Sara C Barron, Mitul P Patel, Justin Gorham, and Martin L Green

The thermochromic phase transition at 68C in vanadium dioxide (VO2) is the most promising candidate for ‘smart’ window activity among transitions in inorganic oxides. In the pure bulk oxide, the transition between the low temperature monoclinic crystal and the high temperature tetragonal (rutile) crystal is accompanied by a dramatic increase in the infrared reflectivity. If the temperature of the transition were shifted to Earth ambient temperatures, this modulation in infrared reflectivity could be used as a building window coating to control solar heating of the building interior. Some transition metal impurities at less than 10 atomic percent have been found to depress the thermochromic transition temperature.

In this study, we employ a combinatorial materials approach to characterizing the effects of transition metal impurities in thin film VO2. Thin film samples with an intentional compositional gradient across the 3” diameter sample are prepared by pulsed laser deposition (PLD) from a V2O5 target and one or two other chemically distinct oxide targets. Thermochromic transitions in these combinatorial libraries are then characterized using a custom-built spectroscopic infrared reflectance tool. The measurement protocol, fully automated in LabVIEW, is to stabilize the sample uniformly at a desired temperature, then to make many local reflectance measurements at different sample positions (and hence, compositions), and then to heat to the next desired temperature. In this way, over an eight hour period, the infrared reflectance of hundreds of chemical compositions can be determined at ~10 temperatures in the range of 15C to 80C.

We find that the tantalum impurities can depress the onset of the thermochromic transition by up to 30C below that of pure VO2 films. Further, in co-doped films incorporating tungsten and niobium, the thermochromic transition can occur at temperatures less than 16C, in the range of ambient temperatures relevant for smart window modulation.