Effects of Substrate Surface in the Design of VO2 Micro- and Nano-Crystals

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.