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

Abstract

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 x O x 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 Al 2 O 3 , 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.

Aliovalent Calcium Doping of Yttrium Oxyhydride Thin Films and Implications for Photochromism

“…Ca-doped yttrium oxyhydride thin films ((Ca z Y 1– z )H x O y , ∼ 300 nm) were prepared by reactive magnetron cosputtering of Ca (MaTecK, 99.9%) and Y (Stanford, 99.99%) metal targets onto 10 × 10 mm 2 quartz plates (MaTecK) at room temperature (∼21 °C) and an Ar/H 2 (7:1 flow) atmosphere. Following from our previous work on REH 3–2 x O x thin films (RE = Sc, Y, Nd, Gd, Dy, and Er), 2 − 5 the combined Ar/H 2 deposition pressure ( p dep ) affects the as-deposited RE dihydride; we found that if p dep is below a critical pressure ( p * ∼ 0.4 Pa for Y), 2 , 4 the film remains a metallic RE dihydride, but above p *, it forms a semiconducting oxyhydride upon ambient air exposure ( Figure 1 ). Similarly, we find that upon air exposure the as-deposited CaY hydride films become semiconducting, although some of the films already seem to incorporate oxygen when measured in the glovebox before air exposure (glovebox: [H 2 O] and [O 2 ] < 0.1 ppm), perhaps due to oxidation from residual gases.…”

“…Rare-earth metal oxyhydrides (REH 3–2 x O x ) are multianion compounds which have gained attention in recent years because they exhibit a photochromic effect . Thin films of REH 3–2 x O x (RE = Sc, Y, Nd, Gd, Dy, and Er) − are transparent semiconductors which “darken”, or become opaque, upon exposure to light with photon energy greater than their band gap ( E incident > E g ). When this light is removed, the materials return to their transparent state.…”

“…Many properties have been evaluated for their influence on photochromism such as the anion and cation compositions , and the defects or inhomogeneities present in the film. , One explanation for photochromism has emerged involving a structural rearrangement, , perhaps by local diffusion, to segregate a metallic phase ,, of high H – content. ,, On the other hand, some suggest the formation of in-gap states by H 2 or OH – generation as well as H – exchange between phases …”

Co-Sputtering Crystal Lattice Selection for Rare Earth Metal-Based Multi Cation and Mixed Anion Photochromic Films

“…Chaykina. et al enhanced the stability of NdO x H y by adding an alumina protective layer via ALD (atomic layer deposition), but the surface encapsulation layer enhanced the bleaching kinetics of the films [ 20 ]. Lu et al improved the generation energy of nanocrystals by doping Sr into CsPbI 3 nanocrystals, replacing Pb to cause a slight lattice shrinkage, and the environmental stability was enhanced [ 21 , 22 , 23 ].…”

Large Polaron Conduction, Photoconductivity, and Photochromism in GdOxH3−2x Oxyhydride Thin Films

“…[2][3][4] Photochromism and photo-conductivity in polycrystalline rare-earth oxyhydride thin films were first reported in 2011 on Y-based compounds. [5] Since then, several other REs (RE=Sc, Y, Nd, Sm, Gd, Dy, Er) [6][7][8][9][10] were found to form photochromic oxyhydride thin films, suggesting that these properties are rather general to the whole material class. Common to all of them is the REO x H 3 − 2x composition and a crystal structure based on the CaF 2 -type lattice.…”