Masao Takahashi

Bio: Masao Takahashi is an academic researcher from Osaka University. The author has contributed to research in topics: Passivation & Thin film. The author has an hindex of 20, co-authored 128 publications receiving 1804 citations. Previous affiliations of Masao Takahashi include Tokyo University of Technology.

Nitric acid oxidation of Si to form ultrathin silicon dioxide layers with a low leakage current density

Abstract: Ultrathin silicon dioxide (SiO2) layers with excellent electrical characteristics can be formed using the nitric acid oxidation of Si (NAOS) method, i.e., by immersion of Si in nitric acid (HNO3) solutions. The SiO2 layer formed with 61 wt % HNO3 at its boiling temperature of 113 °C has a 1.3 nm thickness with a considerably high density leakage current. When the SiO2 layer is formed in 68 wt % HNO3 (i.e., azeotropic mixture with water), on the other hand, the leakage current density (e.g., 1.5 A/cm2 at the forward gate bias, VG, of 1 V) becomes as low as that of thermally grown SiO2 layers, in spite of the nearly identical SiO2 thickness of 1.4 nm. Due to the relatively low leakage current density of the NAOS oxide layer, capacitance–voltage (C–V) curves can be measured in spite of the ultrathin oxide thickness. However, a hump is present in the C–V curve, indicating the presence of high-density interface states. Fourier transformed infrared absorption measurements show that the atomic density of the SiO...

Theoretical calculations on the structures, electronic and magnetic properties of binary 3d transition metal nitrides

Abstract: The electronic structures of a number of binary 3d transition metal and iron nitrides, some of which still need to be synthesized, have been investigated by means of spin-polarized first principles band structure calculations (TB-LMTO-ASA). The chemical bonding in all compounds has been clarified in detail through the analysis of total and local densities-of-states (DOS) and crystal orbital Hamilton populations (COHP). The binary transition metal nitride set includes ScN, TiN, VN, CrN, MnN, FeN, CoN and NiN, both in the sodium chloride as well as in the zinc blende structure type. Antibonding metal-metal interactions for higher electron counts are significantly weaker in the zinc blende type, thus favoring this structural alternative for the later transition metal nitrides.

Ultrathin silicon dioxide layers with a low leakage current density formed by chemical oxidation of Si

Abstract: Chemical oxidation of Si by use of azeotrope of nitric acid and water can form 1.4-nm-thick silicon dioxide layers with a leakage current density as low as those of thermally grown SiO2 layers. The capacitance–voltage (C–V) curves for these ultrathin chemical SiO2 layers have been measured due to the low leakage current density. The leakage current density is further decreased to ∼1/5 (cf. 0.4 A/cm2 at the forward gate bias of 1 V) by post-metallization annealing at 200 °C in hydrogen. Photoelectron spectroscopy and C–V measurements show that this decrease results from (i) increase in the energy discontinuity at the Si/SiO2 interface, and (ii) elimination of Si/SiO2 interface states and SiO2 gap states.

The XAFS beamline BL01B1 at SPring-8.

Abstract: aJASRI, SPring-& Mikazuki, Sayo-gun, Hyougo 679-5198, Japan, bRIKEN, SPring-& Kamigori, Ako-gun, Hyougo 6795198, Japan, CjAERI Kansai, SPring-& Mikazuki, Sayogun, Hyougo 679-5143, Japan, dlnstitute of Scientific and Industrial Reseach, Osaka University, Ibaraki, Osaka 5670047, Japan, eFaculty of Science, Okayama University, Okayama 700, Japan, tDepartment of Molecular Engineering, Kyoto University, Kyoto 606-5801, Japan, gFaculty of Science, Osaka University, Toyonaka, Osaka 560, Japan, "Fundamental Research Laboratry, NEC, Tsukuba, Ibaragi 305, Japan. Emaihurugat@springS.or.jp

Energetics of binary iron nitrides

Abstract: High-temperature solution calorimetry in molten sodium molybdate 3Na 2 O·4MoO 3 was used to determine the energetics of formation of a series of binary iron nitrides: γ′-Fe 4 N, e-Fe 3 N 1+ y ( y =0, 0.10, 0.22, 0.30, 0.33), ζ-Fe 2 N and γ′′-FeN 0.91 . The linear relation Δ H ° f (FeN x )=−65.23 x +13.48 kJ mol −1 was found between the enthalpies of formation from the elements at 298 K of iron nitrides FeN x and their nitrogen content x . Using this linear approximation, the enthalpy of formation of α′′-Fe 16 N 2 has been estimated to Δ H ° f (Fe 16 N 2 )=85.2±46.8 kJ mol −1 .

Stability of polymer solar cells.

Abstract: Organic photovoltaics (OPVs) evolve in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) has in the last decade been increased by almost a factor of ten approaching 10%. A main concern has been the stability that was previously measured in minutes, but can now, in favorable circumstances, exceed many thousands of hours. This astonishing achievement is the subject of this article, which reviews the developments in stability/degradation of OPVs in the last five years. This progress has been gained by several developments, such as inverted device structures of the bulk heterojunction geometry device, which allows for more stable metal electrodes, the choice of more photostable active materials, the introduction of interfacial layers, and roll-to-roll fabrication, which promises fast and cheap production methods while creating its own challenges in terms of stability.

Interaction of nanostructured metal overlayers with oxide surfaces

Abstract: Interactions between metals and oxides are key factors to determine the performance of metal/oxide heterojunctions, particularly in nanotechnology, where the miniaturization of devices down to the nanoregime leads to an enormous increase in the density of interfaces. One central issue of concern in engineering metal/oxide interfaces is to understand and control the interactions which consist of two fundamental aspects: (i) interfacial charge redistribution — electronic interaction, and (ii) interfacial atom transport — chemical interaction. The present paper focuses on recent advances in both electronic and atomic level understanding of the metal–oxide interactions at temperatures below 1000 ∘C, with special emphasis on model systems like ultrathin metal overlayers or metal nanoclusters supported on well-defined oxide surfaces. The important factors determining the metal–oxide interactions are provided. Guidelines are given in order to predict the interactions in such systems, and methods to desirably tune them are suggested. The review starts with a brief summary of the physics and chemistry of heterophase interface contacts. Basic concepts for quantifying the electronic interaction at metal/oxide interfaces are compared to well-developed contact theories and calculation methods. The chemical interaction between metals and oxides, i.e., the interface chemical reaction, is described in terms of its thermodynamics and kinetics. We review the different chemical driving forces and the influence of kinetics on interface reactions, proposing a strong interplay between the chemical interaction and electronic interaction, which is decisive for the final interfacial reactivity. In addition, a brief review of solid–gas interface reactions (oxidation of metal surfaces and etching of semiconductor surfaces) is given, in addition to a comparison of a similar mechanism dominating in solid–solid and solid–gas interface reactions. The main body of the paper reviews experimental and theoretical results from the literature concerning the interactions between metals and oxides (TiO2, SrTiO3, Al2O3, MgO, SiO2, etc.). Chemical reactions, e.g., redox reactions, encapsulation reactions, and alloy formation reactions, are highlighted for metals in contact with mixed conducting oxides of TiO2 and SrTiO3. The dependence of the chemical interactions on the electronic structure of the contacting metal and oxide phases is demonstrated. This dependence originates from the interplay between interfacial space charge transfer and diffusion of ionic defects across interfaces. Interactions between metals and insulating oxides, such as Al2O3, MgO, and SiO2, are strongly confined to the interfaces. Literature results are cited which discuss how the metal/oxide interactions vary with oxide surface properties (surface defects, surface termination, surface hydroxylation, etc.). However, on the surfaces of thin oxide films grown on conducting supports, the effect of the conducting substrates on metal–oxide interactions should be carefully considered. In the summary, we conclude how variations in the electronic structure of the metal/oxide junctions enable one to tune the interfacial reactivity and, furthermore, control the macroscopic properties of the interfaces. This includes strong metal–support interactions (SMSI), catalytic performance, electrical, and mechanical properties.

Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X-ray Absorption Techniques

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Nanocrystalline calcium phosphate ceramics in biomedical engineering

Abstract: Nanocrystalline calcium phosphate based bioceramics are the new rage in biomaterials research. Conventionally, calcium phosphates based materials are preferred as bone grafts in hard tissue engineering because of their superior biocompatibility and bioactivity. However, this group of bioceramics exhibits poor mechanical performance, which restricts their uses in load bearing applications. The recent trend in bioceramic research is mainly concentrated on bioactive and bioresorbable ceramics, i.e. hydroxyapatite, bioactive glasses, tricalcium phosphates and biphasic calcium phosphates as they exhibit superior biological properties over other materials. In recent times, the arena of nanotechnology has been extensively studied by various researchers to overcome the existing limitations of calcium phosphates, mainly hydroxyapatite, as well as to fabricate nanostructured scaffolds to mimic structural and dimensional details of natural bone. The bone mineral consists of tiny HAp crystals in the nano-regime. It is found that nanocrystalline HAp powders improve sinterability and densification due to greater surface area, which could improve the fracture toughness and other mechanical properties. Nano-HAp is also expected to have better bioactivity than coarser crystals. Nanocrystalline calcium phosphate has the potential to revolutionize the field of hard tissue engineering from bone repair and augmentation to controlled drug delivery devices. This paper reviews the current state of knowledge and recent developments of various nanocrystalline calcium phosphate based bioceramics from synthesis to characterization.

Thiolato-complexes of the transition metals

Abstract: A. Introduction B. Complexes with terminal thiolato-ligands (i) Mononuclear homoleptic complexes with monoand bi-dentate thiolates _ (ii) Mononuclear complexes with heteroligands (a) Oxo-complexes (b) Complexes with metal-nitrogen multiple bonds (c) Miscellaneous monomeric thiolato complexes with other heteroligands . (iii) Complexes with sterically hindered thiolato-ligands (a) Chromium, molybdenum and tungsten (b) Manganese, technetium and rhenium (c) Iron, ruthenium and osmium. (d) Cobalt, rhodium and iridium. (e) Copper, silver and gold C. Complexes with CL’-bridging thiolato-ligands (i) Introduction (ii) Dinuclear and linear polynuclear complexes with two p2-thiolato-ligands . . (a) Molybdenum ..... (b) Manganese, technetium and rhenium (c) Iron (d) Cobalt (e) Nickel, palladium and platinum (f) Copper, silver and gold. (g) Zinc, cadmium and mercury (iii) Complexes with three Ir_2-thiolato-ligands (iv) Complexes with quadruple r2-thiolato-ligands (v) Three-dimensional clusters (a) Clusters with tetrahedral metal centres (b) Copper and silver (c) Nickel, palladium and platinum (d) Other cluster types D. Complexes with p’-bridging thiolato-ligands E. Synthesis of thiolato-complexes . .