Thermochromic VO[2] nanorods made by sputter deposition: Growth conditions and optical modeling

Abstract

Reactive dc magnetron sputtering onto glass-based substrates yielded deposits of thermochromic VO[2] with well-developed nanorods and nanowires. Their formation was promoted by high substrate temperature (above ∼500 °C), sufficient film thickness, proper inlet of the reactive gas, dispersed gold “seeds, ” and pronounced substrate roughness. Rutherford back scattering ascertained mass thicknesses, scanning electron microscopy depicted the nanostructures, and glancing incidence X-ray diffraction proved that single-phase VO[2] was normally formed. Spectrophotometric measurements of total and diffuse transmittance and reflectance on VO[2] thin films, at room temperature and ∼100 °C, allowed us to determine complex dielectric functions below and above the “critical” temperature for thermochromic switching (∼68 °C). These data were then used in computations based on the Bruggeman effective medium theory applied to randomly oriented prolate spheroidal structural units to derive the optical properties of the deposits. Experimental and computed data on spectral absorptance were found to be in good qualitative agreement.