Kazuhiro Fukami

Electrodeposition of Noble Metals into Ordered Macropores in p-Type Silicon

The electrodeposition of noble metals, i.e., platinum, palladium, and gold, into macroporous p-type silicon was examined. For platinum and palladium, the electrodeposition proceeded preferentially from the pore bottom to the opening when sodium chloride was used as a supporting electrolyte. When sodium sulfate was used as a supporting electrolyte, the electrodeposition mainly proceeded at pore openings, leading to plugging. For gold electrodeposition, a condition for achieving pore filling from the bottom was not found in either the NaCI or Na 2 SO 4 solutions. Pore depth was another key factor to achieve continuous filling by electrodeposition. As the depth of pores became deeper, the electrodeposition proceeded preferentially from the bottom. The effect of mass transfer in pores was also investigated by changing the concentration of metal ions and applied potential.

Simultaneous observation of nascent plasma and bubble induced by laser ablation in water with various pulse durations

We investigate the effects of pulse duration on the dynamics of the nascent plasma and bubble induced by laser ablation in water. To examine the relationship between the nascent plasma and the bubble without disturbed by shot-to-shot fluctuation, we observe the images of the plasma and the bubble simultaneously by using two intensified charge coupled device detectors. We successfully observe the images of the plasma and bubble during the pulsed-irradiation, when the bubble size is as small as 20 μm. The light-emitting region of the plasma during the laser irradiation seems to exceed the bubble boundary in the case of the short-pulse (30-ns pulse) irradiation, while the size of the plasma is significantly smaller than that of the bubble in the case of the long-pulse (100-ns pulse) irradiation. The results suggest that the extent of the plasma quenching in the initial stage significantly depends on the pulse duration. Also, we investigate how the plasma-bubble relationship in the very early stage affects the shape of the atomic spectral lines observed at the later delay time of 600 ns. The present work gives important information to obtain high quality spectra in the application of underwater laser-induced breakdown spectroscopy, as well as to clarify the mechanism of liquid-phase laser ablation.

Single-Pulse Underwater Laser-Induced Breakdown Spectroscopy with Nongated Detection Scheme

We investigated spatially resolved emission spectra of Al atoms in a very small (∼0.1 mm) laser ablation plasma produced by a single long-pulse (∼100 ns) irradiation of an Al target in water. The spectral feature varied considerably, depending on the position to be measured. The density of the plasma periphery was low enough to neglect the self-absorption effect, even when resonance lines were observed. By properly selecting the position, we successfully obtained well-resolved spectral lines even without time-gated detection. This suggests that time-gating is not necessary anymore in the practical applications of underwater laser-induced breakdown spectroscopy when employing spatially resolved detection system.

Gold Nanostructures for Surface-Enhanced Raman Spectroscopy, Prepared by Electrodeposition in Porous Silicon

Electrodeposition of gold into porous silicon was investigated. In the present study, porous silicon with ~100 nm in pore diameter, so-called medium-sized pores, was used as template electrode for gold electrodeposition. The growth behavior of gold deposits was studied by scanning electron microscope observation of the gold deposited porous silicon. Gold nanorod arrays with different rod lengths were prepared, and their surface-enhanced Raman scattering properties were investigated. We found that the absorption peak due to the surface plasmon resonance can be tuned by changing the length of the nanorods. The optimum length of the gold nanorods was ~600 nm for surface-enhanced Raman spectroscopy using a He–Ne laser. The reason why the optimum length of the gold nanorods was 600 nm was discussed by considering the relationship between the absorption peak of surface plasmon resonance and the wavelength of the incident laser for Raman scattering.

On-Site Quantitative Elemental Analysis of Metal Ions in Aqueous Solutions by Underwater Laser-Induced Breakdown Spectroscopy Combined with Electrodeposition under Controlled Potential

We propose a technique of on-site quantitative analysis of Zn(2+) in aqueous solution based on the combination of electrodeposition for preconcentration of Zn onto a Cu electrode and successive underwater laser-induced breakdown spectroscopy (underwater LIBS) of the electrode surface under electrochemically controlled potential. Zinc emission lines are observed with the present technique for a Zn(2+) concentration of 5 ppm. It is roughly estimated that the overall sensitivity over 10 000 times higher is achieved by the preconcentration. Although underwater LIBS suffers from the spectral deformation due to the dense plasma confined in water and also from serious shot-to-shot fluctuations, a linear calibration curve with a coefficient of determination R(2) of 0.974 is obtained in the range of 5-50 ppm.

Synergetic effects of double laser pulses for the formation of mild plasma in water: toward non-gated underwater laser-induced breakdown spectroscopy.

We experimentally study the dynamics of the plasma induced by the double-laser-pulse irradiation of solid target in water, and find that an appropriate choice of the pulse energies and pulse interval results in the production of an unprecedentedly mild (low-density) plasma, the emission spectra of which are very narrow even without the time-gated detection. The optimum pulse interval and pulse energies are 15-30 μs and about ~1 mJ, respectively, where the latter values are much smaller than those typically employed for this kind of study. In order to clarify the mechanism for the formation of mild plasma we examine the role of the first and second laser pulses, and find that the first pulse produces the cavitation bubble without emission (and hence plasma), and the second pulse induces the mild plasma in the cavitation bubble. These findings may present a new phase of underwater laser-induced breakdown spectroscopy.

In Situ Electrode Surface Analysis by Laser-Induced Breakdown Spectroscopy

A method of in situ elemental analysis of the electrode surface in electrolytic solution is proposed. The method is based on the measurement of emission spectra from the plume induced by the laser ablation of an electrode surface. A Cu-Zn binary system electrodeposited on a Pt plate has been employed to demonstrate the applicability of this method to electrode surfaces, and quantitative analysis of the sample surface has been attempted. Also, some improvement is shown for the pulse-to-pulse fluctuation in the measurement of relative intensity by using several spectral lines instead of a single line for each element.

Self-Organized Periodic Growth of Stacked Hexagonal Wafers in Synchronization with a Potential Oscillation in Zinc Electrodeposition

Electrodeposition of zinc (Zn) from a strongly alkaline Zn(II) solution showed a potential oscillation in the region of the current density exceeding the diffusion-limited one, accompanied by the formation of dendrites with a stacked wafer structure. Scanning electron microscopic inspection showed that the dendrites changed their shape periodically in synchronization with the potential oscillation from thick hexagonal wafers on the negative potential side to thin maple-leaf-like wafers on the positive potential side and inversely. In situ electrochemical quartz crystal microbalance measurements revealed that the rate of Zn deposition also oscillated in synchronization with the oscillation. A mechanism is proposed to explain the oscillation and the dendrite formation by taking into account coupling of autocatalytic Zn-crystal growth, in close relation with Mullins-Sekerka instability, and autocatalytic surface oxidation at the close packed crystal faces leading to surface passivation.

Two-dimensional space-resolved emission spectroscopy of laser ablation plasma in water

We developed a method for two-dimensional space-resolved emission spectroscopy of laser-induced plasma in water to investigate the spatial distribution of atomic species involved in the plasma. Using this method, the laser ablation plasma produced on a Cu target in 5 mM NaCl aqueous solution was examined. The emission spectrum varied considerably depending on the detecting position. The temperature and the atomic density ratio NNa/NCu at various detecting positions were evaluated by fitting emission spectra to a theoretical model based on the Boltzmann distribution. We are successful in observing even a small difference between the distributions of the plasma parameters along the directions vertical and horizontal to the surface. The present approach gives direct information for sound understanding of the behavior of laser ablation plasma produced on a solid surface in water.

Structural considerations on multistopband mesoporous silicon rugate filters prepared for gas sensing purposes

Different designs for producing multiple stopband mesoporous silicon rugate filters via electrochemical anodization are compared. The effects of light absorption and dispersion to visible range filter design are investigated. Thermal oxidation is applied for passivating the chemically reactive porous silicon surface, and the response of the passivated structures to ethanol vapor is examined. Differences in gas sensing properties for the various designs are evaluated and possible reasons for the observed differences are discussed. Methods for sidelobe suppression in multipeak filters are discussed and demonstrated, and their effects in gas sensing applications are estimated.