Katsuhisa Murakami

Doping and hydrogen passivation of boron in silicon nanowires synthesized by laser ablation

Local vibrational modes of boron (B) in silicon nanowires (SiNWs) synthesized by laser ablation were observed at about 618 and 640cm−1 by Raman scattering measurements. Boron doping was performed during the growth of SiNWs. Fano [Phys. Rev. 124, 1866 (1961)] broadening was also observed in the Si optical phonon peak. These results prove that B atoms were doped in the SiNWs. Hydrogen (H) passivation of B acceptors in the SiNWs was also investigated. A broad peak was observed at around 650–680cm−1 after hydrogenation, demonstrating that B dopants were passivated by the formation of the well-known H–B passivation centers.

Observation of Hg diffusion in CdTe by means of 40‐MeV O5+ ion backscattering

The diffusion profile of Hg in CdTe crystals was observed by means of heavy ion (40 MeV O5+) backscattering. The near‐surface region of CdTe immersed in Hg was investigated up to 1.4 μm from the surface. The observed Hg profile indicated that the concentration of Hg atom at the surface reached 4×1020 cm−3 and the distribution was interpreted by a simple diffusion model. Temperature dependence of the diffusion coefficient was determined to be D=5×103u2009exp[(−2.0±0.3) eV/kT] (cm2/s). In the case of CdTe immersed in Hg which contained a small amount of Cd, it was found that Hg diffusion did not occur. These experimental facts suggest that the rate of Hg diffusion is controlled by the diffusion of a Cd vacancy which is introduced by the outdiffusion of Cd atoms from CdTe crystals.

Selective adsorption and patterning of Si nanoparticles fabricated by laser ablation on functionalized self-assembled monolayer

We demonstrate an in situ selective adsorption of Si nanoparticles fabricated by laser ablation on a functionalized self-assembled monolayer (SAM). Si nanoparticles adsorbed on –CH3 terminated a SAM while Si particles did not adsorb on –NH2, –F, –OH, and –COOH, terminated SAMs. The end group of a SAM solely determines the selectivity against Si nanoparticle adsorption. We utilized the screening ability of functionalized SAMs to pattern Si nanoparticles onto desired locations on a Si substrate.

Damage and strain in single-layer graphene induced by very-low-energy electron-beam irradiation

From the analysis of the ratio of D peak intensity to G peak intensity in Raman spectroscopy, electron beam irradiation with energies of 100u2009eV was found to induce damage in single-layer graphene. The damage becomes larger with decreasing electron beam energy. Internal strain in graphene induced by damage under irradiation is further evaluated based on G peak shifts. The dose-dependent internal strain was approximately 2.22% cm2/mC at 100u2009eV and 2.65u2009×u200910−2% cm2/mC at 500u2009eV. The strain induced by the irradiation showed strong dependence on electron energy.

Direct synthesis of large area graphene on insulating substrate by gallium vapor-assisted chemical vapor deposition

A single layer of graphene with dimensions of 20u2009mmu2009×u200920u2009mm was grown directly on an insulating substrate by chemical vapor deposition using Ga vapor catalysts. The graphene layer showed highly homogeneous crystal quality over a large area on the insulating substrate. The crystal quality of the graphene was measured by Raman spectroscopy and was found to improve with increasing Ga vapor density on the reaction area. High-resolution transmission electron microscopy observations showed that the synthesized graphene had a perfect atomic-scale crystal structure within its grains, which ranged in size from 50u2009nm to 200u2009nm.

Hg content and thermal stability of the anodic sulfide films on Hg1−xCdxTe investigated by 30–40‐MeV O5+ ion backscattering

The Hg content of anodic sulfide films on Hg1−xCdxTe (MCT) were investigated for as‐grown and heat‐treated films by using 30–40‐MeV O5+ ion backscattering. Heat treatments were performed in a vacuum for 60 min at various temperatures up to 300u2009°C for the samples with x=0.20 and up to 350u2009°C for the samples with x=0.30 and 0.35. A significant Hg content of about 15 at.u2009% of Cd was found in the as‐grown anodic sulfide film on MCT samples with x=0.20, 0.30, and 0.35. After heat treatment above 260u2009°C for x=0.30 and 0.35 and above 210–240u2009°C for x=0.20, the Hg atoms were removed from the films, and Hg deficient layers were formed near the interface region of the MCT crystal. This suggests that the residual Hg in the anodic sulfide films introduced during the sulfidization process plays an important role toward the degradation of the passivation properties of the sulfide film on MCT.

Profile redistribution of implanted Zn impurities in Si due to explosive crystallization from a surface layer

Abstract Implanted Zn impurities were found to segregate at the interface between ion-implanted amorphous Si and underlying crystal Si after pulsed laser irradiation with energy densities less than that required to cause liquid phase epitaxial regrowth. Both time-resolved reflectivity from the back and front sides indicate a strong correlation between the internal Zn impurity segregation and explosive crystallization. This measurement also gives a direct evidence of coexistence of a liquid Si and polycrystalline nuclei at the surface layer at energy densities from 0.27 to 0.5 J/cm2.

Atomic migration and surface evaporation of Hg in Hg1−x CdxTe crystals observed by 40‐MeV O5+ ion backscattering method

Dissociation and evaporation of Hg from the near‐surface region of Hg1−xCdxTe crystals was investigated, up to several microns, by means of 40‐MeV O5+ backscattering. Samples with x=0.18 and 0.32 were heat treated in a vacuum for 60 min at various temperatures up to 320 and 340u2009°C, respectively. The change of the backscattering spectra with the temperature of heat treatment indicated that the Hg atoms outdiffused partly from the near‐surface region of the crystals at higher temperatures than 280 and 320u2009°C for x=0.18 and 0.32, respectively. This behavior of outdiffusion is in contrast to the case of HgTe crystal.

Cysteine-containing oligopeptide β-sheets as redispersants for agglomerated metal nanoparticles

Oligopeptide β-sheets comprising a fluorenyl methoxy carbonyl (Fmoc) group on its N-terminus and five amino acid residues of cysteine, lysine and valine displays redispersive properties with respect to agglomerated metal nanoparticles (MNPs, M = Au, Cu, Pt and Pd). The ligand-free MNPs prepared by a laser ablation technique in liquid maintain a high dispersion state due to the inherent surface charges delivered by anionic species present in solution, but may agglomerate after the preparation depending on concentration or salinity. We show how the agglomerated MNPs can be returned to the dispersed state by adding the Fmoc-oligopeptide β-sheets in methanol, as characterized by photoabsorption spectroscopy and transmission electron microscopy. Systematic studies in which we vary the concentration, the amino acid sequences and the secondary structures of a series of the oligopeptides clarify that the β-sheet structure is essential for the redispersion of the MNPs, where metal-binding thiol groups are integrated on one side and positively charged amino groups are located on the other side of the β-sheet. A possible mechanism for the redispersion may be that the agglomerated MNPs are subsequently enwrapped by the flexible β-sheets and gradually separated due to the reconstruction of peptide β-sheets under the assembly/disassembly equilibrium.

Hg content of anodic oxide films on Hg1−xCdxTe after heat treatment as measured by 40 MeV‐O5+ ion backscattering

The Hg concentration of anodic oxide films on Hg1−xCdxTe and its change after heat treatment were investigated by using 40 MeV‐O5+ backscattering. Heat treatment in a vacuum for 60 min was performed at various temperatures up to 350 and 400u2009°C, respectively, for the samples with x=0.19 and x=0.32. A significant Hg content in the as‐grown anodic oxide was found in accordance with the recent result of C. M. Stahle, D. J. Thomson, C. R. Helms, C. H. Becker, and A. Simmons [Appl. Phys. Lett. 47, 521 (1985)]. The Hg in the film gradually decreased with increasing heat treatment temperature up to about 290u2009°C and rapidly disappeared above 290u2009°C for both x=0.19 and x=0.32.