三木 一司

Abstract Conventional doping processes are no longer viable for realizing extreme structures, such as a δ-doped layer with multiple elements, such as the heavy Bi, within the silicon crystal. Here, we demonstrate the formation of (Bi + Er)-δ-doped layer based on surface nanostructures, i.e. Bi nanolines, as the dopant source by molecular beam epitaxy. The concentration of both Er and Bi dopants is controlled by adjusting the amount of deposited Er atoms, the growth temperature during Si capping and surfactant techniques. Subsequent post-annealing processing is essential in this doping technique to obtain activated dopants in the δ-doped layer. Electric transport measurement and photoluminescence study revealed that both Bi and Er dopants were activated after post-annealing at moderate temperature.

Laying-down gold nanorods (GNRs) of a monolayer immobilized on a solid substrate was realized with a hybrid method, a combination of three elemental technologies: surface modification, electrophoresis, and solvent evaporation. The self-assembly of CTAB-protected GNRs in the solution was induced by 0.05 mM of EDTA. The assembled GNRs were deposited in a laying-down form on the solid surface during the hybrid method. The final coverage was over 71% on the substrate with an area larger than 0.6 cm(2). The spacing between the sides of the GNRs was fixed to be 4.6 +/- 0.9 nm by the thermal annealing-promoted crystalline packing of the bilayer of CTAB salt-bridged with EDTA. The obtained laying-down GNRs of a monolayer on the gold substrate show a small shift of the transverse LSPR around 550-570 nm (with a width of around 100 nm) and a large red shift of the longitudinal LSPR to be 900-1050 nm (with a width of 500 nm), because of the strong electromagnetic coupling between the GNRs and gold substrate. Therefore it can be used in a wide range of wavelengths for surface enhanced Raman spectroscopy (SERS) applications. The film has a high enhancement factor with 10(5) for R6G.

The self-assembled monolayer (SAM)-modified metallic nanoparticles (MNPs) often exhibit improved chemoselectivity in various catalytic reactions by controlling the reactants’ orientations adsorbed in the SAM; however, there have been a few examples showing that the reaction rate, i.e., catalytic activity, is enhanced by the SAM-modification of MNP catalysts. The critical parameters that affect the catalytic activity, such as the supports, nanoparticle size, and molecular structures of the SAM components, remain uninvestigated in these sporadic literature precedents. Here, we report the mechanistic investigation on the effects of those parameters on the catalytic activity of alkanethiolate SAM-functionalized gold nanoparticles (AuNPs) toward silane alcoholysis reactions. The evaluation of the catalytic reaction over two-dimensionally arrayed dodecanethiolate SAM-functionalized AuNPs with different supports revealed the electronic interactions between AuNPs and the supports contributing to the rate enhancement. Additionally, an unprecedented size effect appeared—the AuNP with a 20 nm radius showed higher catalytic activity than those at 10 and 40 nm. Infrared reflection–absorption spectroscopy revealed that the conformational change of alkyl chains of the SAM affects the entrapment of reactants and products inside the SAM, and therefore brings about the acceleration effect. These findings provide a guideline for further applying the SAM-functionalization technique to stereoselective organic transformations with designer MNP catalysts.

We realize Mn δ-doping into Si and Si/Ge interfaces using Mn atomic chains on Si(001). Highly sensitive X-ray absorption fine structure techniques reveal that encapsulation at room temperature prevents the formation of silicides/germanides while maintaining one-dimensional anisotropic structures. This is revealed by studying both the incident X-ray polarization dependence and post-annealing effects. Density functional theory calculations suggest that Mn atoms are located at substitutional sites, and show good agreement with experiment. A comprehensive magnetotransport study reveals magnetic ordering within the Mn δ-doped layer, which is observed at around 120 K. We demonstrate that doping methods based on the burial of surface nanostructures allows for the realization of systems for which conventional doping methods fail.

We report a simple and facile method for fabricating monolayer colloidal films of alkanethiol-capped gold nanoparticles (AuNPs) on glass substrates. The new method consists of two sequential sonication processes. The first sonication is performed to obtain a well-dispersed state of alkanethiol-capped AuNPs in hexane/acetone in the presence of a substrate. After additional static immersion in the colloidal solution for 5 min, the substrate is subjected to sonication in hexane. By using this method, we succeeded in forming uniform and stable assemblies of midnanometer-sized AuNPs (14, 34, and 67 nm in diameter) over the entire surface of 10 nun square glass substrates in a short processing time of less than 10 min. It was also demonstrated that this method can be applied to a 1.5-in. octagonal glass substrate. The mechanism of monolayer colloidal film formation was discussed based on scanning electron microscopy observations at each preparation step. We found that the second sonication was the key process for uniform and high-surface-coverage colloidal film formation of midnanometer-sized AuNPs. The second sonication promotes the migration of AuNPs on top of the monolayer in contact with the substrate surface, decreasing both the multilayer region and the bare surface area. Eventually, a nearly perfect monolayer colloidal film is formed.

Surface hydrophobization by self-assembled monolayer formation is a powerful technique for improving the performance of organic field-effect transistors (OFETs). However, organic thin-film formation on such a surface by solution processing often fails due to the repellent property of the surface against common organic solvents. Here, a scalable unidirectional coating technique that can solve this problem, named self-assisted flow-coating, is reported. Producing a specially designed lyophobic lyophilic pattern on the lyophobic surface enables organic thin-film formation in the lyophobic surface areas by flow-coating. To demonstrate the usefulness of this technique, OFET arrays with an active layer of poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) are fabricated. The ideal transfer curves without hysteresis behavior are obtained for all OFETs. The average field-effect hole mobility in the saturation regime is 0.273 and 0.221 cm(2).V-1.s(-1) for the OFETs with the channels parallel and perpendicular to the flow-coating direction, respectively, and the device-to-device variation is less than 3% for each OFET set. Very small device-to-device variation is also obtained for the on-state current, threshold voltage, and subthreshold swing. These results indicate that the self-assisted flow-coating is a promising coating technique to form spatially uniform thin films of polymeric organic semiconductors on lyophobic gate insulator surfaces.

We explored the effect of neat C-60 doping on the performance of bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) based on poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM). The power conversion efficiencies (PCEs) were found to gradually decrease as the relative concentration of C-60 increased. This PCE decrease is mainly due to the reduction in short-circuit current density (J(sc)). Atomic force microscopy images revealed that C-60-doped BHJ films consisted of unformed large P3HT crystals and many small PCBM aggregates. This morphology leads to a decrease in the effective P3HT conjugation length and an enhancement of absorption bleaching in neutral P3HT chains. These results explain well the blue-shift absorption spectra, the decrease of absorption coefficients, and the reduction in J(sc) found in C-60-doped BHJ OPVs.

<p>Photo-alignment efficiency of polyimides containing azobenzene in the backbone structure (Azo-PIs) is significantly enhanced by performing a simple alkylamine vapor treatment on the precursor (polyamic acid: Azo-PAA) films prior to photo-alignment. This is due to the efficient photo-induced rotation of Azo-PAA backbone structures in the amine-treated films, which are swollen by absorbing alkylamine molecules. In this study, we have examined the relationships between the amine vapor treatmen t time and the thickness of the amine-treated Azo-PAA film, and also between the amine vapor treatment time and the photo-induced in-plane anisotropy of the Azo-PAA and Azo-PI films.</p>

Higher manganese silicide nanowires have been grown on the Si(001)-2 x 1 surface by the pre-growth of Bi nanolines. Scanning tunnelling microscope (STM) observations show that the nanowire has a linear surface reconstruction with a periodicity of 0.56 nm, and we propose a reconstruction on their surface to reduce the density of dangling bonds, which forms linear structures matching the dimensions from STM. Scanning tunnelling spectroscopy (STS) data agree with previous calculation results and reveal that the nanowires are degenerate semiconductors, with potential application for spintronics.

The technique of producing new nanomaterial structures has been examined by making into a template the bismuth nanolines formed on silicon (001) surfaces. By this method, we succeeded in manganese forming its silicide wire by applying anisotropic strain using the tensile stress which arises between bismuth nanolines. The anisotropic surface stress blocks the formation of embedded structures and stabilizes the nucleation of manganese silicide islands which grow in a preferred direction at the growth temperature of 450 degrees C, forming nanowires with a band gap of approximately 0.6 eV, matching the reported band gap of MnSi1.7 and not that of MnSi.

The electronic structure of the Si(110)-(16 x 2) surface was studied by scanning tunneling microscopy at room temperature (RT) and at 78 K. A combination of point tunneling spectroscopy measurements and local density of states mappings reveal details of the electronic structure of the (16 x 2) reconstruction both in empty and occupied states. Point tunneling spectra show a small band gap indicating that Si(110)-(16 x 2) is a semiconductor. The pentagon, which is the main building block in the Si(110)-(16 x 2) surface, consists of at least four electronic states. The pentagon in empty states is created by the superposition of two states with different origins: a four-lobed pattern similar to that observed in filled states; and another state that causes splitting of one of the lobes. The 78 K data show that the band responsible for the four-lobed shape in filled states (located at -0.2 eV) splits further. We present a very simple structure, calculated by density functional theory, which partially explains the experimental data.

We propose a new method of creating light-emitting point defects, or G-centers, by modifying a silicon surface with hexamethyldisilazane followed by laser annealing of the surface region. This laser annealing process has two advantages: creation of highly dense G-centers by incorporating carbon atoms into the silicon during heating; freezing in the created G-centers during rapid cooling. The method provides a surface region of up to 200 nm with highly dense carbon atoms of up to 4 x 10(19) cm(-3) to create G-centers, above the solubility limit of carbon atoms in silicon crystal (3x10(17) cm(-3)). Photoluminescence measurement reveals that the higher-speed laser annealing produces stronger G-center luminescence. We demonstrate electrically-driven emission from the G-centers in samples made using our new method. Copyright 2011 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [doi:10.1063/1.3624905]

SrTiO3(100)-(root 5 x root 5)-R26.6 surfaces were studied by means of high-resolution scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. By varying the bias voltage in the occupied state, it was possible to observe the arrangement of titanium and oxygen atoms in the unit cells of a (root 5 x root 5) surface superstructure, which revealed that the TiO2 layer is the terminating plane of the (root 5 x root 5) surface. In the STM images, peculiar protrusions were seen at the oxygen fourfold hollow site responsible for root 5 x root 5 periodicity. The protrusions are asymmetrical in contrast, which would be an important consideration in proposing accurate structural models for (root 5 x root 5) surface superstructures. (C) 2011 Elsevier B.V. All rights reserved.

A method for promoting the growth of manganese silicide nanowires on Si(001) at 450 °C is described. The anisotropic surface stress generated by bismuth nanolines blocks the formation of embedded structures and stabilizes the nucleation of manganese silicide islands which grow in a preferred direction, forming nanowires with a band gap of approximately 0.6eV, matching the reported band gap of MnSi1.7. This method may also provide a means to form silicide nanowires of other metals where they do not otherwise form. © 2011 IOP Publishing Ltd.

The formation process of Al magic clusters on the Si(111)-7 x 7 surface was investigated by means of a variable-temperature scanning tunneling microscope (STM) in situ and was interpreted using density-functional theory (DFT) calculations. At a growth temperature of 450 degrees C, Al atoms hopped among the corner, center, and T4 sites and also across the dimer rows on the Si(111)-7 x 7 surface. At low coverage below 0.08 ML, a single Al atom was adsorbed on the corner or center site. When the coverage was increased to 0.08 ML, Al dimers and trimers appeared, and Al magic clusters were also observed. However, no Al tetramers or pentamers were experimentally confirmed. Careful analysis of STM images suggests that Al trimers could be key precursors for the formation of Al magic clusters, and DFT calculations verified this interpretation. Total-energy calculation results using DFT reveal that this is due to the small energy gain from Al trimer to Al tetramer. These results are important for understanding the atomic structure and the formation mechanism of the magic clusters on the Si(111)-7 x 7 surface.

DNAナノファイバをテンプレートに用いて金ナノ粒子の1次元金属ナノアレイを作製した。作製した金属ナノアレイの偏光特性を評価することで、DNAナノファイバ中の金ナノ粒子の配列性を検討した。さらにアスペクト比の異なる金ナノ粒子を用いた結果も検討した。

我々はDNAを1次元集合させることでより長いナノファイバーへと成長させ、さらにそれらを基板表面にアレイ化する簡単な方法を報告した。1本鎖レベルのDNAと異なり、DNAナノファイバーは基板表面に固定された状態でもその高次構造は保たれていると期待される。そこで本研究では作製されたDNAナノファイバーの力学、電気そして光学特性を評価し、1次元ナノ構造体としての応用性を検討する。

本研究では、DNAなどの生体高分子を1次元集合させることでより長いナノファイバーへと成長させ、さらにそれらを基板表面にアレイ化する簡単な方法を開発したので報告する。

By means of variable-temperature scanning tunneling microscopy, we have investigated systematically the formation process of Al magic cluster arrays on the Si(111)-7x7 surface at high temperature in situ. It was found that the magic clusters form only when the Al coverage is over a critical value, similar to 0.08 +/- 0.015 ML. These clusters occupy preferentially on the faulted-half-unit cells of the Si(111)-7x7 surface, but this preference is coverage dependent. By analyzing systematically the spatial distribution of magic clusters below the saturation coverage, an attractive interaction between clusters was found, which prevents a triangular ordering of the nanocluster array.

We report a simple method for fabricating metal nanoarrays with DNA templates. After DNA-stretching and fixation on surfaces, highly aligned and integrated Au nanoarrays were fabricated via electroless depositions of Au on DNA. The direct visualization of metallic nanoarrays and interaction with optical near-filed have been studied by atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). The resulting metallic nanoarrays are attractive prospects in nano-optics and an important step toward the construction of plasmon waveguids.