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SYNTHESIS AND METAL-INSULATOR TRANSITION PROPERTIES OF VANADIUM DIOXIDE NANOSTRUCTURES
Restricted (Penn State Only)
Doctor of Philosophy
Date of Defense:
November 29, 2018
Raymond E. Schaak, Dissertation Advisor
Raymond E. Schaak, Committee Chair
Christine D. Keating, Committee Member
Thomas E. Mallouk, Committee Member
Zhiwen Liu, Outside Member
Nanomaterials provide exciting opportunities for the understanding and realization of their distinctive properties derived from the nanoscale dimensions. Vanadium dioxide (VO2) materials that exhibit metal-insulator transition (MIT) properties, with coupled structural transformations and abrupt changes in electrical and optical properties, serve as an example of such materials with unique fundamental dynamics and distinct properties at reduced dimensions. To illustrate the fundamental transition mechanisms on the nanoscale, and to construct switchable building blocks that can be integrated into nanoelectronic and nano-optical devices, advancements of synthesis routes to nanoscale VO2 materials need to be developed by overcoming intrinsic synthetic challenges associated with the stoichiometry and phase control. In this dissertation, I present such strategies for synthesizing VO2 nanostructures. This dissertation starts with an overview of nanomaterials syntheses in chapter 1, with emphasis on the synthesis of nanoscale heterostructures, post-synthetic composition and phase engineering of nanostructures via cation exchange processes, and the metal-insulator transition properties of the VO2 nanostructures. In chapter 2, I demonstrate a pathway to synthesize complex heterostructures enabled by Ag-Au-S reactive synthons, utilizing the phase segregation and cation exchange based methods introduced in chapter 1. The following chapters focus on strategies for the solution-based synthesis of VO2 nanostructures. Chapter 3 demonstrates a seeded approach for synthesizing VO2-TiO2-VO2 heterostructures, where VO2 domains nucleate and grow epitaxially on TiO2 nanorod seeds. The metal-insulator transition properties of the VO2 domains can be modulated by controlling the VO2 domain sizes as well as the TiO2-VO2 interfacial features. Chapter 4 describes a ZnO-templated synthesis of amorphous, morphologically pre-defined VO2 nanostructures, which can be thermally converted to crystalline VO2 nanostructures exhibiting metal-insulator transition properties. Chapter 5 demonstrates the synthesis of Mo-doped VO2 nanorod structures that are seeded by epitaxial MoO2 nanostructures, with controlled metal-insulator transition temperature as a function of Mo dopant distribution and concentration. This dissertation is summarized by Chapter 6, which provides insights into emerging fields of study enabled by expanded capabilities for synthesizing VO2 nanostructures.
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