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Title Advancing electronic structure characterization of semiconducting oxide nano-heterostructures for gas sensing
URL
Publication Date
Degree PhD
Discipline/Department Materials Science and Engineering
Degree Level doctoral
University/Publisher The Ohio State University
Abstract The last decade has seen a rapid progression of newly-synthesized nano-heterostructures of many shapes, sizes, and compositions in an attempt to improve the properties and performance of gas sensors. Most studies published show improved or otherwise unique performance attributes when combining multiple compositions finely dispersed on the nano-scale. However, these novel structures are created faster than their electronic properties can be understood, leading many to a trial-and-error approach toward finding the right combinations of materials for a specific application. The performance of these materials is highly dependent on the defect states and charge carrier movement at the surfaces and interfaces. The fraction of studies that do attempt to explain the mechanisms behind the improvements often rely on literature values of important properties such as resistivity, band gap, defect state energies and Fermi level, which may or may not be accurate in their nanomaterials. In order to properly develop models to explain the charge carrier movement phenomena at the surfaces and interfaces, these structures must be understood and characterized in their most basic units. Unfortunately, the size and dispersion of these nanomaterials are beyond the spatial resolution limits of the best optical measurement techniques, leading to measurements averaged over several particles. Variations in synthesis and processing between samples and research groups adds additional uncertainty. Furthermore, measuring films of many particles necessitates interpreting results based on the expected average particle size, shape, or composition, rather than the variations actually present.In this work, single-nanowire devices were fabricated in order to assess the sensor properties without many of the confounding variables present in a film of randomly dispersed nanostructures. It was found that the current path through the nano-heterostructures can completely change the response type behavior in core-shell n-p materials. Impedance spectroscopy on single-nanowires helped to show that the resistance modulation of the junctions between nanowires are more sensitive to oxygen content than depletion of the internal or “bulk” region of the nanowires. Furthermore, high-resolution STEM techniques such as valence EELS and cathodoluminescence measured small spatial variations in the band gap and mid-gap defect states in SnO2 nanowires, ZnO nanowires, and TiO2 nanoparticles. In SnO2 and TiO2, emission peaks were designated to specific surface and bulk defects. Additionally, optical and dielectric properties were measured in individual nanostructures as well as spatially across nano-heterostructure interfaces. These direct measurements will help to build better mechanistic models than when relying on literature values from bulk versions of these materials. The photocatalytic ability of these materials to degrade a test dye in an aqueous environment was also investigated and showed the most promising results in ZnO nanowires. Furthermore, the creation of a new…
Subjects/Keywords Materials Science; nanowires; sensor; heterostructure; semiconductor; oxide; SnO2; TiO2; ZnO; VLS; nanomaterial; EELS; cathodoluminescence
Contributors Akbar, Sheikh (Advisor); Morris, Patricia (Advisor)
Language en
Rights unrestricted ; This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.
Country of Publication us
Format application/pdf
Record ID oai:etd.ohiolink.edu:osu1492639729205609
Repository ohiolink
Date Retrieved
Date Indexed 2017-09-19
Grantor The Ohio State University

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