Hiroyasu Yamahara

Enhanced Photocurrent in Rh-Substituted α-Fe2O3 Thin Films Grown by Pulsed Laser Deposition

Rh-substituted α-Fe2O3 (Fe2-xRhxO3; 0.0≤x≤2.0) thin films were grown on α-Al2O3(110) substrates with a Ta-doped SnO2 electrode layer by pulsed laser deposition. Highly oriented epitaxial films with pure corundum structures were successfully fabricated over the entire compositional range. The optical absorption spectra of the films indicate narrowing of the bandgap with increasing Rh content. Consequently, the photoelectrochemical performance was improved in the Rh-substituted films. We found that the optimum Rh content lies at around x=0.2, where the photocurrent is significantly enhanced over a wavelength range of 340–850 nm.

CuO nanowire/microflower/nanowire modified Cu electrode with enhanced electrochemical performance for non-enzymatic glucose sensing

CuO nanowire/microflower structure on Cu foil is synthesized by annealing a Cu(OH)2 nanowire/CuO microflower structure at 250 °C in air. The nanowire/microflower structure with its large surface area leads to an efficient catalysis and charge transfer in glucose detection, achieving a high sensitivity of 1943 μA mM(-1) cm(-2), a wide linear range up to 4 mM and a low detection limit of 4 μM for amperometric glucose sensing in alkaline solution. With a second consecutive growth of CuO nanowires on the microflowers, the sensitivity of the obtained CuO nanowire/microflower/nanowire structure further increases to 2424 μA mM(-1) cm(-2), benefiting from an increased number of electrochemically active sites. The enhanced electrocatalytic performance of the CuO nanowire/microflower/nanowire electrode compared to the CuO nanowire/microflower electrode, CuO nanowire electrode and CuxO film electrode provides evidence for the significant role of available surface area for electrocatalysis. The rational combination of CuO nanowire and microflower nanostructures into a nanowire supporting microflower branching nanowires structure makes it a promising composite nanostructure for use in CuO based electrochemical sensors with promising analytical properties.

High spin polarization at room temperature in Ge-substituted Fe3O4 epitaxial thin film grown under high oxygen pressure

Epitaxial thin films of room-temperature ferrimagnetic (Fe,Ge)3O4 were fabricated using pulsed laser deposition. Films with a single-phase spinel structure were grown under high oxygen pressures (0.01–0.6 Pa). The carrier transport across (Fe,Ge)3O4/Nb:SrTiO3 interface was studied to estimate the spin polarization of (Fe, Ge)3O4. Current–voltage curves of Fe2.8Ge0.2O4/Nb:SrTiO3 junction showed rectifying behavior even at 300 K whereas Fe3O4/Nb:SrTiO3 junction showed ohmic behavior. Calculations based on a model for a Schottky contact with a ferromagnetic component yielded a spin polarization of 0.50 at 300 K for Fe2.8Ge0.2O4, indicating its potential as a promising spin injector.

Solid–liquid-type solar cell based on α-Fe2O3 heterostructures for solar energy harvesting

Photoelectrochemical cells based on Rh-substituted α-Fe2O3 films were fabricated by pulsed laser deposition. The optical bandgap of the films was found to decrease with increasing Rh content. X-ray photoemission spectroscopy analysis revealed that the bandgap narrowing in Rh-substituted films is caused by the hybridization of the Rh t2g band with the valence band of α-Fe2O3 near the Fermi level. As a result, the photoelectrochemical performance was improved in the Rh-substituted films in the visible and near-infrared regions. The photocurrent in films with a high orientation along the [110] direction was significantly larger than that in the polycrystalline films, which is possibly attributed to the anisotropic electrical conduction of α-Fe2O3.

Experimental observation of bulk band dispersions in the oxide semiconductor ZnO using soft x-ray angle-resolved photoemission spectroscopy

The electronic structure of the oxide semiconductor ZnO has been investigated using soft x-ray angle-resolved photoemission spectroscopy (ARPES). The obtained band dispersions within the kx−ky planes reflect the symmetry of the Brillouin zone and show no surface-state-derived flat bands. Band dispersions along the kz direction have also been observed. The obtained band dispersions qualitatively agree with band-structure calculations except for the bandwidth. The observations provide experimental evidence that soft x-ray ARPES enables us to study the bulk band structure of semiconductors.

Long-term potentiation of magnonic synapses by photocontrolled spin current mimicked in reentrant spin-glass garnet ferrite Lu3Fe5−2xCoxSixO12 thin films

(Co,Si)-cosubstituted Lu3Fe5O12 (Lu3Fe5−2xCoxSixO12; 0.00 ≤ x ≤ 1.00) thin films were grown on Y3Al5O12(001) substrates by pulsed laser deposition. Reentrant spin-glass behavior was observed and its robustness against external magnetic fields was improved by increasing the Co–Si content. In contrast, the spectral linewidth of ferromagnetic resonance was broadened by increasing the Co–Si content, which indicates that the Lu3Fe4.8Co0.1Si0.1O12 thin film is an optimal spin-glass spin-wave conductor. The spin pumping voltage of Lu3Fe4.8Co0.1Si0.1O12 exhibited short-term plasticity with photoinduced magnetic anisotropy and long-term potentiation with the photomemory effect, which mimicked the pre- and postsynaptic potentials of biological systems.

Hydroxyl radical and thermal annealing on amorphous InGaZnO4 films for DNA immobilizations.

The effect of hydroxyl radicals (OH) and thermal annealing on an amorphous InGaZnO4 (aIGZO) film surface was investigated for manipulation of DNA immobilization. X-ray photoemission and fluorescence measurements were conducted to reveal the status of surface OH coverage and DNA immobilization, respectively. Systematic examinations concerning OH termination on the film surface suggested that the surface coverage of OH leveling DNA immobilization was related to the local surface potential. Furthermore, OH affinity on the aIGZO film surface was sensitive to thermal annealing. A remarkable change in surface OH coverage was observed for the film surface annealed at high temperature. This behavior was framed by a structural change from amorphous to crystalline state, which regulated DNA immobilization. These results indicate that the OH affinity on aIGZO films is dependent on structural properties such as defects. This study suggests that an amorphous structure is critical for obtaining a high OH surface coverage governing DNA immobilization, and is hence more suitable for biosensing.

Fabrication and characterization of wüstite-based epitaxial thin films: p-type wide-gap oxide semiconductors composed of abundant elements

We report on the growth of FeO (wustite)-based thin films using pulsed laser deposition. Epitaxial films of iron oxide with different crystal structures (corundum, spinel, and rock-salt-type) were separately fabricated by controlling the Si content in the Fe2O3-SiO2 mixture target. The rock-salt-type Fe0.92Si0.08O was found to form a solid solution with MgO, whose bandgap could be widely tuned (2.7–6.0 eV) by altering the MgO fraction. Moreover, Fe0.92Si0.08O-MgO films show p-type semiconducting behavior with high electrical conductivity (0.1–7.8 S/cm) at 300 K. These results indicate that Fe0.92Si0.08O-MgO films are good candidates for transparent p-type semiconductors.

Spin-glass behaviors in carrier polarity controlled Fe3−xTixO4 semiconductor thin films

Carrier-type control of spin-glass (cluster spin-glass) is studied in order to engineer basic magnetic semiconductor elements using the memory functions of spin-glass. A key of carrier-polarity control in magnetite is the valence engineering between Fe(II) and Fe(III) that is achieved by Ti(IV) substitution. Single phases of (001)-oriented Fe3−xTixO4 thin films have been obtained on spinel MgAl2O4 substrates by pulsed laser deposition. Thermoelectric power measurements reveal that Ti-rich films (x = 0.8) show p-type conduction, while Ti-poor films (x = 0.6–0.75) show n-type conduction. The systematic Fe(III) reduction to Fe(II) followed by Ti(IV) substitution in the octahedral sublattice is confirmed by the X-ray absorption spectra. All of the Fe3−xTixO4 films (x = 0.6–0.8) exhibit ferrimagnetism above room temperature. Next, the spin-glass behaviors of Ti-rich Fe2.2Ti0.8O4 film are studied, since this magnetically diluted system is expected to exhibit the spin-glass behaviors. The DC magnetization and AC...

Highly spin-polarized current in Co-substituted Fe3O4 epitaxial thin films at room temperature

Epitaxial thin films of cobalt ferrite CoxFe3−xO4 (x = 0.0, 0.5, and 1.0) were fabricated on α-Al2O3 (001) substrates using pulsed laser deposition. It was found that the coercive force of the cobalt ferrite films could be tuned by changing the Co content. The films prepared under low oxygen pressure (1.0 × 10−6 Pa) showed semiconducting behavior even for x = 1.0. X-ray photoelectron spectroscopy revealed that the Co ions were 2+ for all compositions. On the other hand, it was found that the Fe ions were in the Fe2+/Fe3+ valence state, which may cause small-polaron hopping among the Fe-3d electrons in the films. An anomalous Hall effect was observed in the Co0.5Fe2.5O4 film even at 300 K, suggesting that carriers in the films were highly spin-polarized at room temperature.