Influence of Crystal Structure, Encapsulation, and Annealing on Photochromism in Nd Oxyhydride Thin Films

Thin films of rare earth metal oxyhydrides show a photochromic effect, the precise mechanism of which is yet unknown. Here, we made thin films of NdH_{3–2<i>x</i>}O_<i>x</i> and show that we can change the band gap, crystal structure, and photochromic contrast by tuning the composition (O^2–:H^–) via the sputtering deposition pressure. To protect these films from rapid oxidation, we add a thin ALD coating of Al₂O₃, which increases the lifetime of the films from 1 day to several months. Encapsulation of the films also influences photochromic bleaching, changing the time dependency from first-order kinetics. As well, the partial annealing which occurs during the ALD process results in a dramatically slower bleaching speed, revealing the importance of defects for the reversibility (bleaching speed) of photochromism.