First-Principles Calculations on the Crystal/Electronic Structure and Phase Stability of H‑Doped SrFeO₂

It has been recently reported that H-doping of SrFeO₂ induces an insulator-to-metal transition but generates only a small amount of carrier electrons. To investigate the crystal structure of H-doped SrFeO₂ (SFOH) and the origin of its peculiar transport properties, we performed DFT-based first-principles calculations. Through structural sampling and total energy calculations, we showed that the doped hydrogen atoms exist in hydride form (H^–). Incorporation of the hydride drastically changed the valence state or the d-band configuration of the Fe ions adjacent to the doped hydride, resulting in a metallic density of states in specific hydride configurations. Thermodynamic analysis revealed that the formation of an insulating phase with O-site hydride was energetically preferable, but metallic phases with O-site hydride or interstitial hydride could also be present at reaction temperature, suggesting that SFOH is a mixture of an insulating matrix and metallic domains. This two-phase model accounted for the observed low carrier density as well as the metallic transport properties

Photoelectrochemical Behavior of Self-Assembled Ag/Co Plasmonic Nanostructures Capped with TiO₂

The use of localized surface plasmon resonance induced by Ag nanostructures is a promising way for high-efficiency photoelectric conversion. In plasmonic photoelectric conversion devices, however, the chemical instability of Ag in ambient atmosphere and its immediate deterioration have been a critical issue. Here, we propose a Ag–Co nanostructure array embedded in a TiO₂ matrix as a plasmonic resonator that ensures long-term stability. We also developed an electrochemical process to remove surface Co nanoclusters protecting fresh Ag from exposure to air. This enabled us to “unseal” Ag at the desired time. Furthermore, we confirmed photoelectric conversion using Ag–Co–TiO₂ nanocomposite films in contact with solution; the photoelectric conversion was substantially enhanced by the plasmon resonance of the Ag nanorods. The Ag nanostructures sealed in a TiO₂ matrix are expected to be used in other application fields, such as catalytisis and sensing, in which a fresh Ag surface is needed

Reversible Changes in Resistance of Perovskite Nickelate NdNiO₃ Thin Films Induced by Fluorine Substitution

Perovskite nickel oxides are of fundamental as well as technological interest because they show large resistance modulation associated with phase transition as a function of the temperature and chemical composition. Here, the effects of fluorine doping in perovskite nickelate NdNiO₃ epitaxial thin films are investigated through a low-temperature reaction with polyvinylidene fluoride as the fluorine source. The fluorine content in the fluorinated NdNiO_3–<i>x</i>F_<i>x</i> films is controlled with precision by varying the reaction time. The fully fluorinated film (<i>x</i> ≈ 1) is highly insulating and has a bandgap of 2.1 eV, in contrast to NdNiO₃, which exhibits metallic transport properties. Hard X-ray photoelectron and soft X-ray absorption spectroscopies reveal the suppression of the density of states at the Fermi level as well as the reduction of nickel ions (valence state changes from +3 to +2) after fluorination, suggesting that the strong Coulombic repulsion in the Ni 3d orbitals associated with the fluorine substitution drives the metal-to-insulator transition. In addition, the resistivity of the fluorinated films recovers to the original value for NdNiO₃ after annealing in an oxygen atmosphere. By application of the reversible fluorination process to transition-metal oxides, the search for resistance-switching materials could be accelerated