| dc.description.abstract | Vanadium dioxide (VO2) is an exemplary phase-transition material due to its insulator-to-metal transition, which boasts sharp changes in its lattice vectors (~1%), electrical resistivity (~5 orders of magnitude), and optical constants (Δn up to ~2, Δk up to~5). Responsive to mechanical (~1% strain), thermal (~68°C), electrical (~106-107 V/m), and optical switching (few mJ/cm2, as fast as 26 fs), it enables a wide variety of devices, from tunable metastructures to smart windows.
The phase transition is exquisitely sensitive to many factors: intrinsic and extrinsic strain, dopants, defects, stoichiometry, size, shape, and morphology. This sensitivity presents both an opportunity to fine-tune various phase-transition behaviors (transition temperature, contrast, hysteresis sharpness and width, additional phases) and a challenge to sufficiently control all the relevant factors. Many processes exist to fabricate VO2 thin films, single crystals, and nanoparticles, each subject to its own process parameters and challenges, but most involve a solid substrate on which VO2 is grown. Substrate-surface interactions have a profound impact on the morphology and composition of VO2 thin films and single crystals, making substrate choice not only an important consideration, but a valuable engineering tool. This dissertation shows how substrate affects two specific growth processes—vapor-transport growth of single crystals and dewetting of thin films to form nanoparticles—but the interactions identified are broadly applicable. Lattice match determines preferred orientation; chemical reactions lead to doping and interfacial species; and surface energy controls wetting/dewetting and thus microcrystal size.
Two cases are presented that exemplify the range of VO2 applications: reconfigurable 2D material metastructures requiring large, low-aspect-ratio microcrystals, and passive thermal control films employing VO2 thin films (but potentially benefitting doped nanoparticles). These and other VO2-based structures, as well as scientific studies of VO2 as a representative correlated material, require precise morphological control, for which a thorough understanding of substrate surface interactions is essential. | |
