy in Chemistry University of California, Berkeley Professor Tanja Cuk, Co-Chair Professor Delia Milliron, Co-Chair Electronically doped metal oxide nanocrystals exhibit tunable infrared localized surface plasmon resonances (LSPRs). Semiconductors provide an alternative dielectric environment than metallically bonded solids, such as noble metals, for metallic behavior. The ways in which the electronic structure of the semiconductor and of the dopants used to make them metallic hybridize substantially changes the plasmonic behavior. Choice of dopant element, dopant placement within the nanocrystal, and dopant interaction with other defects in the lattice all lead to changes in the observed optical properties of these nanocrystals. Here, the methods for optical characterization of LSPRs in doped metal oxides are discussed with particular attention directed at how undetermined heterogeneous contributions to ensemble measurements lead to misattributing inhomogenous broadening to poor plasmonic performance. Electronic damping in these materials is incredibly low compared to coinage metals, and they tout the added benefit of spectral tuning through chemical composition rather than morphology. The result is a class of materials that can both have their optical response tuned separately from other application relevant factors like nanocrystal size, and yield high performance LSPR for directing far-field radiation to the near-field. Learning that doped metal oxides have high quality factor LSPR was found through the first single nanocrystal measurements of LSPR made in the mid-IR through the use of near-field optics to interrogate these nanocrystals separately. The result was uncovering substantial nanocrystal-to-nanocrystal variation within batches of nanocrystals making ensemble measurements appear to have broad LSPR, while in fact these materials have high quality factors individually. These measurements were enabled by broadband synchrotron based scattering type- scanning near field optical microscopy (s-SNOM). Broadband s-SNOM in the IR can yield the single nanocrystal optical spectrum and dielectric function of an isolated signal nanocrystal when the proper considerations are made to backgrounding signal over such a wide spectral range. The methodology as well as new understanding of the materials learned through this instrumentation advance are outlined. Finally, the lessons learned about the properties of LSPR in doped metal oxides from single nanocrystal measurements are extended to an adaptation of applying Mie theory to the nanocrystal dielectric function in order to assign reasonable dielectric constants to nanocrystals even from ensemble optical measurements over any energy range, not just those obtained from mid-IR s-SNOM. Further, these advances in assigning optical density to an ensembles of doped metal oxide nanocrystals are applied to understanding how energy relaxation out of the LSPRs occurs in these materials through the use of the two-temperature model, using constants obtained from NIR ultrafast transient absorption measurements. The low free carrier concentrations of metal oxide nanocrystals lead to less efficient heat generation as compared to metallic nanocrystals such as Ag. This suggests that metal oxide nanocrystals may be ideal for applications wherein untoward heat generation may disrupt the application’s overall performance, such as solar energy conversion and photonic gating.

One of the most promising trends in modern nanophotonics is the employment of plasmonic effects in the engineering of advanced device nanostructures. This book implements the binocular vision of such a complex metal-semiconductor system, examining both the constituents and reviewing the characteristics of promising constructive materials.