Metal-Semiconductor Transitions in Nanoscale Vanadium Dioxide - Thin Films, Subwavelength Holes, and Nanoparticles
Donev, Eugenii U.
Large-volume (bulk) vanadium dioxide (VO2) is an unusual material that undergoes a critical transition from insulating-like to metal-like when the temperature is raised above approximately 67 °C. This thesis describes observations of the thermally induced phase transitions of in various VO2-containing structures with critical dimensions from 10 to 1000 nm, including hybrid metal-VO2 nanostructures. Following a review of the properties of VO2 and the latest developments in the decades-long debate over the precise transition mechanism, I present my original research contributions in two broad categories. In the first, I employ the drastic change in infrared (IR) optical properties of VO2 across the metal-semiconductor transition to modulate the transmission of light through hybrid nanostructures composed of VO2 and noble metals, known to support localized and/or propagating surface-plasmon oscillations. Specifically, I demonstrated (i) a size- and polarization-dependent shift of the localized surface-plasmon resonance of gold NPs due to the transition-induced changes in the dielectric functions of a thin VO2 overlayer; and (ii) a novel way to control the so-called extraordinary optical transmission through arrays of subwavelength holes in gold-VO2 and silver-VO2 double-layer films. I also discovered and explained a counterintuitive transmission effect pertaining to VO2: a perforated VO2 film lets more IR light through while in its metallic phase, contrary to the usual behavior of intact VO2 films. In a second category of experiments, I used Raman scattering to reveal critical properties of VO2 nanostructures. In particular, I measured two "firsts": (iii) the phase transition in isolated VO2 NPs, showing that not only size but particle morphology plays a role in determining the ease of nucleation of the transition; and (iv) the size-dependence of the monoclinic-tetragonal structural transformation in arrays of gold-VO2 NPs. The latter was facilitated by yet another size-dependent effect - the enhancement of electromagnetic fields in the vicinity of the gold NPs capping the VO2 NPs, which I correlated to the Mie scattering efficiency of a gold sphere immersed in a composite dielectric medium.