Naoki Kishimoto

Effects of high magnetic field on the morphology of carbon nanotubes and selective synthesis of fullerenes

Carbon nanotubes and fullerenes were synthesized by arc discharge in a He gas atmosphere under a high magnetic field up to 10 T. Morphology and the yield of these nanotubes were investigated by scanning electron microscopy and transmission electron microscopy (TEM). The yield and type of fullerenes were also evaluated by ultraviolet visual spectroscopy. TEM images revealed that thin collapsed nanotubes having larger curvature were synthesized under a high magnetic field, in particular at 10 T. Furthermore, the ratio of C70 to C60 concentration was found to vary depending on the magnitude of the magnetic field during arc discharge. Based on these results, the effects of magnetic field on the synthesis of carbon nanotubes and fullerenes are discussed.

Photoconductivity evolution due to carrier trapping by defects in 17 MeV‐proton irradiated silicon

Damage concentration dependence of dark and photoconductivity has been studied in crystalline Si, irradiated with 17 MeV proton. The photoconductivity spectra consist of a broad peak due to a band‐to‐band transition and a tail spectrum on the lower energy side. Two kinds of tail spectra are observed in impurity‐doped and/or damaged specimens, and the latter threshold is deeper than the former. While the main peak of the doped specimen does not greatly change up to a certain irradiation fluence φC, it steeply decreases beyond the φC, which depends on the shallow‐impurity concentration. The tail spectrum of the shallow impurity simultaneously vanishes at the φC and another tail spectrum grows above the φC. As compared to fluence dependence of photocurrent decay time, it is clarified that the drastic decrease in the main peak results from a drop in the decay time or the carrier density, not in the carrier mobility. The defect density for the critical fluence φC has good correlation with the dopant concentrat...

Dispersion of nonlinear dielectric function of Au nanoparticles in silica glass

We have investigated wavelength dispersion of photo-induced nonlinear dielectric function of Au nanoparticle materials. Transient transmission and reflection spectra were sequentially measured by the pump-probe method with a femtosecond laser system. The dispersion of real and imaginary parts of the nonlinear dielectric function of Au:SiO(2) nanoparticle material in the vicinity of the surface plasmon resonance was evaluated from these transient spectra with total differential. A local electromagnetic field factor and interband transition in Au nanoparticles directly dominate the dispersion.

Stable photoconductivity in metastable a-Si:H under high-energy proton irradiation

Abstract To study the effects of high-energy protons on optoelectronic properties of a-Si:H, in situ measurements of photon- and proton-induced conductivities have been conducted using 17 MeV-proton irradiation. The protons and monochromized light ranging from 0.7 to 2.4 eV are intermittently irradiated samples of a-Si:H. The band-to-band photoconductivity (PC) of a-Si:H showed reversible response to the light, including some Staebler–Wronski effect. Although the PC of crystalline Si (c-Si) greatly decreased with increasing the proton fluence, that of a-Si:H maintained PC-response, which slightly increased to a dose of 6×1013 ion/cm2 and then decreased at the larger doses. Radiation-induced conductivity (RIC) occurred during the beam-on, with a time-constant of 60 s. Also, dark conductivity had a persistent excited conductivity lasting for hours, after terminating the beam. While the RIC-response of a-Si:H was metastable, PC stability of a-Si:H was 103 times better than that of c-Si. A mechanism of the stable PC is discussed with the metastability induced by the protons.

White luminescence from silica glass containing red/green/blue luminescent nanocrystalline silicon particles

Silica glasses containing blue/green/red luminescent nanocrystalline silicon (nc-Si) particles that consist of monolayer and/or three-layer structures were fabricated by a radio-frequency sputtering technique and postannealing treatment. These silica glasses showed very broad luminescence spectra with a peak at 460 nm (blue light), 550 nm (green light), and 800 nm (red light). When these samples were irradiated by using a xenon lamp with an optical bandpass filter of 313 nm, the luminescence colors from these silica glasses were a white light. The white luminescence of the sample with the three-layer structure exhibited the high luminance value of 1.5 cd/m2. This value was ascribed to the adjustment of sizes and densities of blue/green/red luminescent nc-Si particles, and the lowering of densities of Pb centers (nonradiative recombination centers) at the nc-Si particle/silica glass interface layer.

Electronic excitation and optical responses of metal-nanoparticle composites under heavy-ion implantation

Measurements of optical transmission and ion-induced photon emission (IIPE) during implantation of copper ions into silica glass (a-SiO2) have been combined in order to study formation of Cu nanoparticles. It has been shown that in situ measurements are more advantageous than conventional experimental approaches of examining the properties of ion-implanted nanocomposites solely after implantation. Series of experiments have been done to prove that the band of IIPE at 545–550 nm originates from Cu+ solutes in a-SiO2. The combination of in situ optical techniques provides means of monitoring Cu nanoparticles and Cu solutes via the optical absorption in the range of the surface-plasmon resonance (SPR) of nanoparticles and the IIPE of Cu+ solutes. It has been shown that a simple linear approximation can be used to separate optical bands of defects, Cu solutes, and nanoparticles. Ion-induced transient optical absorption has been found in the ranges of SPR and defect bands. The transient optical response in the...

High-Current Heavy-Ion Accelerator System and Its Application to Material Modification

A high-current heavy-ion accelerator system has been developed to realize intense particle fluxes for material modification. The facility of a tandem accelerator attained 1 mA-class ion current both for negative low-energy ions and positive high-energy ions. A negative ion source of the key device is of plasma-sputter type, equipped with multi-cusp magnets and Cs supply. The intense negative ions are either directly used for material irradiation at 60 keV or further accelerated up to 6 MeV after charge transformation. Application of negative ions, alleviating surface charging, enables us to conduct low-energy high-current irradiation to insulating substrates. Since positive ions over MeV are irrelevant to the Coulomb repulsion, the facility as a whole meets high-current irradiation into insulators over a wide energy range. Application of high flux ions provides technological merits not only in efficient implantation but also in different material kinetics. Other characteristics of the system are co-irradiation of intense laser and in-situ detection of kinetic processes. For the material modification, we present nanoparticle fabrication in insulators, and synergistic phenomena by co-irradiation of ions and photons.

Radiation-induced differential optical absorption of metal nanoparticles

A method of measuring the temperature of metal nanoparticles under ion bombardment is proposed. Optical absorption in the range of the surface plasmon resonance of metal nanoparticles was measured during implantation of 3MeV Cu2+ ions into silica glass to derive a difference in optical absorption between beam on and off regimes. The radiation-induced differential (RD) spectra were similar to the spectra of thermomodulation (TM) and quite different from the spectra of nonlinear optical response measured by the pump-probe method. Increasing amplitude of RD and TM spectra was assigned to an increase of lattice temperature of Cu nanoparticles.

Saturation of nonlinear optical absorption of metal-nanoparticle composites

Metal-nanoparticle composites consisting of copper and gold nanoparticles embedded in insulators (Cu:SiO2, Au:SiO2, and Cu:Al2O3) have been fabricated by metal-ion implantation. Nonlinear optical (NLO) absorption of the nanocomposites has been examined by the Z-scan method with a tunable femtosecond laser. We have measured (a) NLO absorption spectra near the surface-plasmon resonance and (b) laser-intensity dependence of NLO absorption, α(I). The results are compared to the data reported on NLO response for various laser pulse durations. By analyzing the laser-intensity dependence of Z-scan signals, the saturation intensity of light, IS, the coefficient of NLO absorption, β, and the radius of the beam waist, w0, are derived. This approach takes into account the saturation of optical absorption, whereas the conventional approach using the linear approximation α(I)≈α0+βI cannot do it. The analysis shows that transient optical absorption, |α(I)−α0|, decreases with shortening the laser pulse duration. The dec...

Fabrication of amorphous carbon nitride films by hot-wire chemical vapor deposition

Abstract Amorphous carbon nitride (a-C 1− x N x :H) films were fabricated by hot-wire chemical vapor deposition (HWCVD) using a mixture of C 2 H 2 and NH 3 gases. The nitrogen concentration of a-C 1− x N x :H films increased as the ratio of the NH 3 flow to the total gas flow rate increased. Infrared absorption peaks due to vibration of the triple bond between carbon and nitrogen (CN) were not observed for a-C 1− x N x :H films prepared by HWCVD, suggesting that the amount of CN bonding was reduced by this method.