Near-Ultraviolet Light-Driven Photocathodic Activity for (001)-Oriented BiOCl Thin Films Synthesized by Mist Chemical Vapor Deposition.

Semitransparent and homogeneous bismuth oxychloride (BiOCl) thin films with (001) preferred orientation were synthesized on polycrystalline Sn:In2O3-glass substrates by mist chemical vapor deposition. The films showed photocathodic activity even under near-ultraviolet light within the band gap due to the in-gap states induced by oxygen vacancies. Higher synthesis temperatures resulted in a significant increase of photocurrent density under ultraviolet light. While the longer lifetime of photocarriers led to an increase of internal quantum efficiency, the larger band-edge absorption significantly contributed to the higher external quantum efficiency.

A high-TC heavy rare earth monoxide semiconductor TbO with a more than half-filled 4f orbital.

Terbium monoxide, TbO, with an unusual valence state of Tb2+ ([Xe]4f85d1) has been known to exist as a gas phase. In this study, we report a solid phase rock-salt structured TbO in the form of an epitaxial thin film. This TbO is a narrow gap ferromagnetic semiconductor with a bandgap of ∼0.1 eV and high Curie temperature of 231 K, and exhibited negative magnetoresistance of -14% at 9 T and 2 K. Its magnetization steeply increased with the appearance of a wasp-waisted hysteresis curve significantly below the Curie temperature at about 20 K. These results suggested that both 4f and 5d magnetic sublattices were formed at a single magnetic Tb2+ site in TbO.

Synthesis of Stoichiometric SrTiO3 and Its Carrier Doping from Air-Stable Bimetallic Complexes.

We demonstrate the synthesis of perovskite oxide SrTiO3 with ideal cation stoichiometry and homogeneous La doping using air-stable Sr-Ti and La-Ti bimetallic peroxo complexes with a 1:1 cation ratio. Phase-pure SrTiO3, La2Ti2O7, and LaTiO3 were successfully synthesized by thermal decomposition of those complexes. La-doped SrTiO3 was obtained by mixing the Sr-Ti and La-Ti complexes in an acid solution, followed by thermal decomposition. La-doped SrTiO3 showed systematic chemical trends in the lattice constant and electrical conduction. Not only those bulk polycrystals but also cation-stoichiometric SrTiO3 epitaxial thin films were grown with an atomically flat surface from the Sr-Ti complex.

Epitaxial growth of bismuth oxyhalide thin films using mist CVD at atmospheric pressure.

We report the epitaxial growth of bismuth oxyhalide BiOX (X = Cl, Br, and I) thin films using mist chemical vapour deposition at atmospheric pressure. The thin films grown under optimum conditions possessed atomically flat surfaces and high crystallinity, where the lattice constants of BiOX were controlled by epitaxial strain.

Tetragonality induced superconductivity in anti-ThCr2Si2-type RE2O2Bi (RE = rare earth) with Bi square nets.

We report a series of layered superconductors, anti-ThCr2Si2-type RE2O2Bi (RE = rare earth), composed of electrically conductive Bi square nets and magnetic insulating RE2O2 layers. Superconductivity was induced by separating the Bi square nets as a result of excess oxygen incorporation, irrespective of the presence of magnetic ordering in RE2O2 layers. Intriguingly, the transition temperature of all RE2O2Bi including nonmagnetic Y2O2Bi was approximately scaled by unit cell tetragonality (c/a), implying a key role in the relative separation of the Bi square nets to induce superconductivity.

Anion-Substitution-Induced Nonrigid Variation of Band Structure in SrNbO3- xN x (0 ≤ x ≤ 1) Epitaxial Thin Films.

Pervoskite oxynitrides exhibit rich functionalities such as colossal magnetoresistance and high photocatalytic activity. The wide tunability of physical properties by the N/O ratio makes perovskite oxynitrides promising as optical and electrical materials. However, composition-dependent variation of the band structure, especially under partially substituted composition, is not yet well understood. In this study, we quantitatively analyzed the composition-dependent variation of band structure of a series of SrNbO3- xN x (0 ≤ x ≤ 1.02) epitaxial thin films. Electrical conductivity decreased along with the increase of N content x as a result of an increase in Nb valence from 4+ to 5+. Optical measurements revealed that the N 2p band is formed at a critical composition between 0.07 < x < 0.38, which induces charge-transfer transition (CTT) in the visible-light region. These variations in the band structure were explained by first-principles calculations. However, the CTT energy slightly increased at higher N contents (i.e., lower carrier density) on contrary to the expectation based on the rigid-band-like shift of the Fermi level, which suggests a complex combination of the following band-shifting effects induced by N-substitution: whereas (1) reduction of the Burstein-Moss effect causes CTT energy reduction, (2) enhancement of hybridization between Nb 4d and N 2p orbitals and/or (3) suppression of many-body effects enlarge the band gap energy at larger N content. The band structure variation in perovskite oxynitride as presently elucidated would be a guidepost for future material design.

Superconductivity in Anti-ThCr2Si2-type Er2O2Bi Induced by Incorporation of Excess Oxygen with CaO Oxidant.

Recently, superconductivity was induced by expanding interlayer distance between Bi square nets in anti-ThCr2Si2-type Y2O2Bi through incorporation of excess oxygen with increased nominal amount of oxygen. However, such oxygen incorporation was applicable to only Y2O2Bi among R2O2Bi ( R = rare earth metal), probably due to a larger amount of oxygen incorporation for Y2O2Bi. In this study, the interlayer distance in Er2O2Bi was increased by cosintering with CaO, which served as an oxidant, indicating that excess oxygen was incorporated in Er2O2Bi. As a result, superconductivity was induced in Er2O2Bi at 2.2 K.

Superconductivity of Rock-Salt Structure LaO Epitaxial Thin Film.

We report a superconducting transition in a LaO epitaxial thin film with the superconducting transition onset temperature ( Tc) at around 5 K. This Tc is higher than those of other lanthanum monochalcogenides and opposite to their chemical trend: Tc = 0.84, 1.02, and 1.48 K for LaX (X = S, Se, Te), respectively. The carrier control resulted in a dome-shaped Tc as a function of electron carrier density. In addition, the Tc was significantly sensitive to epitaxial strain in spite of the highly symmetric crystal structure. This rock-salt superconducting LaO could be a building block to design novel superlattice superconductors.

New Lutetium Oxide: Electrically Conducting Rock-Salt LuO Epitaxial Thin Film.

C-rare earth structure lutetium sesquioxide Lu2O3 has been recognized as a high-k widegap insulator with closed shell Lu3+ ions. In this study, rock-salt structure lutetium monoxide LuO with unusual valence of Lu2+ (4f145d1), previously known as the gaseous phase, was synthesized as an epitaxial thin film by the pulsed laser deposition method. In contrast with transparent and highly insulating Lu2O3, LuO possessed a dark-brown color and high electrical conductivity concomitant with strong spin-orbit coupling as a manifestation of Lu 5d electron carriers.

Strain Engineering for Anion Arrangement in Perovskite Oxynitrides.

Mixed-anion perovskites such as oxynitrides, oxyfluorides, and oxyhydrides have flexibility in their anion arrangements, which potentially enables functional material design based on coordination chemistry. However, difficulty in the control of the anion arrangement has prevented the realization of this concept. In this study, we demonstrate strain engineering of the anion arrangement in epitaxial thin films of the Ca1-xSrxTaO2N perovskite oxynitrides. Under compressive epitaxial strain, the axial sites in TaO4N2 octahedra tend to be occupied by nitrogen rather than oxygen, which was revealed by N and O K-edge linearly polarized X-ray absorption near-edge structure (LP-XANES) and scanning transmission electron microscopy combined with electron energy loss spectroscopy. Furthermore, detailed analysis of the LP-XANES indicated that the high occupancy of nitrogen at the axial sites is due to the partial formation of a metastable trans-type anion configuration. These results are expected to serve as a guide for the material design of mixed-anion compounds based on their anion arrangements.

Lithium ion battery peformance of silicon nanowires with carbon skin.

Silicon (Si) nanomaterials have emerged as a leading candidate for next generation lithium-ion battery anodes. However, the low electrical conductivity of Si requires the use of conductive additives in the anode film. Here we report a solution-based synthesis of Si nanowires with a conductive carbon skin. Without any conductive additive, the Si nanowire electrodes exhibited capacities of over 2000 mA h g(-1) for 100 cycles when cycled at C/10 and over 1200 mA h g(-1) when cycled more rapidly at 1C against Li metal. In situ transmission electron microscopy (TEM) observation reveals that the carbon skin performs dual roles: it speeds lithiation of the Si nanowires significantly, while also constraining the final volume expansion. The present work sheds light on ways to optimize lithium battery performance by smartly tailoring the nanostructure of composition of materials based on silicon and carbon.