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.@en

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 NdH3-2xOx 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.

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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.@en

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 H _3-2x where 0.5 ≤ x ≤ 1.5. We propose an anion-disordered structure model based on the face-centered cubic unit cell where the O ^2- 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.

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Thin films of yttrium oxy-hydride have interesting, reversible photochromic properties, the origin of which is poorly understood. To investigate the role of point defect mobility, we probed the effect of lattice contraction as induced by the addition of zirconium. Interestingly, we find no loss of photochromic contrast for Y _1-z Zr _z O _x H _y films with a small Zr cationic fractions (z < 0.15). At larger fractions the photochromic contrast is reduced. Zirconium is found to slow down the bleaching process, which suggests that the mobility of point defects may play a role in the thermal bleaching process. However, we cannot rule out substitution of zirconium in the YO _x H _y lattice which may also affect the photochromic properties.

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