Osamu Watanabe

Positive onset potential and stability of Cu2O-based photocathodes in water splitting by atomic layer deposition of a Ga2O3 buffer layer

The Cu2O-based photocathode is considered as one of the most promising photocathodes for high performance water splitting under sunlight. However, the relatively negative onset potential for H2 production of these photocathodes impedes further optimization of the solar-to-fuel conversion efficiency. Here, a thin Ga2O3 buffer layer is introduced between the Cu2O absorber layer and the TiO2 protective layer by atomic layer deposition to increase the photovoltage. For the optimized TiO2 deposition temperature, the Pt/TiO2/Ga2O3/Cu2O electrode achieves a high cathodic photocurrent of −2.95 mA cm−2 at 0 V vs. RHE and an extremely positive onset potential of 1.02 V vs. RHE (defined as the potential where photocathodic current reaches 20 μA cm−2 under air-mass 1.5 global illumination), benefiting from a buried p–n junction and a favorable band alignment. The Pt/TiO2/Ga2O3/Cu2O electrodes exhibit a stable cathodic current for 2 h under continuous illumination of a 500 W Xe lamp for the TiO2 deposition temperatures below 180 °C.

Use of an IC Recorder as a voice output memory aid for patients with prospective memory impairment

This study evaluated the use of an IC Recorder as a voice output memory aid for patients with prospective memory impairment. The IC Recorder can output about 300 previously recorded messages at programmed times. The spoken messages prompted various daily tasks for eight patients with acquired memory impairments. One of these tasks, such as diary writing, or a letter-writing exercise was selected for each patient as a main task, and its completion was logged. These completion data demonstrated that the messages output by the IC Recorder were highly effective in prompting the main tasks for five of the eight patients. This study suggests that the IC Recorder has great potential as a voice output memory aid to assist patients with prospective memory impairment.

Unconventional salt trend from soft to stiff in single neurofilament biopolymers.

We present persistence length measurements on neurofilaments (NFs), an intermediate filament with protruding side arms, of the neuronal cytoskeleton. Tapping mode atomic force microscopy enabled us to visualize and trace at subpixel resolution photoimmobilized NFs, assembled at various subunit protein ratios, thereby modifying the side-arm length and chain density charge distribution. We show that specific polyampholyte sequences of the side arms can form salt-switchable intrafilament attractions that compete with the net electrostatic and steric repulsion and can reduce the total persistence length by half. The results are in agreement with present X-ray and microscopy data yet present a theoretical challenge for polyampholyte interchain interactions.

Non-inductive plasma current start-up by EC and RF power in the TST-2 spherical tokamak

Non-inductive plasma current start-up by EC and RF power was carried out on the TST-2 device. Low frequency RF (21 MHz) sustainment was demonstrated, and the obtained high βp spherical tokamak configuration has similar equilibrium values as the EC (2.45 GHz) sustained plasma. Equilibrium analysis revealed detailed information on three discharge phases: (i) in the initial current formation phase, the plasma current increases with the stored energy, and the current is in the same order as that predicted by theory. (ii) In the current jump phase, the current density profile, which is peaked near the outboard boundary, is not deformed but increases slowly and the initial closed flux surface appears when the current reaches a maximum. (iii) In the current sustained phase, equilibrium is characterized by the hollowness of the current density profile, and it determines the fraction of the current inside the last closed flux surface to the total current. Both EC and RF injections show a similar equilibrium. While MHD instabilities often terminate the RF sustained plasma, no such phenomenon was observed in the EC sustained plasma.

Virus-templated photoimprint on the surface of an azobenzene-containing polymer.

A photoimprint-based immobilization process is presented for cylindrical viruses on the surface of an azobenzene-bearing acrylate polymer by using atomic force microscopy (AFM). Tobacco mosaic virus (TMV), 18 nm in diameter and ca. 300 nm in length, was employed as a model virus. First, a droplet of an aqueous solution containing TMV was placed on the acrylate polymer surface. After drying the droplet, the polymer surface was irradiated with light at a wavelength of 470 nm from blue-light-emitting diodes. Finally, the surface was washed by aqueous solution with detergents. The polymer surface was observed at each step by AFM. TMV was shown to embed itself gradually on the polymer surface during photoirradiation in a time scale of tens of minutes because of the formation of the surface groove complementary to the shape of TMV. Analysis of immobilization efficiency of TMV on the polymer surface by the immunological enzyme luminescence indicated that efficiency increased proportional to the photoirradiation time. In these experimental conditions, the absorption band of the azobenzene moiety remained constant before and after the photoirradiation. These results show that TMV is physically held on the complementary groove formed on the polymer surface by the photoirradiation.

Two-step refractive index changes by photoisomerization and photobleaching processes in the films of non-linear optical polyurethanes and a urethane-urea copolymer

Photoinduced refractive index and absorption spectrum changes in the films of polyurethanes and a urethane-urea copolymer with large second-order non-linear coefficient d have been studied. These changes were explained by a two-step reaction scheme including a trans–cis photoisomerization followed by a photobleaching reaction. The former process was reversible and the latter was irreversible, so that a thermally stable refractive index was obtained. The changes were accelerated by heating. A channel waveguide was fabricated by the processes and light propagation through the waveguide was observed.

Stable Second-Order Optical Nonlinearity of Urethane-Urea Copolymers

Second-order nonlinear optical properties of corona-poled urethane-urea copolymers have been studied by the second-harmonic generation (SHG) method. As compared with a polyurethane, the copolymers exhibited higher optimum poling temperature and better temporal stability of SHG coefficients (d33). For one of the copolymers, d33 at a fundamental wavelength of 1064 nm was 190 pm/V, and remained at 75% of the initial value after heat treatment at 100° C for 300 h.

Simultaneous enhancement of photovoltage and charge transfer in Cu2O-based photocathode using buffer and protective layers

Coating n-type buffer and protective layers on Cu2O may be an effective means to improve the photoelectrochemical (PEC) water-splitting performance of Cu2O-based photocathodes. In this letter, the functions of the buffer layer and protective layer on Cu2O are examined. It is found that a Ga2O3 buffer layer can form a buried junction with Cu2O, which inhibits Cu2O self-reduction as well as increases the photovoltage through a small conduction band offset between the two semiconductors. The introduction of a TiO2 thin protective layer not only improves the stability of the photocathode but also enhances the electron transfer from the photocathode surface into the electrolyte, thus resulting in an increase in photocurrent at positive potentials. These results show that the selection of overlayers with appropriate conduction band positions provides an effective strategy for obtaining a high photovoltage and high photocurrent in PEC systems.

Development of a plasma current ramp-up technique for spherical tokamaks by the lower hybrid wave

Spherical tokamaks (STs) have the advantage of high beta capability, but the realization of a compact reactor requires the elimination of the central solenoid (CS). The possibility of using the lower hybrid wave (LHW) to ramp up the plasma current (Ip) from zero to a high enough level required for fusion burn in ST is examined theoretically and experimentally. Excitation of a travelling fast wave (FW) by the combline antenna installed on TST-2 was confirmed by a finite element analysis, but efficient current drive requires excitation of the LHW, either directly by the antenna or by mode conversion from the FW. The analysis using the TORLH full-wave solver indicates that core current drive by LHW is possible in the low-density, low Ip plasma formed by electron cyclotron heating (ECH). It is important to keep the density low during Ip ramp-up, and the wavenumber must be reduced as Ip increases. Initial results from TST-2 demonstrate that RF power in the LH frequency range (200 MHz) can achieve initial Ip formation, and is more effective than ECH for further ramp-up of Ip. Ip ramp-up to over 12 kA was achieved by combining ramp-up of the externally applied vertical magnetic field and ramp-up of the RF power. The significant asymmetry observed between co-current drive and counter-current drive is attributed to the presence of RF driven current. An optimized LHW antenna with appropriate polarization and wavenumber spectrum controllability is being designed. The success of the TST-2 experiment would provide a scientific basis for quantitatively evaluating the required CS capability for a low-aspect-ratio reactor.

Parametric decay instability during high harmonic fast wave heating experiments on the TST-2 spherical tokamak

A degradation of heating efficiency was observed during high harmonic fast wave (HHFW) heating of spherical tokamak plasmas when parametric decay instability (PDI) occurred. Suppression of PDI is necessary to make HHFW a reliable heating and current drive tool in high ? plasmas. In order to understand PDI, measurements were made using a radially movable electrostatic probe (ion saturation current and floating potential), arrays of RF magnetic probes distributed both toroidally and poloidally, microwave reflectometry and fast optical diagnostics in TST-2. The frequency spectrum usually exhibits ion-cyclotron harmonic sidebands f0 ? nfci and low-frequency ion-cyclotron quasi-modes (ICQMs) nfci. PDI becomes stronger at lower densities, and much weaker when the plasma is far away from the antenna. The lower sideband power was found to increase quadratically with the local pump wave power. The lower sideband power relative to the local pump wave power was larger for reflectometer compared with either electrostatic or magnetic probes. The radial decay of the pump wave amplitude in the SOL was much faster for the ion saturation current than for the floating potential. These results are consistent with the HHFW pump wave decaying into the HHFW or ion Bernstein wave (IBW) sideband and the low-frequency (ICQM). Two additional peaks were discovered between the fundamental lower sideband and the pump wave in hydrogen plasmas. The frequency differences of these peaks from the pump wave increase with the magnetic field. These decay modes may involve molecular ions or partially ionized impurity ions.