Atsuo Kasuya

Size-Selective Growth and Stabilization of Small CdSe Nanoparticles in Aqueous Solution

Using cysteine and its derivatives as capping molecules, we investigated the influence of the physical structure and chemical nature of capping molecules on the selective growth and stabilization of small CdSe nanoparticles (NPs) in aqueous solution at room temperature. Our investigations revealed specific roles for each functional group of cysteine, and we could correlate this structure and nature of the capping molecules with the size, size restriction, size distribution, and stability of the NPs. For selective growth and stabilization of the NPs in aqueous solution, their capping molecules should have at least one functional group with strong nucleophilicity as well as another free, charged functional group. Capping molecules acting as a monodentate ligand were more effective than those acting as a bidentate ligand for restricting the NPs to a smaller size, whereas the former was less effective than the latter for getting a narrower NP size distribution. Capping molecules with relatively bulky spatial geometry near the ligand-NP interface resulted in the formation of NPs with poor short- and long-term stabilities, whereas those having relatively compact spatial geometry near the interface led to NPs with at least moderate short-term stability. We saw that capping molecules having relatively compact outermost spatial geometry led to NPs with excellent long-term stability, whereas those having relatively bulky outermost spatial geometry produced NPs with at most only moderate long-term stability. Our results clearly showed general trends for the possibility of selective growth of stable semiconductor NPs with particular sizes in aqueous solution.

ZnO clusters: Laser ablation production and time-of-flight mass spectroscopic study

Zinc oxide clusters have been produced by laser ablation of bulk powder zinc peroxide in vacuum and investigated by time-of-flight mass spectroscopy. Experimental results revealed unpredicted and hitherto unknown (ZnO)n clusters of enhanced stability (magic clusters) at n=34, 60 and 78. Cage-like structures for the magic clusters have been suggested, supported by first-principles calculations.

Optimization of a two-compartment photoelectrochemical cell for solar hydrogen production

A two-compartment photoelectrochemical cell consisting of a CdS photoanode immersed in aqueous sulfide solution, Nafion membrane, platinum cathode and sulfuric acid solution as the dark-compartment electrolyte was constructed. The effects of the concentrations of the electrolytes, membrane surface and cathode materials on the performance of the cell were studied to reach high quantum yield of hydrogen production. Under optimized conditions, light to hydrogen conversion efficiency up to 12% was observed under sun light illumination.

Synthesis of Au–silica core–shell particles by sol–gel process

Abstract This paper describes an amine free sol–gel method for silica coating of Au nanoparticles and shows an X-ray image of a colloid solution of the silica coated Au (Au–SiO2) particles. The Au nanoparticles that had a size of 16·9±1·2 nm were prepared through a conventional citrate reduction method. The silica coating was performed with a sol–gel reaction of tetraethylorthosilicate (TEOS) catalysed with NaOH in the presence of Au nanoparticles. The silica shell thickness was varied from 37 to 68 nm for TEOS concentrations of 1×10−3–20×10−3M at 4·3×10−5M of Au, 10·7M of H2O and 1·0×10−3M of NaOH. The optical properties of the Au–SiO2 particle colloid solution were related to the refractive index around the Au particles and the intensity of scattering from silica shells. The as prepared colloid solution could be concentrated up to an Au concentration of 4·3×10−2M with salting out. The concentrated colloid solution showed a high contrast X-ray image.

The solid state reaction of Fe with the Si(111) vicinal surface: splitting of bunched steps

The solid state reaction of deposited Fe (four monolayers, ML) with vicinal Si(111) substrate induced by subsequent thermal treatment has been studied using scanning tunnelling microscopy. At the lower range of annealing temperatures up to 400u2009°C the bunched steps of bare substrate are reproduced by the surface of the covering iron silicide layer. At 400u2009°C the onset of three-dimensional growth of iron silicide islands is observed. In comparison to the samples covered with smaller amounts of Fe it appears at a lower annealing temperature. Above 500u2009°C the bunched steps split into lower ones but more densely distributed due to proceeding reactions between Fe-rich iron silicide and Si substrate. As a consequence, at 700u2009°C the well-developed three-dimensional nanocrystallites of iron silicide are randomly distributed on the Si surface. This observation is in contrast to the formation of a regular array of iron silicide crystallites upon deposition of 2xa0ML of Fe.

Preparation of amine free silica-coated Agl nanoparticles with modified Stöber method

Abstract In recent years, the authors studied on fabrication of silica-coated Agl (Agl–SiO2) nanoparticles with a Stöber method toward new X-ray contrast agents. In the Stöber method, amines are often used as catalysts for silica formation, so that it is probable that amine is left in the silica-coated particles. Since amines are harmful to the human body, amine free particles are desired for medical use. From this viewpoint, amine free Agl–SiO2 nanoparticles were prepared with a modified Stöber method using NaOH as a catalyst instead of amine in the present work. The Agl nanoparticles were prepared from AgClO4 and KI with the use of 3-mercaptopropyltrimethoxysilane (MPS) as a silane coupling agent and NaOH catalyst for alkoxide hydrolysis. The silicacoating was performed at 4·5 × 10−5M MPS, 15–22·5M water, 0·0008–0·0024M NaOH and 0·0004–0·009M tetraethylorthosilicate at an Agl concentration of 0·001M. Agl–SiO2 particles as small as ca. 30 nm could be successfully fabricated at the concentrations of 4·5 × 10−5M MPS, 20M water, 0·0011M NaOH, 0·001M Agl and 0·004M tetraethylorthosilicate.

Vacancy Formation Energy at Metal-Silicon Interface Region

We studied the vacancy formation energy in silicon crystals evaporated with various metals. Specimens were cut out from a high-purity FZ. Si crystal. They were evaporated with various metals and heated in hydrogen gas followed by quenching in water. Instead of vacancy (V) concentration, we measured the optical absorption coefficient due to VH4, which is a complex of one V and four H atoms. The vacancy formation energy in these specimens was found to be much smaller than that in high-purity specimens and that in specimens doped with metallic impurities in an isolated state (solid solution). Moreover, after heating for a short time, the vacancy concentration exhibited a spatial distribution decreasing with distance from the interface, which suggests that the vacancy source is the interface region.

Silica-coating of fluorescent polystyrene microspheres by a modified Stöber method and their stability against photobleaching

Abstract Submicron-sized polystyrene spheres incorporating fluorescence dyes (fluorescent microspheres) were coated with silica by means of a seeded polymerization technique based on the Stöber method. Silica-coating of the fluorescent microspheres with a size of 100 nm was performed in the presence of 0 - 10 g/l polyvinylpyrrolidone (PVP), 1 - 13 mol/l water, 0 - 0.8 mol/l aq. ammonia and 0.00038 - 0.2 mol/l tetraethoxyorthosilicate (TEOS). The addition of PVP was found to suppress the generation of free silica particles and produced no shell-free fluorescent microspheres improving the uniformity of shell thickness of silicacoated fluorescent microspheres. Silica shell thickness increased from 11 to 63 nm with increasing TEOS concentration at 10 g/l PVP, 0.4 mol/l aq. ammonia and 11 mol/l water. Silica-coated fluorescent microspheres showed more stable fluorescence to laser-irradiation than uncoated fluorescent microspheres.

Annihilation of Acceptor–Hydrogen Pairs in Si Crystals Due to Electron Irradiation

We observed the annihilation of boron–hydrogen (BH) pairs and gallium–hydrogen (GaH) pairs during electron irradiation of Si crystals. BH and GaH pairs were generated by annealing of specimens co-doped with B or Ga and H. They were then irradiated with 3 MV electrons at room temperature. Intensities of optical absorption peaks due to BH and GaH pairs were observed at about 7 K. BH pairs and GaH pairs were found to decrease in one stage and two stages, respectively, with the increase of irradiation dose. These decreases were interpreted to be due to interactions between those pairs and self-interstitials.

Fast Boron Diffusion in Si Crystal under Electron Irradiation at Room Temperature Indicated by the Enhanced Formation of Boron-Hydrogen Pairs

We observed the enhanced formation of boron-hydrogen (BH) pairs during electron irradiation of Si crystals co-doped with boron and hydrogen. Such formation was not observed in case of GaH pairs. These results indicate that the BH formation during electron irradiation is due to enhancement of B motion but not hydrogen motion. The optical absorption spectrum of BH pairs observed in the above experiment was the same as that formed by annealing of similar specimens, in which B occupies a substitutional site, B(s). Hence, the observed spectrum is due to B(s)H pairs in both experiments, even though the above B motion occurs due to motion of interstitial B.