Makoto Hirasawa

Size-dependent crystallization of Si nanoparticles

Crystallization temperature dependency on Si nanoparticles size was studied by using Raman scattering spectroscopy. Si nanoparticles synthesized by pulsed laser ablation were annealed at various temperatures while they were suspended in helium background gas, and then were classified by a differential mobility analyzer. After the size classification, Si nanoparticles showed a narrow size distribution which enabled investigation of the size-dependent crystallization. The temperature threshold for the transition from amorphous to crystalline (Tc) decreased as the particle size decreased: the Tc values of the 10, 8, 6, and 4nm particles were 1273, 1173, 1073 and 773K, respectively.

Tunable, narrow-band light emission from size-selected Si nanoparticles produced by pulsed-laser ablation

We have demonstrated narrow-band visible light emission from size selected silicon nanoparticles (np-Si), with a wavelength controlled by size tuning. The np-Si were synthesized by pulsed-laser ablation of a silicon single-crystal target in high-purity He background gas. A postannealing process improved morphology and crystallinity. Using a differential mobility analyzer, nanoparticles were classified with a diameter tunable from 3 to 6 nm. Monodispersed np-Si deposited on substrate exhibited a sharp photoluminescence band. The energy of this band increased from 1.34 to 1.79 eV with decrease in particle size, and narrowed to approximately 0.22 eV full width at half maximum due to highly resolved size-selection and improvement in crystallinity. The results suggest that tunable, narrow-band light emitting np-Si produced by gas phase synthesis have good possibilities for application as optoelectronic devices.

Fabrication of silicon nanostructured films by deposition of size-selected nanoparticles generated by pulsed laser ablation

Nanostructured films composed of size-controlled silicon nanoparticles were fabricated by pulsed laser ablation in helium background gas. The size distribution of the silicon nanoparticles in the gas phase was controlled and measured by the electrostatic size classification technique using a differential mobility analyzer coupled with an electrometer. The microstructures and crystallinity of the as-deposited and size-classified nanostructured films were analyzed by scanning electron microscopy and Raman spectroscopy, respectively. The effects of the background gas pressure and particle size on the morphology of the films are discussed.

Decomposition of toluene with surface-discharge microplasma device

A surface-discharge microplasma device (SMD) was developed for the decomposition of volatile organic compounds (VOCs) in the gas phase. The device is composed of a microscale-patterned electrode, a dielectric substrate, and a ground electrode. As a result of localized dielectric barrier discharge (DBD), surface-discharge microplasma was generated when a piezoelectric-transformed high voltage (66.7 kHz, 3.5 kV) was applied to the microscale-patterned electrode. The discharge current and voltage characteristics of the DBD were analyzed under atmospheric conditions. Toluene decomposition rate was evaluated by gas chromatography–mass spectrometry and nondispersive IR-absorption CO2 analysis. A decomposition efficiency of more than 99% was achieved in batch experiments. When the SMD was operated in a flow reactor, 30–80% of toluene was reduced with the percentage depending on residence time. The carbon balance between the toluene starting material and the CO2 product indicates that toluene was almost completely decomposed into CO2 by atomic oxygen in the microplasma.

Evaluation of Morphology and Size Distribution of Silicon and Titanium Oxide Nanoparticles Generated by Laser Ablation

Nanometer-sized particles of silicon and titanium oxide were generated by irradiating solid targets using a nanosecond pulsed-Nd : YAG laser in a low pressure atmosphere. A low pressure differential mobility analyzer (LP-DMA) was used to classify the size of the generated particles. The LP-DMA and electron microscopes (SEM and TEM) were used to measure the change in the size distribution and morphology of the generated particles with laser power density and system pressure. The size distribution of both silicon and titanium oxide ranged from two to one hundred nanometers in diameter depending on the laser power density and pressure. From the high resolution TEM observation and electron diffraction, it was found that the generated titanium oxide nanoparticles were composed of a ‘core’ of faceted metallic single crystals with an oxide layer shell’.

Effect of in situ annealing on structure and optical properties of ZnTe nanoparticles produced by pulsed laser ablation

An improvement in morphology, crystallinity, and optical property of ZnTe nanoparticles produced by pulsed laser ablation (PLA) was achieved by in situ annealing. ZnTe nanoparticles produced in argon gas ambience by PLA were annealed in the gas flow at a temperatures Ta ranging from 300 °C to 800 °C and size-selected by a differential mobility analyzer. The bimodal size distribution of the ZnTe nanoparticles changed to unimodal at Ta = 600 °C. In this condition, the shape of the monodispersed ZnTe nanoparticles, classified into around 20 nm, became uniformly spherical and their crystallinity estimated by x-ray diffraction was extremely improved. These improvements by the in situ annealing were examined for ZnTe nanoparticles produced from off-stoichiometric target. Although the optical property of ZnTe nanoparticles produced from a zinc rich target was improved, those produced from a tellurium rich target could not be improved. It was found that the effect of in situ annealing on optical properties of ZnTe nanoparticles was dependent upon its content.

Decomposition of Volatile Organic Compounds Using Surface-Discharge Microplasma Devices

Five volatile organic compounds (VOCs), n-octane, ethyl acetate, toluene, p-xylene and ethyl benzene, were decomposed with a newly developed surface-discharged microplasma device (SMD). The SMD consisted of a micropatterned electrode on one side of a mica substrate and an inductive electrode printed on the other side. A piezoelectric, transformed high-voltage (66.7 kHz, 3.5 kV) was applied to four SMDs placed in a batch reactor containing VOC in a gas mixture; the aim was to generate a surface-discharged microplasma through a localized dielectric barrier discharge for the decomposition of the VOCs. The decay in VOC concentration (CVOC) during the discharge was evaluated by gas chromatography–mass spectrometry. For all the five VOCs, the decomposition rates can be treated as first-order reactions against CVOC after a discharge time of 30 min. Decomposition rate was dependent on the type of compound; the reaction rates of aromatic compounds were approximately twice as large as those of aliphatic compounds. Ion concentration measurements during the microplasma operation revealed that reaction rate showed linear relationship with the VOC ion concentration, which suggests that the ionization of the VOCs closely correlates with the rate-determining steps of decomposition reactions.

Anisotropic growth of NiO nanorods from Ni nanoparticles by rapid thermal oxidation

NiO nanorods with extremely high crystallinity were grown by rapid thermal oxidation through exposure of Ni nanoparticles (NPs) heated above 400° C to oxygen. Oxidation proceeds by nucleation of a NiO island on a Ni NP that grows anisotropically to produce a NiO nanorod. This process differs completely from that under mild oxidation conditions, where the surface of the NPs is completely covered with an oxide film during the early stage of oxidation. The observed novel behaviour strongly suggests an interfacial oxidation mechanism driven by the dissolution of adsorbed oxygen into the Ni NP sub-surface region, subsequent diffusion and reaction at the NiO/Ni interface. The early oxidation conditions of metal NPs impose a significant influence on the entire oxidation process at the nanoscale and are therefore inherently important for the precise morphological control of oxidized NPs to design functional nanomaterials.

Synthesis of size-selected silicon nanoparticles by laser ablation

Abstract Using a laser ablation process under low pressure conditions in combination with a LP-DMA, we were able to fabricate size-selected charged silicon nanoparticles. By measuring the concentration and estimating the time required to cover the surface, it was found that this process can be used in QD material processing. The effect of system pressure and laser conditions on the size distribution of particles will be also discussed in the presentation.

Gas-phase generation of noble metal-tipped NiO nanorods by rapid thermal oxidation

The thermal oxidation of alloy nanoparticles (NPs) composed of nickel and a noble metal was investigated by high-resolution electron microscopic observations of the NPs oxidized in a gas phase under different oxidation conditions. When Ni0.8Au0.2 NPs were heated with oxygen from room temperature, oxidation progressed to form Au–NiO core–shell structures, however, the Au core spilled out by breaking the NiO shell at high temperatures. In contrast, when the alloy NPs were subjected to rapid thermal oxidation, which was enabled by heating the NPs at high temperatures (≥500 °C) and then abruptly exposed to oxygen, oxidation advanced anisotropically such that a NiO island protruded and built up to form a NiO nanorod. This resulted in the formation of Au-tipped NiO nanorods in which a hemispherical Au tip bonded to a NiO nanorod via a Au {111}/NiO{100} interface. We found that the relative sizes of Au and NiO in Au-tipped NiO nanorods were easily and widely controlled by changing the Au mole fraction (0.05–0.8) of the alloy NPs. Similarly, rapid thermal oxidation of Ni–Pt NPs generated Pt-tipped NiO nanorods in which a spherical Pt tip was half-embedded in a NiO nanorod. The present gas-phase approach has great potential for fabricating functional asymmetric hybrid nanostructures in clean conditions.