Kenjiro Oura

King Mongkut's Institute of Technology Ladkrabang, Bangkok, Bangkok, Thailand

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Publications (281)474.88 Total impact

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    ABSTRACT: Various surface processes, such as thin film growth or etching, are usually performed by introducing various gases into a vacuum chamber. In order to monitor such surface processes in situ, we have developed an ion scattering and recoiling spectroscopy apparatus equipped with a differential pumping system. The system was applied for real time monitoring of hydrogen-mediated growth of Ge films on Si substrates under a hydrogen gas pressure of 10-4 Torr.
    Surface Review and Letters 04/2012; 07(05n06). · 0.28 Impact Factor
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    ABSTRACT: Protective-layer-coated single-walled carbon nanotubes (SWNTs) with palladium nanoparticle decoration (Pd-SiO(2)-SWNTs) were fabricated and their sensing properties for hydrogen (H(2)) were investigated. SWNTs were coated with a 3-4 nm thick SiO(2) layer by pulsed laser deposition and subsequently decorated with Pd nanoparticles by electron beam evaporation. Even though the SWNTs were completely surrounded by a protective layer, Pd-SiO(2)-SWNTs responded to H(2) down to a concentration of 1 part per million. Compared with the Pd nanoparticle-decorated SWNTs without a protective layer (Pd-SWNTs), Pd-SiO(2)-SWNTs exhibited highly stable sensor responses with variations of less than 20%; Pd-SWNTs showed a variation of 80%. The density of the Pd-SWNTs significantly decreased after the sensing test, while that of the Pd-SiO(2)-SWNTs with the netlike structure remained unchanged. The hydrogen sensing mechanism of the Pd-SiO(2)-SWNTs was attributed to the chemical gating effect on the SWNTs due to dipole layer formation by hydrogen atoms trapped at the Pd-SiO(2) interface. Moreover, the relationship between H(2) concentration and sensor response can be described by the Langmuir isotherm for dissociative adsorption.
    Nanotechnology 02/2011; 22(5):055501. · 3.84 Impact Factor
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    ABSTRACT: We fabricated a highly stable and sensitive gas sensor based on single-walled carbon nanotubes (SWNTs) protected by metal-oxide coating layer. SWNTs were deliberately decorated with 2--5 nm of oxygen-deficient SiOx and AlOx by pulsed laser deposition, followed by annealing under Ar/H2 ambient. Surpassing the as-grown SWNTs, the metal-oxide coated SWNTs showed an excellent sensing stability with a variation of sensor response less than 7--8%. The obtained sensitivity to NO2 was also improved. Moreover, the relationship between NO2 concentration and sensor response can be described by the Frumkin--Temkin isotherm.
    Applied Physics Express - APPL PHYS EXPRESS. 01/2009; 2(9).
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    ABSTRACT: We demonstrated highly sensitive detection of carbon monoxide (CO) down to 1 ppm at room temperature using platinum-decorated single-walled carbon nanotubes (Pt-SWNTs). The obtained sensitivity to CO was 3--4 orders higher than the values reported for functionalized SWNTs, and was achieved by the controlled deposition of Pt nanoparticles on SWNTs. For 1--10 ppm of CO, the sensor response linearly increased with CO concentration, affording the quantitative detection of CO in a low-concentration range. Furthermore, Pt-SWNTs exhibited detection selectivity to CO against H2. The sensing mechanism was attributed to electron donation to the SWNTs as a result of CO oxidation on the Pt catalyst surface.
    Applied Physics Express - APPL PHYS EXPRESS. 01/2008; 1(1).
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    ABSTRACT: Vertically aligned carbon nanotubes (CNTs) with a high number density of 2× 1011 cm-2 were successfully synthesized by catalyst-assisted thermal chemical vapor deposition using catalyst preheated in C2H2 atmosphere. The synthesized CNTs were found to possess high linearity and good crystallinity. The catalyst nanoparticles with a high number density and a narrow size distribution were efficiently formed by preheating in C2H2 as compared with those treated in H2 and He. The obtained results can be attributed to the decrease in the migration length of Fe catalyst due to the incorporation of carbon during the preheating process.
    Japanese Journal of Applied Physics 01/2008; 47:1941-1943. · 1.07 Impact Factor
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    ABSTRACT: We investigated the properties of field emission from a pillar array of aligned carbon nanotube (CNT) bundles, which were fabricated on a Si substrate by thermal chemical vapor deposition. To explore the influence of the pillar arrangement on its field emission, the ratio of interpillar distance (R) to pillar height (H), R/H, was investigated as a function of H by changing H while maintaining R at 100 μm. The most-enhanced field concentration was obtained at R/H ∼ 6, although we previously reported that the optimal configuration of pillars for R = 250 μm was a ratio of R/H = 2. These results show that the optimal R/H for greatest field emission from the CNT pillar array is dependent on R.
    Diamond and Related Materials 01/2008; · 1.71 Impact Factor
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    ABSTRACT: The authors fabricated a screen-printed field electron emitter using purification-free and length-controlled carbon nanotubes (CNTs). They used vertically aligned CNTs grown on Si substrates by thermal chemical vapor deposition as the source material for fabricating CNT paste. The length of CNTs was controlled by adjusting the growth time. The amounts of amorphous carbon and catalyst in the source material were less than 1 and 0.5 wt % , respectively, which obviated the need to purify the CNTs. The emitter fabricated using source CNTs with a length of over 80 μ m showed good reproducibility of current density (J) –electric field (E) characteristics. With a low threshold field E<sub> th </sub> of 1.5 V /μ m , J=1 mA / cm <sup>2</sup> was produced. The emitter exhibited good emission stability for 100 h . It was found that the length distribution of the standing CNTs was determined in a precise manner when long CNTs were used as the source material, which led to a highly reproducible fabrication of field emitters.
    Applied Physics Letters 10/2007; · 3.79 Impact Factor
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    ABSTRACT: The authors report on the field emission from an aligned carbon nanotube CNT bundle grown by thermal chemical vapor deposition. The CNT bundle showed a low-threshold electric field of 2.0 V / m that produced a current density of 10 mA/ cm 2 , sustainable evolution of current density up to 2.8 A / cm 2 at 2.9 V / m, and good emission stability without degradation for 200 h of continuous dc emission. By calculating the electric-field distribution, it was found that the electric field was significantly higher at the edge of the CNT bundle than at the center. The excellent field-emission properties of the aligned CNT bundle were attributed to the edge effect and the high-density structure. © 2007 American Institute of Physics. A carbon nanotube 1 CNT is an ideal field-emitting ma-terial because of its high geometric aspect ratio, small radius of curvature at the tip apex, high electrical conductivity, high mechanical strength, and chemical inertness. 2 So far, the use-fulness of electron sources using a CNT emitter as a cold cathode has been demonstrated for vacuum electronic de-vices such as field-emission displays, 3,4 backlight sources, 5 and x-ray tubes. 6 For such device applications, CNT emitters are required to posses a low driving voltage and durability for a long lifetime. In recent years, CNT emitters have been fabricated by two main methods: chemical vapor deposition 7 CVD and screen printing. 5 In both methods, it is important to control the length and the intertube distance of CNTs to reduce the screening effect in adjacent CNTs. 8 For an aligned CNT array, it has been predicted that the field emission be-comes maximum when the ratio of the intertube distance to the height of each CNT is approximately 2. 9 However, the fabrication of an array with this ideal arrangement using iso-lated CNTs requires an expensive nanolithography tech-nique. Alternatively, the facile methods for patterning of CNTs such as shadow masking, 4,10 soft lithography, 11 nano-contact printing, 12 and nanoimprint lithography 13 are actually adaptable for making the ideal CNT arrays from the aspect of large area and low cost applications. Recently, using a pillar array of aligned CNT bundles grown by CVD with photoli-thography, we have achieved fabricating an architecture that satisfies the optimal ratio of interpillar distance to pillar height and also possesses the advantages of simplicity and inexpensiveness. 14 The pillar array of CNT bundles showed highly efficient field emission. However, apart from the re-duction of the screen effect, the field-emitting potentiality of an aligned CNT bundle itself, which is a component of an efficient field emitter, has not been sufficiently investigated. In this study, we investigated the field emission from an aligned CNT bundle grown by CVD. The CNT bundle dis-played a low-threshold electric field E th of 2.0 V / m that produced a current density J of 10 mA/ cm 2 , sustainable evolution of current density up to 2.8 A / cm 2 at 2.9 V / m, and excellent emission stability over 200 h of continuous operation. By calculating the electrostatic potential using a finite element method, it was found that the electric field at the edge region of the bundle was higher than that of central region, indicating that the highly efficient field emission can be attributed to the edge effect. The aligned CNT bundle was synthesized by thermal CVD combined with photolithography. Patterns of 50 m diameter of the Fe 1 nm /Al 10 nm multilayer catalyst were fabricated on a stainless-steel substrate SUS304 by photolithography and electron-beam evaporation at room temperature. Subsequently, the sample was annealed at 650 ° C for 1 h to form catalyst nanoparticles. For this sample, thermal CVD was carried out at a growth tempera-ture of 650 ° C under a pressure of 600 Pa maintained using flowing acetylene C 2 H 2 and helium He gases with a total flow rate of 100 SCCM SCCM denotes cubic centimeter per minute at STP. The volume ratio of C 2 H 2 to He was 20%. Using this process, five CNT bundles were fabricated sepa-rately at a distance of more than 3 mm from each other. Therefore, the screening effect 9 is negligible, and field emis-sion from individual CNT bundle can be evaluated. The aligned CNT bundle was characterized by high-resolution scanning electron microscopy SEM and trans-mission electron microscopy TEM. The field-emission properties were measured using parallel plates with 1 mm separation in a high-vacuum system base pressure: 10 −7 Pa. The two-dimensional numerical calculation of the electric field on a CNT bundle was performed using a finite element method.
    Applied Physics Letters 04/2007; · 3.79 Impact Factor
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    ABSTRACT: Using scanning tunneling microscopy, it has been found that a one-monolayer Tl∕Ge(100)2×1 surface demonstrates a fascinating possibility to form antiphase boundaries (APB’s) by re-bonding underlying Ge dimers. This is in contrast to adsorbate-free Si(100) and Ge(100) surfaces and one-monolayer Tl∕Si(100) system, where APB formation does not occur. Energetics of APB formation in various systems, including adsorbate-free Si(100) and Ge(100) surfaces and Tl∕Si(100) and Tl∕Ge(100) systems, have been evaluated using first-principles total-energy calculations, which reveal that Tl∕Ge(100) is the only system where APB formation is the energetically favorable process.
    Physical review. B, Condensed matter 01/2007; 76(19). · 3.77 Impact Factor
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    ABSTRACT: We investigated the effect of electrical aging on field electron emission from a screen-printed carbon nanotube (CNT) film. After maintaining the field-emission current density at 20 mA/cm2 for 3 h in the DC mode, it was observed that initially long CNTs became short and initially lying CNTs stood up. As a result, the field-emission uniformity and lifetime were markedly improved. From the analysis of the corresponding Fowler-Nordheim plots using a temperature-dependent formula, the shortening of CNTs by electrical aging was found to originate from the thermal evaporation of carbon atoms at the tip of CNTs during field electron emission.
    Japanese Journal of Applied Physics 01/2007; 46:867-869. · 1.07 Impact Factor
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    ABSTRACT: Using scanning tunneling microscopy, phase formation and temperature-driven phase transitions in Tl/Ge(1 0 0) system have been studied. Evolution of Tl overlayer structure has been considered for three temperature ranges, including around room temperature (RT), high-temperature (HT) (350–450 K) and low-temperature (LT) (20–100 K) ranges. Upon RT growth, a 2 × 1-Tl phase develops in submonolayer range and is completed at around 1 ML of Tl. Cooling of the RT-deposited Tl overlayer results in formation of a set of various LT structures. These are 1D chains, 5 × 4-Tl and “stroked” phases observed in submonolayer range and a long-period c(12 × 14)-Tl phase developed at around 1 ML. All transitions between these RT and LT structures are reversible. At doses beyond 1 ML, RT deposition of Tl onto Ge(1 0 0) leads to the growth of second-layer Tl stripes, forming arrays with a 1 × 4 periodicity. Meanwhile, structure of the first layer also changes and it displays a set of various reconstructions, c(2 × 8), c(10 × 6) and c(10 × 7). All these structures remain unchanged upon cooling to LT. Growth at HT as well as heating of RT-deposited Tl overlayer irreversibly produces 3 × 2-Tl phase whose rows become decorated by second-layer Tl stripes at prolonged Tl deposition.
    Surface Science 01/2007; 601(3):595-602. · 1.84 Impact Factor
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    ABSTRACT: Using scanning-tunneling microscopy and first-principles total-energy calculations, we have considered the structural properties of the so-called doped clusters formed by depositing additional 0.05 monolayer of In onto the 4×3-periodicity magic-cluster array in the In∕Si(100) system. Low-temperature STM observations have revealed that most of the doped clusters have an asymmetric shape. According to the total-energy calculations, these clusters have plausibly Si6In8 composition. In such a cluster, one of the In atoms is mobile and can hop between four equivalent sites within a cluster. The hopping between sites, located in the different 2a×3a halves of the cluster, is characterized by the barrier of about 0.7 eV, and this hopping becomes frozen at 55 K. In contrast, the hopping between the neighboring sites within the same cluster half persists up to very low temperatures, as the barrier height here is an order of magnitude lower. Due to the above structural properties, the doped asymmetric Si6In8 cluster can be treated as a promising switch, logic gate, or memory cell of the atomic-scale size.
    Physical Review B 09/2006; 74(12). · 3.66 Impact Factor
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    ABSTRACT: We investigated the impact of the growth morphology of single-walled carbon nanotubes (SWNTs) on gas sensing performance. An SWNT film was directly synthesized on alumina substrate by thermal chemical vapour deposition. Different morphologies of the SWNTs in terms of density, diameter distribution and orientation were obtained by varying the growth temperature. Vertically aligned SWNTs with a high density were grown at 750 °C, while horizontally lying SWNT networks with a low density were grown in the temperature range 800–950 °C. The sensor response of the resultant SWNTs to NO2 was characterized at room temperature. It was found that the density of SWNTs strongly dominates sensor performance; the SWNT networks with the lowest density exhibited the highest sensor sensitivity. This was evidenced by characterization of density-controlled SWNTs synthesized using different thicknesses of an Fe/Al multilayer catalyst. The high sensor sensitivity for low-density SWNT networks is likely to be attributed to suppression of the formation of SWNT bundles and reduction of narrow-band-gap conduction paths, resulting in the enhancement of the adsorption probability and chemical gating efficiency of gas molecules on SWNTs.
    Nanotechnology 08/2006; 17(17):4424. · 3.84 Impact Factor
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    ABSTRACT: The solid-phase reactive epitaxial growth processes and structures of Er/Si(100) thin films were investigated by coaxial impact-collision ion scattering spectroscopy (CAICISS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The as-deposited Er film grown at room temperature was transformed into crystalline rectangular-shaped islands after annealing at 900 °C. These islands have a hexagonal AlB2-type structure and the epitaxial relationship is determined to be ErSi2(011¯0)[0001]//Si(100)[011¯]. It has been revealed that the surface of the Er silicide island is terminated with an Er plane.
    Thin Solid Films 06/2006; · 1.87 Impact Factor
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    ABSTRACT: Thermal-CVD was carried out for the low-temperature growth of carbon nanofibers (CNFs) using a CuNi alloy catalyst film with a thickness of 5 nm on Si in a gas mixture of C2H2 and He (C2H2/He=3/12 sccm). The experimental results obtained using the CuNi alloy catalyst film were compared with those obtained using the Fe, Ni, and FeNi catalyst films with the same thickness of 5 nm. It was shown that an amorphous CNF with a diameter of 20 nm can be grown even at 400 °C using the CuNi catalyst film, but not using the Fe, Ni and FeNi catalysts. A reduction in the growth temperature of CNFs was considered to be achieved using small CuNi catalyst particles with a comparatively smaller surface energy than FeNi catalyst particles.
    Japanese Journal of Applied Physics 01/2006; 45(No. 6A):5329-5331. · 1.07 Impact Factor
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    ABSTRACT: We have improved field emission uniformity in a screen-printed paste composed of double-walled carbon nanotubes (DWNTs), which were ground by planetary ball milling (PBM). Results showed that after grinding pristine DWNTs, carbonaceous particles that affected the roughness of the DWNT paste were atomized and long DWNTs were cut. Ground DWNTs were dispersed uniformly on a stainless steel substrate, thereby improving field-emission uniformity. This simple grinding technique is expected to be applicable to low-cost and high-yield processing that produces a DWNT paste for field emitters.
    Japanese Journal of Applied Physics 01/2006; 45:1044-1046. · 1.07 Impact Factor
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    ABSTRACT: We demonstrated ultrasensitive detection of ozone using single-walled carbon nanotube (SWNT) networks directly grown on a conventional sensor substrate. Ozone was detected down to the ppb level (˜ 6 ppb) at room-temperature operation with a fast response. Using an electrical breakdown technique, gas sensitivity was improved. This result clearly indicates that owing to its high sensitivity, simplicity in fabrication and compact size, an SWNT sensor provides a feasible route to ultrasensitive ozone detection surpassing existing methods.
    Japanese Journal of Applied Physics 01/2006; 45:3669-3671. · 1.07 Impact Factor
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    ABSTRACT: Carbon nanofibers have synthesized a low temperature using DC Ar plasma and Fe-Phthalocyanine, and a characteristic difference of the synthesized CNF according to the location of the substrate was investigated. The carbon nanofibers had about 100nm diameter and up to length. These were grown in random orientation. There are two shapes in the CNFs, screw and straight line shapes. Furthermore, we found the selective growth of nanofibers on the scratched substrates. The density of CNFs synthesized on the position (a) were higher than that synthesized on the position (b) [See the Fig. 2]. Also, the length of CNFs was different. In the shape, CNFs with screw and straight line shape were synthesized in the position (a), but. only CNFs with straight line shape were synthesized in the position (b). The difference have an important effect on the field emission characteristics.
    Journal of the Korea Industrial Information System Society. 01/2006; 11(4).
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    ABSTRACT: A detailed structural analysis has been carried out on vertically aligned carbon nanotubes (CNTs) formed by thermal chemical vapor deposition (CVD) with an Fe/Al multilayer catalyst on a Si substrate. From the results, it was confirmed that the CNTs are composed of two configurations, namely high-density vertically aligned CNTs formed on the catalyst, followed by randomly oriented thin CNTs with low crystallinity formed on the high-density CNTs. Growth kinetics was proposed by considering the dependence of the CNT growth rate on the size of catalyst grains.
    Japanese Journal of Applied Physics 01/2006; 45(No. 11):8988-8990. · 1.07 Impact Factor
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    ABSTRACT: Using scanning tunneling microscopy, reversible phase transitions have been detected in the modulated pseudomorphic In monolayer on the Si(111) surface. It has been found that the room-temperature quasihexagonal √7×√3 structure is transformed into the √7×√7 structure during cooling in the temperature range from 265 to 225 K. Further cooling results in developing long-range modulations in the In layer, including formation of the chevron-type structure with 6√7×√7 periodicity, the ordered arrays with regular antiphase domain boundaries with local 3√7×√7, and 2√7×√7 periodicity and the chained-ring structure with 10/3√3×40/3√3 periodicity, which is believed to originate from the 5/3√3×5/3√3 structure occurring at room temperature near surface defects and at domain boundaries of the original √7×√3-In phase.
    Physical Review B 01/2006; 74(3). · 3.66 Impact Factor

Publication Stats

1k Citations
474.88 Total Impact Points

Institutions

  • 2011
    • King Mongkut's Institute of Technology Ladkrabang
      • College of Nanotechnology
      Bangkok, Bangkok, Thailand
  • 1976–2011
    • Osaka University
      • • Research Center for Ultra-High Voltage Electron Microscopy
      • • Division of Electronic and Information Engineering
      • • Graduate School of Engineering
      • • Department of Electrical and Electronics Engineering
      • • Department of Information Systems Engineering
      Suika, Ōsaka, Japan
  • 2003
    • Japan Fine Ceramics Center
      Nagoya, Aichi, Japan
  • 2000
    • Far Eastern State Medical University
      Chabarowsk, Khabarovsk Krai, Russia
  • 1991–1997
    • Osaka Institute of Technology
      Ōsaka, Ōsaka, Japan