Narumi Ohta

Giant optical second-harmonic generation in single and coupled microcavities formed from one-dimensional photonic crystals

The nonlinear optical properties of one-dimensional all-solid-state photonic-crystal microcavities (MCs) are experimentally studied by second-harmonic generation (SHG) spectroscopy in both the frequency and the wave-vector domains. The studied single and coupled MCs are formed by the alternating of mesoporous silicon layers of different porosities. When the fundamental radiation is in resonance with the MC mode the second-harmonic intensity is enhanced by a factor of approximately 102. The resonant SHG response is compared with the off-resonance response, as the fundamental wavelength is outside the photonic bandgap. The splitting of the modes of two identical coupled MCs is observed in the wave-vector domain spectrum of enhanced SHG. The SHG enhancement is attributed to the combined effects of the spatial localization of the fundamental field in the MC spacer and the fulfillment of the phase-matching conditions. The confinement of the resonant fundamental field is probed directly at the MC cleavage by a scanning near-field optical microscope. The role of the phase matching that is associated with the giant effective dispersion in the spectral vicinity of the MC mode is deduced from a comparison with the SHG peaks at both edges of the photonic bandgap.

Porous silicon-based ferroelectric nanostructures

A procedure is suggested for the preparation of porous silicon-based ferroelectric nanostructures. It is demonstrated that the method of chemical deposition from solutions provides for the penetration of the initial components of the solution into the matrix pores, and subsequent annealing leads to the crystallization of the ferroelectric phase. The diagnostics of the ferroelectric properties is performed using the method of generation of second optical harmonic. The spectral characteristics of the prepared ferroelectric nanostructures are investigated.

Dye sensitization of synthetic p-type diamond electrode

A synthetic semiconductive p-type diamond electrode was photosensitized by the Ru(bpy)32+ molecule in an electrolyte solution. The photoexcited molecule injects a hole into the valence band. However, under a certain potential, charge recombination occurred through interband states; the hole on the top of the valence band recombined with the photoreduced Ru(bpy)31+. The mechanism of this charge recombination was studied by measuring the transient photopotential and the intrinsic sub-band gap photoresponse of the electrode.

Electrochemical modification of surface morphology of Au/Ti bilayer films deposited on a Si prism for in situ surface-enhanced infrared absorption (SEIRA) spectroscopy.

Surface-enhanced infrared absorption (SEIRA)-active Au/Ti bilayer films sputter deposited on Si substrates have been prepared by an electrochemical annealing (ECA) treatment for the first time. The application of Au/Ti bilayer films on Si substrates to the spectroscopic technique is a promising alternative to the conventional technique using directly deposited Au films on Si substrates, offering excellent adhesive durability of the deposited metal films. However, Au/Ti bilayer films have never been selected for the spectroscopy technique because the films in the as-prepared state exhibit relatively smooth surface morphology: the excitation of the localized surface plasmon is vital to achieving SEIRA enhancements but could hardly be observed on the smooth morphology. It is shown by ex situ scanning tunneling microscopy measurements that the unfavorable smooth morphology of the as-prepared Au/Ti bilayer films can be modified by the ECA treatment to a reasonably rough, island-structure morphology similar to that of the conventional SEIRA-active Au films. In situ infrared absorption spectroscopy of adsorbed sulfate anions has been conducted on the Au/Ti bilayer film both before and after ECA treatment. The spectroscopy measurements demonstrate that the SEIRA activity of the film after being subjected to the treatment is significantly improved so that the technique could detect adsorbates on the film electrodes even with the submonolayer coverage. As an additional benefit, the ECA treatment has brought about a substantial increase in the fraction of Au(111) domains on the polycrystalline Au film surfaces. Accordingly, this approach enables us to prepare SEIRA-active Au films having sufficient adhesion to the Si substrates as well as the highly preferred (111) orientation.

Deprotonation of a dinuclear copper complex of 3,5-diamino-1,2,4-triazole for high oxygen reduction activity

A dinuclear copper(II) complex of 3,5-diamino-1,2,4-triazole is one of the highly active copper-based catalysts for the oxygen reduction reaction (ORR) in basic solutions. Our in situ X-ray absorption near edge structure measurements revealed that deprotonation of the triazole ligand might cause coordination geometrical changes, resulting in the enhancement of the ORR activity.

Ferroelectrics templated in nanoporous silicon membranes

A new method is proposed of fabrication of ferroelectric nanostructures using porous silicon as a template. It is shown that with the pore size of about 20 nm conventional sol-gel technique provides embedding of guest material only into the top layer of the host. Optical second harmonic generation is used as a probe to distinguish between paraelectric and ferroelectric phases of embedded nanoparticles.

Layered (1−x−y)LiNi[sub 1/2]Mn[sub 1/2]O[sub 2]⋅xLi[Li[sub 1/3]Mn[sub 2/3]] O[sub 2]⋅yLiCoO[sub 2] (0≤x=y≤0.3 and x+y=0.5) Cathode Materials

An exploration of new cathode materials along the two lines having the nominal compositions of (1 - x - y)LiNi1/2Mn1/2O2.xLi[Li1/3Mn2/3]O-2.yLiCoO(2) (0 less than or equal to x = y less than or equal to 0.3 and x + y = 0.5) was performed in the phase diagram of LiNi1/2Mn1/2O2-Li[Li1/3Mn2 3]O-2-LiCoO2 by a simple solid-state reaction. XRD results identified that all materials are pure phases with layered structure, indicating good structural compatibility between three layered compounds, LiNi1/2Mn1/2O2, Li[Li1/3Mn2/3]O-2, and LiCoO2. Li[Li1/3Mn2/3]O-2 and LiCoO2 contents in these electrode materials presented important effects on structure and electrochemical properties. Our results demonstrated that the suitable structural integration of Li[Li1/3Mn2/3]O-2 and LiCoO2 components into LiNi1/2Mn1/2O2 structure is a practicable way to prepare new electrode materials with the desired electrochemical performances even via a simple solid-state reaction. Among the materials investigated in this work, a reversible capacity of 180-200 mAh/g was obtained for the materials with 0.3 less than or equal to x + y less than or equal to 0.6 and excellent cycling performance was observed with a large x (x greater than or equal to 0.25) value in the series of x + y = 0.5

All Solid-State Photoelectrochemical Cell with RbAg4 I 5 as the Electrolyte

The photoelectrochemical reaction was investigated at a type of photosensitizing interface between a solid electrolyte and a semiconducting electrode. RbAg 4 I 5 thin film was used as a solid electrolyte and n-Si single-crystal plate as a photoanode. This cell showed an incident photon flux to photocurrent conversion efficiency higher than 70% in a wide range of wavelengths from visible to near-infrared lights (500-900 nm).

Porous amorphous silicon film anodes for high-capacity and stable all-solid-state lithium batteries

Owing to its high theoretical capacity of ~4200 mAh g−1 and low electrode potential (<0.35 V vs. Li+/Li), utilising silicon as anode material can boost the energy density of rechargeable lithium batteries. Nevertheless, the volume change (~300%) in silicon during lithiation/delithiation makes stable cycling challenging. Since some of the capacity fading mechanisms do not function in solid electrolytes, silicon anodes exhibit better cycling performance in solid electrolytes than liquids. Nonetheless, capacity can fade rapidly because of the difficulties in maintaining mechanical integrity in thick/bulky electrodes, especially when high active material loading is employed to deliver practically useful areal capacity. By contrast, silicon nanostructures can relieve deformation-induced stress and enhance cycling performance. Here we report enhanced cycling performances achieved using nanostructured silicon films and inorganic solid electrolyte and show that amorphous porous silicon films maintain high capacity upon cycling (2962 mAh g−1 and 2.19 mAh cm−2 after 100 cycles).Silicon is a promising anode material owing to its high theoretical capacity, but can be unstable over charge/discharge cycles owing to a large volume change during cycling. Here, the authors report improved stability (>99.8% efficiency over 100 cycles) using porous silicon films with inorganic solid electrolyte.

Characterization of n-Si ∕ RbAg4I5 Interfaces by Photocurrent Measurements and Electrochemical Impedance Spectroscopy

A photoelectrochemical cell with an n-Si/RbAg 4 I 5 interface showed a high open-circuit potential (V o c = 0.69 V) and a high incident photon-to-current conversion efficiency (>70%) in a wide range of wavelengths from visible to near-infrared light (500-900 nm). An electronic structure model at the interface was proposed from electrochemical data involving electrochemical impedance spectra to elucidate the origin of the high V o c . The flatband potential was determined to be -0.09 V obtained from a Mott-Schottky plot, which led to band-edge potentials of -0.33 and 0.79 V for the conduction band and the valence band, respectively. The redox potential of the I - /I 2 couple (0.65 V), which donates electrons to photogenerated holes in the valence band, was higher but close to the band-edge potential of the valence band, resulting in the high V o c .