Optical and electronic properties of amorphous WO₃ thin film irradiated by laser in air

To study the potential in optical recording, the laser induced chromism of amorphous WO₃ thin films has been investigated. The original film can be colored from grey to purple by one pulse of the KrF excimer laser at 248 nm and bleached to dark grey by one pulse of Nd-YAG laser at 1.06 μm in air. Spectroscopy measurements were applied to study the films at the three alternate states: original, colored and bleached. The changes of refractive index (n) and extinction coefficient (k) from colored states to bleached states, could be measured by ellipsometry spectroscopy that showed increasing and decreasing tendency, respectively in luminous range at colored state. X-ray photoelectron spectroscopy (XPS) was used to study the chemical states. With some W⁵⁺ ions existed in the original films by pulsed laser deposition, more lower states ions, such as the W³⁺ ions were produced along with decreasing W⁶⁺ states in films colored by KrF excimer. Laser bleaching was accompanied with decreasing of W⁵⁺, W³⁺ states and increasing of W⁶⁺ states in films. The purple color was thought due to the polaron transition between W³⁺ states and W⁴⁺ states or, W⁵⁺ states and W⁴⁺ states. Scanning tunneling spectroscopy (STS) showed that the colored films had more characteristics of n type semiconductor after coloring. We attribute the coloring and bleaching process to photochemical activation and photothermal oxidation, respectively. Raman spectroscopy showed slight crystallization after coloration in films. From temperature field calculation, the crystallization is suggested most likely caused by ion intercalation instead of thermal crystallization. The color states in films are very stable after long-time exposure in air, as well as in oxygen.