Repeatable Photoinduced Insulator-to-Metal Transition in Yttrium Oxyhydride Epitaxial Thin Films

Abstract

Currently, no known material retains its photoinduced metallic conductivity over a long time while also exhibiting repeatable metal-to-insulator recovery. Here, we demonstrate such a highly repeatable photoinduced insulator-to-metal transition in yttrium oxyhydride (YO x H y ) epitaxial thin films. The temperature (T) dependence of the electrical resistivity (ρ) of the films transforms from the insulating to the metallic state (dρ/dT > 0) under ultraviolet laser illumination. The YO x H y film recovers its original insulating state when heated (125 °C) under an Ar atmosphere and regains metallic conductivity when subsequently subjected to ultraviolet laser illumination again, showing a repeatable photoinduced insulator-to-metal transition. First-principles calculations show that the itinerant carriers originate from the variations in the charge states of the hydrogen atoms that occupy octahedral interstitial sites. This study indicates that tuning the site occupancy (octahedral/tetrahedral) of the hydrogen atoms exerts a significant effect on the photoresponse of metal hydrides.

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

“…On the other hand, there can be a large difference in the properties of octahedral versus tetrahedral H – , making the decrease in the population of certain H – potentially significant for the observed properties of the material. Not only are the octahedral H – likely the first to leave the structure upon oxidation, , but they are often cited as more mobile than tetrahedral H – either due to the lower formation energy for an octahedral H vacancy or their weaker electrostatic interactions with O 2– (more distance) . Having these mobile H – could be an essential ingredient to the formation and dissolution of a “darkened” phase.…”

“… 5 , 7 One explanation for photochromism has emerged involving a structural rearrangement, 8 , 9 perhaps by local diffusion, 10 to segregate a metallic phase 1 , 11 , 12 of high H – content. 7 , 11 , 12 On the other hand, some suggest the formation of in-gap states by H 2 13 or OH – 14 generation as well as H – exchange between phases. 15 …”

“…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 …”

Correlating Photoconductivity and Optical Properties in Oxygen‐Containing Yttrium Hydride Thin Films

“…Recently published works [17,21] have shown that epitaxial thin films of YHO (originally insulating) exhibit metal-like temperature dependence of resistivity after UV laser exposure. In our study, polycrystalline YHO thin films photodarkened by 455 nm LED have showed similar behavior, that is, the resistance of the photodarkened film increased with temperature as expected for a metal (see Figure S2, Supporting Information).…”

Selective Epitaxial Growth of Ca2NH and CaNH Thin Films by Reactive Magnetron Sputtering under Hydrogen Partial Pressure Control

“…Furthermore, hydrogen may exist in three different charge states, i.e., positive H + , neutral H 0 , and negative H – . The hydrogen charge states in hydrogen-containing materials play a major role in their physical, mechanical, and electrochemical properties. , Conventionally, the electrochemical conversion between H + and H 0 has been extensively used in energy storage devices, such as fuel cells and H 2 storage systems. , Recently, H – has attracted considerable attention due to its novel applications in hydride-ion batteries, NH 3 synthesis catalysts, − and photochromic materials. − …”