Photochromic YO_xH_y Thin Films Examined by <i>in situ</i> Positron Annihilation Spectroscopy

Abstract

Doppler broadening positron annihilation spectroscopy depth profiles were collected on photochromic YOxHy thin films. In situ UV illumination of photochromic semiconductor YOxHy films leads to an increase in S-parameter and a large reduction in W -parameter, possibly caused by a change in the charge state of vacancies or the growth of hydrogen-rich metallic Y(Ox)Hy clusters, albeit that vacancy formation or changes in positronium formation during illumination might also play a role. Intriguingly, both the S-and W -parameters increase during thermal bleaching, indicating that another process takes place. The Doppler parameters do not return to their initial values after complete thermal bleaching, suggesting that persistent local rearrangements of vacancies and possibly hydride ions have occurred during the full photodarkening-thermal bleaching cycle. Positron annihilation lifetime spectroscopy shows that a small fraction of positronium is formed in as-deposited YOxHy films, indicating that the films contain some nanopores.

Influence of Cation (RE = Sc, Y, Gd) and O/H Anion Ratio on the Photochromic Properties of REOxH3–2x Thin Films

“…After a full cycle of photodarkening and bleaching, we observe irreversible changes in the S/W parameters of all samples, evidence that permanent light-induced structural modifications occur in Y oxyhydrides sputtered at any deposition pressure. In particular, we find the same trend originally reported by Plokker et al, where upon in-situ illumination S increases and W decreases, suggesting a permanent formation of positron trapping sites that might include negatively charged cation monovacancies ( V Y ) or neutrally charged vacancy clusters (e.g., V Y – V H , V H – V H , V O – V H ). , Visually, we note that the direction of the change in the S/W plot is similar for all samples. This is quantified by the so-called R parameter ( R = Δ S /Δ W ), a defect-specific value that is independent of the concentration of defects and on the positron trapping efficiency .…”

Preferential Orientation of Photochromic Gadolinium Oxyhydride Films

“…It is then oxidized by exposure to air, resulting in photochromic polycrystalline oxyhydride films [11]. Several studies further attempted at each time to reveal the mechanisms of photochromism in rare-earth metal oxyhydrides through time resolved X-ray diffraction using synchrotron radiation [12], composition analysis [3,9,13] and positron annihilation spectroscopy (PAS) [14,15]. However, the answer for the exact mechanism for rare-earth metal oxyhydrides still debated.…”

Temperature‐Dependent Photochromic Performance of Yttrium Oxyhydride Thin Films

“…Of the rare‐earth‐metal oxyhydrides, yttrium oxyhydride (YHO) has been a strong subject of recent experimental [ 2,9–12 ] and theoretical [ 13,14 ] investigations. Thin films of YHO exhibit photochromic behavior, in which a decrease in the transmittance, and consequently an enhancement of light absorption, of the films (referred to as photodarkening) is observed upon illumination with photons with energies between 1.8 and 3.1 eV.…”