Low Temperature Formation of Crystalline VO2 Domains in Porous 1 Nanocolumnar Thin Films for Thermochromic Applications

TL;DR

This study demonstrates low-temperature oxidation of porous VOx thin films to form crystalline VO2 domains, enhancing thermochromic properties for smart window applications.

Contribution

It reveals how controlled oxidation below 300°C induces VO2 formation and improves optical and electrical performance in nanocolumnar VOx films.

Findings

VO2 domains form at oxidation temperatures as low as 260°C.

Optimal properties achieved at [O]/[V]=1.5 and 280°C oxidation.

Optical modulation of nearly 50% in the near-infrared range.

Abstract

The formation of VO2 crystalline domains in amorphous substoichiometric nanocolumnar VOx thin films subjected to an oxidation process at temperatures below 300{\deg}C has been studied. It is obtained that values of [O]/[V] above 1.9 lead to the sole formation of V2O5 after oxidation, while values below 1.9 favor the formation of VO2, V3O7 and V2O5 crystalline domains for temperatures as low as 260{\deg}C. Moreover, it is found that the adsorption of oxygen and its incorporation into the film network produce a relevant volume expansion in a so-called swelling mechanism that makes pores shrink. Under some specific conditions, the low temperature oxidation does not only trigger the formation of VO2 domains but also a drastic reduction of oxygen-deficient amorphous VOx in the films, which clearly improves the overall transparency and thermochromic modulation capabilities. The changes in the optical and electrical properties of these films during the metal-insulator transition have been studied, finding the best performance when the stoichiometry of the film before oxidation is [O]/[V]=1.5 and the oxidation temperature 280{\deg}C. These conditions yield a relatively transparent coating that presents an optical modulation in the near-infrared range of nearly 50% and a drop of electrical resistivity of more than two orders of magnitude. A tentative model based on the volume increase experienced by film upon oxidation is proposed to link the structural/chemical features of the films and the formation of VO2 domains at such relatively low temperatures.