Effect of the tungsten and molybdenum oxidation states on the thermal coloration of amorphous WO3 and MoO3 films

Transmission and X-ray photoelectron spectroscopy (XPS) was used to investi gate the optical and electronic changes caused by thermal annealing in UHV of amorphous WO3 and MoO3 thin films suitable for electrochromic applicatio ns. The core level XPS measurements showed that annealing in UHV progressiv ely reduces the oxides to the lower oxidation states and causes changes in their color. Their luminous transmittance was related to their state of oxi dation at each annealing temperature during thermal coloration. Only the O2 p peak characterizes the valence band spectra of the oxides before colorati on. After annealing, the 5d and 6s states of tungsten and 4d, 5s of molybde num are partially filled by electrons released due to the reduction of the oxides. The MoO3 film exhibits deep blue coloration below 400 degreesC, ass ociated to a continuous distribution between Mo6+ and Mo5+ states. For the WO3 film the W5+ state, which appears above 400 degreesC, plays the dominan t role in deep coloration. At lower temperatures distinct W5+ states are fo rmed with a limited contribution to coloration. (C) 2001 Elsevier Science B .V. All rights reserved,