[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: Different adsorbate/silicon systems Si(100) and Si(111) surfaces were investigated to study structural and transport properties of the surface phases on silicon. Scanning tunneling microscopy (STM), reflection-high-energy-electron diffraction and electrical resistance measurement techniques were used for the investigation. It was shown that the superstructures formed in a few of the topmost atomic layers on a single crystal surface, morphology of the surface and conductive islands grown on the surface cause drastic changes in the electrical resistance.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: Hydrogen interaction with single-walled carbon nanotubes (SWNTs) was explored using an SWNT thin-film sensor and thermal desorption spectroscopy (TDS). It was revealed that the adsorption of atomic hydrogen on SWNTs exhibits nonactivated and thermally activated adsorption states obeying first-order kinetics, in which the nonactivated adsorption is dominant. The subsequent desorption of hydrogen molecules was found to follow first-order kinetics with an activation energy of 1.7 eV. These results explain the theoretical prediction that the adsorbed H atoms are paired on the SWNT surface.
[Show abstract][Hide abstract] 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 04/2008; 47(4):1941-1943. DOI:10.1143/JJAP.47.1941 · 1.13 Impact Factor
[Show abstract][Hide abstract] 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 04/2008; 17(4-5):556-558. DOI:10.1016/j.diamond.2007.09.014 · 1.92 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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/mu 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/mu 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. (c) 2007 American Institute of Physics.
[Show abstract][Hide abstract] 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 02/2007; 46(2):867-869. DOI:10.1143/JJAP.46.867 · 1.13 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 11/2006; 45(11-No. 11):8988-8990. DOI:10.1143/JJAP.45.8988 · 1.13 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: The density control of vertically aligned carbon nanotubes (CNTs) has been studied using a thermal chemical vapor deposition (CVD) method in correlation with the thicknesses of the Fe/Al multilayer catalyst on a Si substrate. The density of CNTs could be controlled through the thicknesses of both the Al layer and Fe catalyst layer. The low density of the Fe catalyst grains formed by thermal treatment, caused the formation of CNTs with random orientations, and resulted in a decrease in the CNT height. The optimum film thickness of the Fe/Al multilayer catalyst for obtaining long CNTs (220 mum) was determined for the case of a substrate temperature of 700 °C.
Japanese Journal of Applied Physics 07/2006; 45(7-No. 7):6043-6045. DOI:10.1143/JJAP.45.6043 · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Vertically aligned carbon nanofiber (CNF) films were successfully grown on glass substrates at 450 °C with metal buffer layers by inductively coupled plasma chemical vapor deposition (ICP-CVD). The diameter and number density of the aligned CNFs can be controlled by changing the type and thickness of the metal buffer layers deposited on the glass substrates. The metal buffer layers play an important role in reducing the thermal expansion coefficient difference between the catalyst metal film and the glass substrate, resulting in the enhancement of the formation of catalyst nanoparticles so as to grow the aligned CNFs with high number density.
Japanese Journal of Applied Physics 06/2006; 45(6A):5326-5328. DOI:10.1143/JJAP.45.5326 · 1.13 Impact Factor
[Show abstract][Hide abstract] 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)//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; 508(1):82-85. DOI:10.1016/j.tsf.2005.08.385 · 1.76 Impact Factor