Laser coloration and bleaching of amorphous WO₃ thin film

Laser-induced chromism of amorphous WO₃ thin films prepared by pulsed laser deposition (PLD) has been investigated. The original films could be colored from light brown to purple by a single pulse of KrF excimer laser irradiation at 248 nm and subsequently bleached to brown by a single pulse of Nd-yttrium-aluminum-garnet laser at 1.06 μm in air. Spectroscopic investigations were applied to the films at three different states: original, colored, and bleached. The measurements by ellipsometry spectroscopy showed an increase in the refractive index (n) and decrease in extinction coefficient (k) in the luminous range of films from the colored state to the bleached state. Scanning tunneling microscopy and Raman spectroscopy showed slight crystallization in the films after coloration, with both the grain dimension and the surface roughness around tens of nanometers. The films were very stable to maintain the same color after long-time exposure in air, or in oxygen. X-ray photoelectron spectroscopy was used to study the chromism mechanisms. While some W⁵⁺ states were introduced in the original films by PLD, W⁴⁺ states were produced when the films were colored by the KrF excimer laser, along with the decrease of W⁵⁺ states and the increase of W⁶⁺ states. In contrast, laser bleaching was accompanied with increasing W⁴⁺ states in the films. Therefore, it is believed that the purple color is due to the polaron transition between the W⁴⁺ and W⁵⁺ states. Photochemical activation and photothermal oxidation are ascribed to the coloring and the bleaching processes, respectively.