Photochromic mechanism in oxygen-containing yttrium hydride thin films: An optical perspective

Abstract

Oxygen-containing yttrium hydride thin films exhibit photochromic behavior: Transparent thin films reversibly switch from a transparent state to a photodarkened state after being illuminated with UV or blue light. From optical spectrophotometry and ellipsometry measurements of the transparent state and photodarkened state, it is concluded that the photochromic effect can be explained by the gradual growth, under illumination, of metallic domains within the initial wide-band-gap semiconducting lattice. This conclusion is supported by Raman measurements. DOI: 10.1103/PhysRevB.95.201301 Oxygen-containing yttrium hydride films exhibit photochromic (PC) behavior, i.e., the optical properties of the films change reversibly when illuminated by light of adequate energy (wavelengths in the blue or UV range). Early works by Hoekstra et al. [1] reported photoconductivity in yttrium hydrides at low temperature, and Ohmura et al. [2,3] accidentally discovered PC behavior in yttrium hydride films subjected to high pressure. In addition, Huiberts et al. observed for the first time the gasochromic behavior of yttrium hydride thin films [4]. Later, Mongstad et al. [5,6] reported PC behavior in transparent oxygen-rich yttrium hydride films under atmospheric conditions and at room temperature. In the latter case, however, the yttrium hydride films were directly obtained by reactive magnetron sputtering rather than by the subsequent hydrogenation of a predeposited metallic Y layer. Oxygen-rich yttrium hydride is not the only oxygen-containing hydride which exhibits interesting physical properties. For instance, Miniotas et al. have reported gigantic resistivity and band-gap changes in oxygen-containing gadolinium hydride [7].The mechanism of the PC behavior in oxygen-rich yttrium hydride is still unclear and seems to have no relation to the PC mechanism reported for transition-metal oxides [8]. In the present Rapid Communication, the properties of oxygen-rich transparent semiconducting yttrium hydride thin films-hereafter referred to as YH x O sc w , where the superscript sc refers to their semiconducting character-and of opaque metallic yttrium hydride thin films-from now on referred to as YH y O m z , where y < x and where the superscript m refers to their metallic character-have been investigated by grazing incidence x-ray diffraction (GIXRD), Raman spectroscopy, ellipsometry, and spectrophotometry. Both sets of films, YH x O sc w and YH y O m z , were deposited onto soda-lime glass substrates by sputter deposition at a hydrogen/argon ratio = 0.18 and 0.13, respectively, and then exposed to air where they oxidize. A detailed description of the deposition method can be found in our previous work [9] Chandran et al. [8], which reports changes of the hydrogen species in oxygen-containing yttrium hydride after illumination, suggesting the release of electrons and the formation of a metallic phase.Figure 1(a) shows GIXRD patterns-obtained by using Cu Kα radiation at a fixed angle of incidence of 2• in a Bruker Siemens D5000 diffr...

Spectroscopic Ellipsometry and Microstructure Characterization of Photochromic Oxygen‐Containing Yttrium Hydride Thin Films

“…Films deposited at at low Γ , such as S1, are known to incorporate smaller amounts of oxygen when exposed to air, and hence retain the characteristics of YH 2 , i.e., a smaller lattice parameter of ≈5.2 Å and an opaque appearance . In fact, the oxygen atomic content this films has been found to be five times smaller than in films such as S4, whereas aluminum‐coated films exhibit an oxygen concentration 10 times smaller than their homologous uncapped films . At low enough Γ , the metallic YH 0.067 phase can be obtained (see S1 in Figure ), as observed before by Chandran et al also in the case of oxygen‐containing yttrium hydride thin films.…”

“…The measurement of T at the photodarkened state has been performed in‐situ using an Ocean Optics spectrophotometer QE65000 and the solar simulator as light source. According to our previous work, the photochromic change of the optical properties of the YH x :O y thin films can be quantitatively modeled by assuming a gradual growth, under illumination, of metallic domains within the initial wide‐band gap transparent material, in accordance to the Maxwell–Garnett approximation. Thus, the only parameter allowed to vary in the re‐fit of the photodarked films was the filling factor, i.e., the volume fraction of the metallic phase inclusions.…”

“…Apart from the long‐term stability, there are many open questions regarding YH x :O y including the origin of the photochromic behavior. According to our previous work, the photodarkening is caused by the formation, upon illumination, of metallic domains within the transparent material …”

Photochromism of rare-earth metal-oxy-hydrides

“…We find a value of 5.38 6 0.01 Å for YO x H y which is within the range of values reported in earlier publications. 7,8 Remarkably, the MO x H y lattice constants follow the trend of the lanthanide contraction of the bixbyite-M 2 O 3 compounds (open triangles). 17 Hence, cation substitution allows for tuning of the unit cell dimensions of rare-earth based MO x H y and could thereby enable tailoring of optical and electrical properties.…”

“…6 This polycrystalline semiconductor has an optical bandgap of about 2.6 eV and an fcc crystal structure with a lattice constant of 5.35-5.40 Å . 7,8 The initially transparent material photodarkens in a wide spectral range covering the visible (VIS) and near infrared (NIR) with a time constant in the order of minutes. 6 Hence, YO x H y is a promising material for application in energy saving smart windows and other chromogenic devices.…”