Seiichiro Nakabayashi

Determination of performance on tunnel conduction through molecular wire using a conductive atomic force microscope

Performance of nonresonant tunnel conduction through a self-assembled monolayer of conjugated molecules fabricated on gold (111) was determined by virtue of nanometer-scale electrical probe measurement using a conductive atomic force microscope. Electrical measurements with nanometer spatial resolution enabled mapping of tunnel current as well as efficiency of tunnel conduction through molecular wire by analyzing length dependence on current. A series of conjugated molecules with different numbers of oligothiophene rings proved to possess a high tunnel-conduction efficiency.

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.

In situ optical second harmonic generation studies of electrochemical deposition of tellurium on polycrystalline gold electrodes

The electrodeposition of tellurium (Te) on polycrystalline gold electrodes was studied using in situ optical second harmonic generation (SHG) at two different excitation wavelengths. On excitation at 1064 nm, the SH signal decreased dramatically with the first Te underpotential deposition (upd) and increased slightly with the second upd and the bulk deposition of Te. There was a linear correlation between the square root of the SH intensity from the surface and the surface coverage of Te, although the proportionality constant of this correlation for the first Te upd was different from those for the second upd and bulk deposition. This is possibly because of the stronger interaction of the first upd layer of Te than the second and bulk layer of Te with the substrate surface. However, on excitation at 585 nm, the behavior of the SH signal was more complicated, especially during the desorption process of the first Te monolayer. An increase in the SH intensity was observed with the first Te upd, but this increase in SHG still continued in the potential region in which no deposition of Te occurred. These results are interpreted by considering the existence of an optical resonance at the Au surface covered with a Te monolayer.

Reaction pathway of four-electron oxidation of formaldehyde on platinum electrode as observed by in situ optical spectroscopy

Abstract When a platinum electrode is dipped into aqueous sulfuric acid solution containing formaldehyde, the surface is covered by CO, which is formed by the dissociative adsorption of formaldehyde. A second harmonic generation voltammogram pumped by a 532 nm laser pulse and photo-reflectance voltammogram probed by a 670 nm laser beam are sensitive to the surface CO and surface oxide, respectively. The experiments demonstrate that the anodic oxidation pathway of the formaldehyde through CO adsorption proceeds by coupling with the surface oxide formation and that the CO controls the dual reaction pathway, the anodic and cathodic path of the reaction, and induces a non-linear self-sustained oscillation. The kinetics of the dissociative adsorption is discussed based on the time course of the current induced by the laser desorption of CO.

Anomalous branching ratio in the femtosecond surface chemistry of O2Pd(111)

In this paper we examine the surface chemistry of O 2 /Pd(111) under conditions of dense electronic excitation generated by femtosecond laser pulses. In contrast to the cases of conventional photo-activation and thermal activation, with femtosecond excitation the branching ratio for desorption/dissociation is found to favor desorption strongly. The results are discussed in terms of a recently developed model for desorption induced by multiple electronic transitions (DIMET).

Experimental evidence for the hydrogen evolution site in photocatalytic process on Pt/TiO2

Abstract A hydrogen-deuterium isotope effect on the title reaction is observed. The separation factor of the reaction is found to be 5.3 ± 0.5, which is similar to the separation factor reported on Pt electrode, but different from that observed on TiO 2 electrode. The Pt sites are assigned to the reduction site on the catalyst.

Nonlinear Iron Electrochemical Oscillator: Coupling and Photo Effects

The constant potential oxidation of iron in aqueous sulfuric acid solution induces a self-sustained nonlinear oscillation of the current. Here, two independently controlled iron electrodes become synchronized to each other as the distance between the electrodes is reduced. The temporal pattern of the synchronization is a function of the potential difference between the electrodes. The oscillation frequency of the iron electrode can be changed by irradiation with visible light.

Magnetic field effect on electrochemical oscillations during iron dissolution

A fairly low magnetic field of ca. 30 mT was found to affect the period and the amplitude of the self-sustained current oscillation of an iron electrode. This was observed much distinctively when the direction of the applied field was normal to the electrode surface, i.e., non-magnetohydrodynamic (MHD) configuration. The Flade potential of the iron electrode was not affected by the magnetic field intensity of up to 4 T. The observed magnetic field effect was attributed to the depression of the natural convection in the vicinity of the electrode surface which was caused by the two local paramagnetic body forces, the magnetic field gradient force and/or the concentration gradient force.

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.

Coherence and coupling during oscillatory metal electrodissolution

The spatio‐temporal behavior during periodic current oscillations in the Fe H2SO4 system is studied for a one-dimensional electrode surface. It is found that during the oscillatory response the electrode surface is never in the same state at the same time but the surface potential forms an accelerating front. The mode of communication between the individual regions of the reacting surface is explored by dividing the system into two identical coupled oscillators. The response is studied by either dividing the electrode surface with an insulating film of infinitesimal thickness or by a physical partition. The behavior of the coupled oscillators can be simple in-phase synchronization, k:n phase locking, out-of-phase locking or complex, depending on the length of the insulating film or the use of the partition and the applied potential. The results are compared with a model considering a simple coupling through the electrode potentials of the single oscillators. It is proposed that the coupling patterns can be attributed to the electrical coupling and the distribution of the frequency of the individual oscillators.