UC Berkeley

Atomic doping, a cornerstone of material science, is paramount in the behavior and properties of solid-state materials. Significant strides have been taken, illuminating the indispensable role of atomic doping in engineering processes, impacting phase transitions, crystal growth, annealing, and a myriad of applications including batteries, catalysis, and hydrogen storage. Of these, atomic doping in correlated materials has been widely investigated due to its intimate connection with the unique electronic and magnetic characteristics inherent in CMs. In particular, vanadium dioxide (VO2) holds the metal-insulator transition (MIT) at T_MIT^0 = 67℃, together with insulator to metal transition (IMT) below T_MIT^0 driven by chemical dopants (e.g. H, Li, W). As a result, the diffusion of dopants in VO2 presents an intriguing avenue for fundamental research as the phase transition is correlated with atomic doping. Simultaneously, pave the way for diverse MIT-based applications such as proton-based transistors, temperature-adaptive coatings, and optoelectronic devices. Efforts have been devoted to the production of VO2 powders, wires, and films, paving the way for both scientific and applied studies from nanometer scale to mass production based on a single material. The objective of the dissertation presented herein is to scrutinize the interplay between atomic doping and MIT of VO2, and to devise functional devices leveraging the distinctive behaviors of VO2. Various techniques have been employed to prepare crystals of VO2 for different purposes. Chapter 2 elucidates the preparation of VO2 in the form of thin films, microbeams, and nanowires. Single-crystalline VO2 nanowires or microbeams show a more abrupt, intrinsic MIT than epitaxial films, thus are great platforms for the study of fundamental physics, such as the interplay between phase transition and atomic doping; On the other hand, epitaxial VO2 thin films can be deposited on a range of substrates, hence attract more attention due to the potential for various device applications. Taking advantage of the less abrupt phase transition of VO2 thin films and atomic doping, multiple devices are developed based on the transitioned electrical and optical properties. In Chapter 3, a chemical analogy of super-elasticity along the atomic doping axis is presented. The second derivatives of Gibbs free energy (specific heat and compressibility) diverge at the transition point, resulting in an effect known as super-elasticity along the pressure axis, or super-thermicity along the temperature axis. Here we report a chemical analogy of these singularity effects along the atomic doping axis, where the second derivative of Gibbs free energy (chemical susceptibility) diverges at the transition point, leading to an anomalously high energy barrier for dopant diffusion in co-existing phases, an effect we coin as super-susceptibility. Single-crystalline VO2 microbeams were synthesized by Vapor-Liquid-Solid (VLS) in the initial stage, followed by microfabrication, hydrogen treatment, and characterization of the phase transition. The diverging chemical susceptibility was then demonstrated through the Arrhenius plot of hydrogen diffusivity together with density functional theory (DFT) calculations. As the mechanism is fundamentally based on the thermodynamics of the phase transition, the effect is expected to exist universally, albeit to different extents, in all first-order phase transitions. Chapter 4 introduces a novel thermo-sensing material for uncooled thermal detectors, designed based on the MIT of VO2 and deposited via pulsed laser deposition (PLD). Interlayer W diffusion in VO2 is employed to overcome the limitation posed by the narrow temperature range within which MIT operates in pure or uniformly doped VO2. Moreover, these microbolometers detect thermal radiation from an object based on the Stefan–Boltzmann law, relying on the engineering designs and properties (e.g. temperature coefficient of resistance) of the sensing material. Existing methods to advance the performance of the uncooled detectors are focused on the engineering side, with the efforts approaching the end of the roadmap. In contrast, our efforts are focused on the material side, revealing the advancement of gradient doped WVO2 in comparison with the industrial vanadium oxide (VOx). A thermally isolated, suspended square-shaped multilayer structure microbolometer is proposed, followed by thermal imaging of real objects using our self-designed and fabricated bolometer devices. Furthermore, several other applications based on the unique MIT and atomic doping (diffusion) in VO2 are presented in Chapter 5, underscoring the vast potential for integrating atomic diffusion into real-life products.