Photochromic Properties of Rare-Earth Oxyhydrides

Abstract

The experiments presented in this thesis have provided new insights regarding the photochromic properties of rare earth oxyhydrides (REOxHy). We discovered that thin films of rare-earth (Y, Dy, Er, Gd, Nd) oxyhydrides show unique photochromic properties under ambient conditions. We showed that by direct current reactive magnetron sputtering of rare earth metal targets in an Ar and H2 atmosphere a metallic dihydride thin films can be made. However, above a certain critical deposition pressure, a semiconductor REOxHy thin film is formed upon exposing the films to air. These films show photochromic properties. Compared to YOxHy, the optical bandgaps of the lanthanide-based oxyhydrides are smaller, while photochromic contrast and kinetics show large variation among different cations. The photon energy required to obtain a photochromic effect is given by the optical band gap of the material, as shown in the energy threshold measurements. Photon energies larger than the band gap are required to photo-darken the rare-earth oxyhydrides. The photochromic process is reversible and bleaching occurs through thermal bleaching and possibly through interaction with the light of longer wavelengths (optical bleaching). However, the latter needs to be confirmed with further experiments. We have shown for the first time also that semiconducting YH3 does not exhibit photochromic properties, revealing the importance of the presence of both oxide and hydride ions for the photochromic effect in rare-earth oxyhydrides.