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Advancing electronic structure characterization of semiconducting oxide nano-heterostructures for gas sensing

Miller, Derek
http://orcid.org/0000-0003-1354-3568
http://rave.ohiolink.edu/etdc/view?acc_num=osu1492639729205609

Abstract Details

2017, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
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 open-access database is described which will enable researchers in the field to rapidly analyze relationships between several test variables and performance attributes of resistive-type gas sensor materials utilizing individual data points pulled from published papers. The demonstration of these advanced characterization techniques should inform and encourage future research to deconstruct their mechanistic explanations to the most fundamental scale. Future studies should systematically test variations of these nano-heterostructures in carefully controlled ways to find the optimal morphology and composition for each test environment. These results should be paired with direct measurements of the electronic structure and properties of the primary and secondary materials in order to build a complete framework for explaining the mechanisms for improved performance. Insights gained from these studies will inform bottom-up design of new nano-heterostructures optimized toward specific sensing applications.
Sheikh Akbar (Advisor)
Patricia Morris (Advisor)
David McComb (Committee Member)
387 p.
Materials Science
nanowires; sensor; heterostructure; semiconductor; oxide; SnO2; TiO2; ZnO; VLS; nanomaterial; EELS; cathodoluminescence

Recommended Citations

Citations

  • Miller, D. (2017). Advancing electronic structure characterization of semiconducting oxide nano-heterostructures for gas sensing [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492639729205609

    APA Style (7th edition)

  • Miller, Derek. Advancing electronic structure characterization of semiconducting oxide nano-heterostructures for gas sensing. 2017. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1492639729205609.

    MLA Style (8th edition)

  • Miller, Derek. "Advancing electronic structure characterization of semiconducting oxide nano-heterostructures for gas sensing." Doctoral dissertation, Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492639729205609

    Chicago Manual of Style (17th edition)

osu1492639729205609
374
© 2017, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.