Thin films of rare-earth (RE)–oxygen–hydrogen compounds prepared by reactive magnetron sputtering show a unique color-neutral photochromic effect at ambient conditions. While their optical properties ...have been studied extensively, the understanding of the relationship between photochromism, chemical composition, and structure is limited. Here we establish a ternary RE–O–H composition-phase diagram based on chemical composition analysis by a combination of Rutherford backscattering and elastic recoil detection. The photochromic films are identified as oxyhydrides with a wide composition range described by the formula REO x H3–2x where 0.5 ≤ x ≤ 1.5. We propose an anion-disordered structure model based on the face-centered cubic unit cell where the O2– and H– anions occupy tetrahedral and octahedral interstices. The optical band gap varies continuously with the anion ratio, demonstrating the potential of band gap tuning for reversible optical switching applications.
Rare-Earth oxyhydrides (REH3-2xOx) are characterized by photodarkening when illuminated by photons having an energy exceeding that of the band gap. We propose that the film is segregated in hydrogen ...rich and hydrogen poor areas. Upon illumination, the excited electrons reduce the three-valent cations inducing an insulator to metal transition in the hydrogen rich entities. These small metallic oxyhydride clusters are responsible for the enhanced optical absorption. In the surrounding semiconductor matrix the photoexcitation induces a transition from p to n-type conductivity. This persistent photoconductivity is due to trapping of the holes by hydride ions. As a result, the Fermi level rises above the conduction band inducing a Burstein-Moss effect and a large increase in the conductivity.
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 NdH3–2x O x and show that we can change the band gap, ...crystal structure, and photochromic contrast by tuning the composition (O2–: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.