本多 信一

Single-wall carbon nanotubes (SWCNTs) are promising materials for electronic applications, such as transparent electrodes and thin-film transistors. However, the dispersion of isolated SWCNTs into solvents remains an important issue for their practical applications. SWCNTs are commonly dispersed in solvents via ultrasonication. However, ultrasonication damages SWCNTs, forming defects and cutting them into short pieces, which significantly degrade their electrical and mechanical properties. Herein, we demonstrate a novel approach toward the large-scale dispersion of long and isolated SWCNTs by using hydrodynamic cavitation. Considering the results of atomic force microscopy and dynamic light-scattering measurements, the average length of the SWCNTs dispersed via the hydrodynamic cavitation method is larger than that of the SWCNTs dispersed by using an ultrasonic homogenizer.

Abstract The chemical vapor transport method was used in this research to synthesize MoS₂ bulk. Through mechanical exfoliation, we limited the thickness of MoS₂ flakes from 1 to 3 μm. In order to fabricate a p–n homogeneous junction, we used oxygen plasma treatment to transform the MoS₂ characteristics from n-type to p-type to fabricate a p–n homogenous junction and demonstrate the charge neutrality point shift from −80 to +102 V successfully using FET measurement. The MoS₂ p–n homogeneous junction diode showed an excellent p-n characteristic curve during the measurements and performed great rectifying behavior with 1–10 V_pp in the half-wave rectification experiment. This work demonstrated that MoS₂ flake had great potential for p-n diodes that feature significant p–n characteristics and rectifying behavior.

To clarify the nature of defects presented in neutron (n)-irradiated highly oriented pyrolytic graphite (HOPG), in situ X-ray diffraction (XRD) observation at room temperature (RT) and high pressure was conducted with synchrotron radiation (SPring-8). We focused on the graphite (002) [G(002)] peak under compression to 18.1 GPa and also under decompression. For comparison, unirradiated HOPG was also placed in the same high-pressure cell. We found that the G(002) peak can be represented by two components, the S and L peaks, for the n-irradiated HOPG, whereas it can be represented by only one component for the unirradiated HOPG. The d-spacing for the n-irradiated and unirradiated HOPG samples gradually decreased with increasing pressure. At 18.1 GPa, the d-spacing of the S peak of the irradiated sample became almost the same as that of the unirradiated one, but that of the L peak was larger. Under decompression, the behavior of the d-spacing was almost opposite to that under compression, and the d-spacing was restored to its value before compression. Also, taking account of the changes in the peak widths, we referred to and considered irradiation-induced defects of interstitial-type defects existing between the basal planes and in-plane defects of dislocation dipoles as possible defects that affect the changes in the G(002) peak.

In situ X-ray diffraction observation was done for neutron-irradiated and un-irradiated highly oriented pyrolytic graphite (HOPG) samples with synchrotron radiation to clarify the effect of irradiation-induced defects on the transformation to diamond under high-pressure and high-temperature treatment. At 16 GPa, no remarkable change appeared for the irradiated HOPG with increasing the temperature up to 800 °C. At temperatures of 1200 °C and 1400 °C, hexagonal diamond was formed, along with the formation of cubic diamond. This is probably due to annealing of the irradiation defects that led to partial restoration of the structure to the original HOPG and then enables the formation. On the other hand, in un-irradiated HOPG, hexagonal diamond was formed at 400 °C, which changed to cubic diamond at 1200 °C or higher. We guess that irradiation defects promote the nucleation of cubic diamond in the irradiated sample and then contribute to the formation of isotropic polycrystalline diamond or amorphous diamond.

© 2019 John Wiley & Sons, Ltd. Interaction of highly charged ions (HCIs) with surfaces produce various specific phenomena as a consequence of the potential energy that HCI possesses. In the present study, we have observed photon emission, structural, magnetic, and electronic modification on various carbon-based materials such as carbon nanotube by the impact of HCIs using an electron beam ion source named Kobe EBIS installed at the Kobe University. In order to study the potential effect, HCIs of Arq+ (q = 6–16) with the intensity of 0.1–1 nA are projected on the surface with a constant kinetic energy (16 keV). For photon emission measurements, we observed spatial and spectral distribution of visible light emission from the surface during irradiation with HCIs. On the other hand, the structural modification of multi-walled carbon nanotubes (MWCNTs) irradiated with HCIs has been analyzed using a transmission electron microscopy and Raman spectroscopy. Irradiation effects on the resistivity of single MWCNT supported on micrometer scale bridge pattern were also measured. We have also measured magnetic structure of highly oriented pyrolytic graphite irradiated with HCIs using electron spin resonance at low temperature. At the present paper, we will review our recent experimental results on the interaction of HCI with various carbon-based materials.

© 2019 The Japan Society of Applied Physics. The study used multi-layer graphene, synthesized by thermal chemical vapor deposition, as the material for a homo-junction pn device formation. Due to the oxygen adsorption in air, the synthesized pristine graphene behaved as a p-type semiconductor. In order to modulate the graphene semiconductor type, the nitrogen plasma treatment with radio-frequency power as a parameter was used to transfer the semiconductor characteristics. The graphene-based field-effect transistor transfer characteristics were subsequently measured to confirm whether the modified graphene was p-type or n-type. Different nitrogen doping levels affected the diode properties differently. It demonstrated that the rectification effect of the formed pn device was clearly observed using this simple strategy. We also successfully produced this graphene lateral pn device applying a half-wave rectification circuit. The effective formed pn device can be further applied in bipolar junction transistor development.

Inexpensive and sensitive graphite electrodes were fabricated by applying flame annealing to pencil-graphite rods (PGRs) as electrodes for water electrolysis cells. The resin (polymer, binder) on the surface of PGR was removed by flame annealing to make it porous, and the graphite electrodes with high activity and low cost were obtained. By flame annealing the PGR, although the PGR electrode became active upon water electrolysis, the PGR electrode became instable for long-time operation. The effects of flame annealing on PGR for water electrolysis were analyzed by SEM, FT-IR spectroscopy, Raman spectroscopy, NEXAFS, and electrochemical impedance spectroscopy (EIS).

© 2019 The Japan Society of Applied Physics. The electric characteristics of individual multi-wall carbon nanotubes (MWNTs) irradiated with highly charged ions were evaluated. Each MWNT was located on a highly doped Si substrate with a source/drain contact, forming a back-gate FET configuration. We used highly charged Ar ions (Ar8+, Ar11+, and Ar14+) extracted from an electron beam ion source. We measured the current-voltage curves of the MWNTs irradiated at fluences in the 1011-1013 cm-2 range. The Ar8+ irradiation caused a 10% increase in the resistivity at room temperature at a fluence of 6 × 1011 cm-2. The resistivity at room temperature for the MWNT samples with a high fluence of 1013 cm-2 reached values higher than five times the original values. The current-voltage characteristics at low temperature became nonlinear and Coulomb oscillations were observed in the gate voltage dependence of the drain current. One of the samples exhibited Coulomb diamond characteristics particular to single quantum dot at 1.6 K.

© 2018 The Japan Society of Vacuum and Surface Science. Electron spin resonance (ESR) measurements were performed on highly oriented pyrolytic graphite (HOPG) samples irradiated with highly charged ions (HCIs). The interaction between a HCI and surfaces results in emission of photons in the range of visible to X-ray, hundreds of secondary electrons, sputtering of secondary ions and modification of surface structure in nanometer scale. In the present experiments, HCIs were produced by electron beam ion source (EBIS) and Ar8+ and Ar14+ were used for the irradiation. ESR measurement provides information on unpaired electrons of the sample. We investigated the dependence of defect formation on charge state and fluence of incident HCIs using ESR. The L1 line appeared in HOPG samples irradiated with HCIs at the low temperature region, and the intensity became larger at higher charge state and higher fluence.

© 2018 Author(s). The super hard material of "compressed graphite" (CG) has been reported to be formed under compression of graphite at room temperature. However, it returns to graphite under decompression. Neutron-irradiated graphite, on the other hand, is a unique material for the synthesis of a new carbon phase, as reported by the formation of an amorphous diamond by shock compression. Here, we investigate the change of structure of highly oriented pyrolytic graphite (HOPG) irradiated with neutrons to a fluence of 1.4 × 1024 n/m2 under static pressure. The neutron-irradiated HOPG sample was compressed to 15 GPa at room temperature and then the temperature was increased up to 1500 °C. X-ray diffraction, high-resolution transmission electron microscopy on the recovered sample clearly showed the formation of a significant amount of quenchable-CG with ordinary graphite. Formation of hexagonal and cubic diamonds was also confirmed. The effect of irradiation-induced defects on the synthesis of quenchable-CG under high pressure and high temperature treatment was discussed.

Highly oriented pyrolytic graphite (HOPG) is a unique source material for the synthesis of new types of diamond. It can transform to layered nano-polycrystalline diamond (NPD) under static high pressure and high temperature (HPHT) and to “amorphous diamond” by introducing structural defects by neutron irradiation followed by shock compression. Here, we investigated the structural change of the neutron-irradiated HOPG through a HPHT treatment at 2300 °C and 15 GPa by Raman, X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray absorption near edge structure (XANES) analyses. The recovered sample consists of randomly oriented nanodiamonds (50–100 nm), showing clearly a different microtexture from those of the layered NPD and “amorphous diamond” reported by the previous studies. This is likely as a result of competing effects of the introduction of irradiation-induced defects, which provided the preferential nucleation sites for cubic diamond, and their partial recovery upon annealing during the HPHT treatment. The present result suggests that NPDs with various crystalline structures can potentially be synthesized from neutron-irradiated HOPG by controlling the density and distribution of the defects introduced.

Low-energy Ar ions (0.5-2 keV) were irradiated onto vertically aligned multiwalled carbon nanotube (VA-MWCNT) films, and the chemical states, orientation, and oxygen content of the irradiated VA-MWCNT films were investigated by soft X-ray absorption spectroscopy (XAS). After irradiation, two additional peaks appeared in the XAS spectra in the C K region, indicating a significant change in the electronic structure. Furthermore, it was found that the VA-MWCNT films were oxidized after irradiation. The results of chemical-state analyses of the XAS spectra in both the C K and 0 K regions indicated the formation of carboxyl groups (-COOH) on the surface of the VA-MWCNT films upon irradiation. The oxygen content of the VA-MWCNT films increased with increasing ion energy and fluence. (C) 2016 Elsevier B.V. All rights reserved.

Chemical doping with hetero-atoms is an effective method used to change the characteristics of materials. Nitrogen doping technology plays a critical role in regulating the electronic properties of graphene. Nitrogen plasma treatment was used in this work to dope nitrogen atoms to modulate multilayer graphene electrical properties. The measured I-V multilayer graphene-base field-effect transistor characteristics (GFETs) showed a V-shaped transfer curve with the hole and electron region separated from the measured current-voltage (I-V) minimum. GFETs fabricated with multilayer graphene from chemical vapor deposition (CVD) exhibited p-type behavior because of oxygen adsorption. After using different nitrogen plasma treatment times, the minimum in I-V characteristic shifted into the negative gate voltage region with increased nitrogen concentration and the GFET channel became an n-type semiconductor. GFETs could be easily fabricated using this method with potential for various applications. The GFET transfer characteristics could be tuned precisely by adjusting the nitrogen plasma treatment time.

The carbon nanotube (CNT) has replaced palladium oxide (PdO) as the electrode material for surface-conduction electron-emitter (SCE) applications. Vertically aligned CNT arrays with a delta-star arrangement were patterned and synthesized onto a quartz substrate using photolithography and thermal chemical vapor deposition. Delta-star shaped VACNT arrays with 20 degrees tips are used as cathodes that easily emit electrons because of their high electrical field gradient. In order to improve the field emission and secondary electrons (SEs) in SCE applications, magnesium oxide (MgO) nanostructures were coated onto the VACNT arrays to promote the surface-conduction electron-emitter display (SED) efficiency (11). According to the definition of eta in SCE applications, in this study, the eta was stably maintained in the 75-85% range. The proposed design provides a facile new method for developing SED applications. (C) 2017 Elsevier B.V. All rights reserved.

© 2016 Elsevier B.V. Tungsten-substituted molybdenum disulfide (Mo1 − xWxS2) shows excellent semiconductor properties related to the adjustable band gap structure. Mo1 − xWxS2 is an n-type semiconductor that has high carrier mobility, adjustable electrical characteristics. Graphene, the thinnest two-dimensional material, shows good thermal conductivity and high carrier mobility. The oxygen adsorption effects of graphene permit precise control of its Fermi level and semiconductor properties. We combined Mo1 − xWxS2 films with graphene oxide (GO) in this study to fabricate a pn interface and thoroughly research the energy band and electrical properties for a heterojunction diode. According to the results, the Mo1 − xWxS2 electron affinity decreases with increasing x value of tungsten (W) composition (x = 0.0–1.0, Δx = 0.2). The I–V characteristics of the Mo1 − xWxS2/GO heterojunction diodes can be accurately tuned by conduction band bowing effect with different W composition of the Mo1 − xWxS2 films. By applying our fabricating method, the Mo1 − xWxS2 with different W composition provides a new application of semiconductor device. In this study we using Mo1 − xWxS2 films and GO to fabricate a heterojunction diode, it shows different electrical properties with different x value of Mo1 − xWxS2. The Mo1 − xWxS2 electron affinity decreases with increasing of W composition x value (x = 0.0–1.0, Δx = 0.2). The Mo1 − xWxS2/GO heterojunction diode I–V characteristics can be accurately tuned using the conduction band bowing effect with different W compositions. Mo1 − xWxS2 with different W compositions using our fabrication method provides new semiconductor device applications.

We investigated the photoconductive characteristics of tungsten-substituted molybdenum disulfide (Mo1-xWxS2) series materials synthesized by the chemical vapor transport (CVT) method using different x values of W composition (x = 0-1, Delta x = 0.1). We applied the Mo1-xWxS2 series materials to photoconductive detectors and then measured photocurrent at different laser source wavelengths: 405, 532, 633, and 808 nm. The band gap bowing effect and absorbance indicated the photoconduction mechanism in the Mo1-xWxS2 series materials. The proportion of x influenced the conductivity and absorbance, and induced the energy-gap bowing effect of Mo1-xWxS2. (C) 2017 The Japan Society of Applied Physics

The interaction between ultra-intense laser light and vertically aligned carbon nanotubes is investigated to demonstrate efficient laser-energy absorption in the ps laser-pulse regime. Results indicate a clear enhancement of the energy conversion from laser to energetic electrons and a simultaneously small plasma expansion on the surface of the target. A two-dimensional plasma particle calculation exhibits a high absorption through laser propagation deep into the nanotube array, even for a dense array whose structure is much smaller than the laser wavelength. The propagation leads to the radial expansion of plasma perpendicular to the nanotubes rather than to the front side. These features may contribute to fast ignition in inertial confinement fusion and laser particle acceleration, both of which require high current and small surface plasma simultaneously. Published by AIP Publishing.

Amorphous carbon nanorods (CNRs) were deposited directly using radio frequency magnetron sputtering. The synthesized CNR electrochemical properties were investigated using graphene as the current collector for an electric double layer capacitor. The CNRs were vertically aligned to the graphene to achieve higher specific surface area. The capacitor performance was characterized using electrochemical impedance spectroscopy, cyclic voltammetry, and galvanostatic charge-discharge testing in 1 M KOH electrolyte at 30 degrees C, 40 degrees C, 50 degrees C, and 60 degrees C. The CNR specific capacitance was observed to increase with increasing measurement temperature and could reach up to 830 F/g at 60 degrees C. Even after extensive measurements, the CNR electrode maintained good adhesion to the graphene current collector thereby suggesting electrode material stability. (C) 2014 Elsevier B.V. All rights reserved.

Carbon nanotubes (CNTs) were grown successfully onto a glass substrate using thermal chemical vapor deposition (TCVD) with C2H2 gas at 700 degrees C. The synthesized CNTs exhibited good crystallinity and a vertically aligned morphology. The vertically aligned CNTs (VACNTs) were patterned with a honeycomb configuration using photolithography and characterized using field emission (FE) applications. Owing to the electric field concentration, the FE current density of VACNTs with honeycomb configuration was higher than that of the un-patterned VACNTs. Ti was coated onto the VACNT surface utilizing the relatively lower work function property to enhance the FE current density. The FE current density reached up to 7.0 mA/cm(2) at an applied electric field of 2.5 V/mu m. A fluorescent screen was monitored to demonstrate uniform FE VACNTs with a honeycomb configuration. The designed field emitter provided an admirable example for FE applications. (C) 2013 Elsevier B.V. All rights reserved.

Amorphous carbon nanorods were synthesized from hollow multiwalled carbon nanotubes (MWCNTs) by low-energy Ar ion irradiation at room temperature. The obtained nanorods were investigated by Raman spectroscopy, transmission electron microscopy (TEM), soft X-ray photoelectron spectroscopy (XPS), and soft X-ray absorption spectroscopy (XAS). It was found that the diameter of the MWCNTs significantly increased with increasing the fluence. Finally, the original hollow structure and the graphite (002) TEM diffraction spots of MWCNTs completely disappeared and a broadening of Raman spectra occurred, indicating the amorphization of MWCNTs. The increase in the diameter by the irradiation can be explained by the bending and the randomization of the broken carbon hexagonal networks, and the accumulation of knocked-on atoms. The XPS and XAS measurements also support the formation of amorphous carbon nanorods. (C) 2014 The Japan Society of Applied Physics

Low-energy Ar ions (0.5-2 keV) were irradiated to multi-layer graphenes and the damage process, the local electronic states, and the degree of alignment of the basal plane, and the oxidation process upon ion irradiation were investigated by Raman spectroscopy, soft X-ray absorption spectroscopy (XAS) and in situ X-ray photoelectron spectroscopy (XPS). By Raman spectroscopy, we observed two stages similar to the case of irradiated graphite, which should relate to the accumulations of vacancies and turbulence of the basal plane, respectively. XAS analysis indicated that the number of sp(2)-hybridized carbon (sp(2)-C) atoms decreased after ion irradiation. Angle-resolved XAS revealed that the orientation parameter (OP) decreased with increasing ion energy and fluence, reflecting the turbulence of the basal plane under irradiation. In situ XPS shows the oxidation of the irradiated multi-layer graphenes after air exposure. (C) 2013 Elsevier B.V. All rights reserved.

We have investigated the interaction of ultra intense laser light with a carbon nanotube (CNT) target. The experimental results show an increased electron acceleration and a very low laser reflection as compared to non-structured targets. In addition, interferograms show very weak plasma expansion in front of the CNT target whereas the flat target creates a considerable amount of preformed plasma. A 2-D PIC calculation indicates that high laser absorption is possible via a Brunel mechanism following the ponderomotive heating in the expanded plasma between nanotubes. © Owned by the authors, published by EDP Sciences, 2013.

Controlling defect structure in multiwalled carbon nanotube (MWCNT) is essential to realization of MWCNT devices. Here, we show that the diagram of the Raman intensity ratio of the G to D peaks and the G peak width can reveal two damaging stages of MWCNT films. In a transition period, additional peaks appeared in the X-ray absorption spectra, thereby indicating some significant change in the electronic structure. Also, a remarkable increase occurred in the diameter of the MWCNTs in the latter stage, suggesting the formation of dislocation dipoles which may relate to the change in the properties of field-emission devices. (C) 2012 The Japan Society of Applied Physics

Protective-layer-coated single-walled carbon nanotubes (SWNTs) with palladium nanoparticle decoration (Pd-SiO2-SWNTs) were fabricated and their sensing properties for hydrogen (H-2) were investigated. SWNTs were coated with a 3-4 nm thick SiO2 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-SiO2-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-SiO2-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-SiO2-SWNTs with the netlike structure remained unchanged. The hydrogen sensing mechanism of the Pd-SiO2-SWNTs was attributed to the chemical gating effect on the SWNTs due to dipole layer formation by hydrogen atoms trapped at the Pd-SiO2 interface. Moreover, the relationship between H-2 concentration and sensor response can be described by the Langmuir isotherm for dissociative adsorption.

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 SiO(x) and AlO(x) by pulsed laser deposition, followed by annealing under Ar/H(2) 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 NO(2) was also improved. Moreover, the relationship between NO(2) concentration and sensor response can be described by the Frumkin-Temkin isotherm. (C) 2009 The Japan Society of Applied Physics

The density of states (DOS) of single-walled carbon nanotubes (SWNTs) grown on a metal tip apex was investigated by scanning tunneling spectroscopy (STS). SWNTs were synthesized directly on a Ptlr tip by thermal chemical vapor deposition and utilized as a scanning tunneling microscopy (STM) tip. DOS corresponding to both metallic and semiconducting SWNTs were obtained. The STS spectra displayed sharp peaks originating from van Hove singularities, which revealed the energy gap and diameter. Furthermore, the chirality of SWNTs was determined by comparing the experimental result with theoretical calculations. (C) 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.035005

The facet surface of a Ge nanocrystal prepared on Si(111) was investigated by scanning tunneling microscopy and spectroscopy (STM/STS) using a metal-coated carbon nanotube tip. Because of the small radius and high aspect ratio of the tip, we observed a steep facet surface in detail. It was revealed that the surface formed a Ge(105) 1x2 structure. STS spectra obtained on the facet surface showed density of states (DOS) originating from the surface and bulk states, which shifted depending on the height. First-principles calculation revealed that the shift arose from the intermixing of Si with Ge. (C) 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.035002

The solid-phase epitaxial growth process and surface structure of MnSi oil Si(1 1 1) were investigated by coaxial impact-collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). For the Si(1 1 1) sample deposited with 30 monolayers (ML) of Mn at room temperature, the intermixing of Mn and Si gradually started at 100 degrees C and reached equilibrium at approxinnately 400 degrees C. At this equilibrium state, the Mn atoms were transformed into crystalline MnSi film. Further annealing caused the desorption of Mn atoms. We identified the structure of MnSi as cubic B20 and the crystallographic orientation relationships as Si(1 1 1)//MnSi(1 1 1) and Si[(1) over bar 2 (1) over bar]//MnSi[(1) over bar 1 0]. The MnSi(1 1 1) surface was found to have a dense Si terminating layer oil its topmost Surface. On the other hand, 3 ML of Mn deposited on Si(1 1 1) reacted with Si even at room temperature and formed a pseudomorphic structure. This structure was transformed into MnSi after annealing A filmlike morphology with protrusions was observed for the sample with 30 ML of Mn, while island growth occurred for the sample with 3 ML of Mn. (C) 2008 Elsevier B.V. All rights reserved.

The adsorption kinetics of NO2 on a single-walled carbon nanotube (SWNT) thin-film sensor was investigated. To avoid the influence of ambient air, the adsorption property of SWNTs was explored under high vacuum. By virtue of the suppression of the influence of residual gases and the cleanness of the SWNT surface in vacuum, the SWNTs exhibited high sensitivity with a detection limit of lower than 1 ppb. On the basis of the Langmuir adsorption isotherm, the sticking probability and adsorption energy of NO2 molecules on SWNTs were experimentally estimated. [DOI: 10.1143/JJAP.47.8145]

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 1.7eV. These results explain the theoretical prediction that the adsorbed H atoms are paired on the SWNT surface. (C) 2008 The Japan Society of Applied Physics.

We characterized the surface cleanliness. structure. and composition oh GaN(0001) grown by liquid phase epitaxy (LPE) using coaxial impact-collision ion scattering spectroscopy (CAICISS). Contamination oil the as-received LPL-GaN(0001) vvas removed by annealing at 900 degrees C, but annealing was unable to remove the O atoms from the as-grown surface, indicating that the as-grown sample incorporated O atoms. Although the as-grown sample contained O atoms, its structure perfection was maintained. These facts were explained by localization of O atoms near dislocations. The etched surface exhibited high crystalline quality, which was comparable to that of the sample grown by vapor phase epitaxy (VPE).

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 mu m. The most-enhanced field concentration was obtained at R/H similar to 6, although we previously reported that the optimal configuration of pillars for R = 250 mu m 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. (C) 2007 Elsevier B.V. All rights reserved.

We fabricated a Bayard-Alpert-type ionization gauge using a screen-printed field electron emitter with carbon nanotubes (CNTs) as an electron source. Vertically aligned CNTs grown on Si substrates by thermal chemical vapor deposition was used as the source material for CNT paste to make the screen-printed field emitter. The fabricated gauge responded linearly to the pressure in the range from 10(-4) to 10(-8) Torr,and exhibited a sensitivity of 13Torr(-1), which is competitive with the commercial gauge using a hot-filament electron source. The ion current of the gauge also showed good stability for more than 15 h, indicating the high quality of the electron emitter. This work demonstrated the feasibility of the vacuum electronic device using the CNT field emitter.

Vertically aligned carbon nanotubes (CNTs) with a high number density of 2 x 10(11) cm(-2) were successfully synthesized by catalyst-assisted thermal chemical vapor deposition using catalyst preheated in C(2)H(2) 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 C(2)H(2) as compared with those treated in H(2) 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.

The facet surfaces of a self-organized ErSi2 nanocrystal were observed by the STM with a metal-coated carbon nanotube tip. The 10nm-scale radius and high aspect ratio of the tip was able to provide detailed images of the facet surfaces. The stereographic STM imaging showed that the nanocrystal had (1 0 5) facets of the tetragonal ErSi2 on all facet surfaces with a 1 x 1 periodicity. Bias-dependent STM images revealed that the facet surface has a bulk-truncated structure consisting of Si dimers and Er atoms. Moreover, scanning tunneling spectroscopy exhibited a difference in density of states between top and facet surfaces. (c) 2008 Elsevier B.V. All rights reserved.

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 H(2). The sensing mechanism was attributed to electron donation to the SWNTs as a result of CO oxidation on the Pt catalyst surface. (C) 2008 The Japan Society of Applied Physics.

We demonstrated the accurate imaging of steep ridges by scanning tunneling microscopy (STM) with a carbon nanotube (CNT) tip coated with a PtIr thin layer. Compared with the conventional tungsten tip, the PtIr-coated CNT tip could trace the shape of steep ridges (140 mn in width, 50 nm in height) more precisely with reduced artifacts originating from the finite shape of the tips. We also estimated the tip radius from the line profiles in the STM image, and proved that the tunneling current exactly flowed through the apex of the PtIr-coated CNT without bending or tilting of the tip during STM.

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 mu m showed good reproducibility of current density (J)-electric field (E) characteristics. With a low threshold field E-th of 1.5 V/mu m, J=1 mA/cm(2) 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.

Stockholm, Sweden, July 2-6, 2007

We performed four-terminal conductivity measurements on a CoSi2 nanowire (NW) at room temperature by using PtIr-coated carbon nanotube (CNT) tips in a four-tip scanning tunneling microscope. The physical stability and high aspect ratio of the CNT tips made it possible to reduce the probe spacing down to ca. 30 nm. The probe-spacing dependence of resistance showed diffusive transport even at 30 nm and no current leakage to the Si substrate.

We present a method for the fabrication of a passivated probe with a nanoscale conductive region. For an electrochemically sharpened W tip wholly coated with inner PtIr and outer SiO2 thin layers by pulsed laser deposition, the local removal of the SiO2 layer from the tip apex was achieved by electron-beam irradiation in a transmission electron microscope. The bared area of the tip apex was less than 50 nm in diameter. Scanning tunneling microscope imaging using the tip revealed that the active area at the apex was conductive. Moreover, the passivated tip showed stability to a biological culture solution.

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.

San Francisco, USA, April 9-13, 2007

Using scanning tunneling microscopy, phase formation and temperature-driven phase transitions in Tl/Ge(100) 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 x 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 ID chains, 5 x 4-Tl and "stroked" phases observed in submonolayer range and a long-period c(12 x 14)-Tl phase developed at around 1 ML. All transitions between these RT and LT structures are reversible. At doses beyond I ML, RT deposition of Tl onto Ge(100) leads to the growth of second-layer Tl stripes, forming arrays with a 1 x 4 periodicity. Meanwhile, structure of the first layer also changes and it displays a set of various reconstructions, c(2 x 8), c(10 x 6) and c(10 x 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 x 2-Tl phase whose rows become decorated by second-layer Tl stripes at prolonged Tl deposition. (c) 2006 Elsevier B.V. All rights reserved.

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/cm(2) for 3 It 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.

We have established a fabrication process for conductive carbon nanotube (CNT) tips for multiprobe scanning tunneling microscope (STM) with high yield. This was achieved, first, by attaching a CNT at the apex of a supporting W tip by a dielectrophoresis method, second, by reinforcing the adhesion between the CNT and the W tip by electron beam deposition of hydrocarbon and subsequent heating, and finally by wholly coating it with a thin metal layer by pulsed laser deposition. More than 90% of the CNT tips survived after long-distance transportation in air, indicating the practical durability of the CNT tips. The shape of the CNT tip did not change even after making contact with another metal tip more than 100 times repeatedly, which evidenced its mechanical robustness. We exploited the CNT tips for the electronic transport measurement by a four-terminal method in a multiprobe STM, in which the PtIr-coated CNT portion of the tip exhibited diffusive transport with a low resistivity of 1.8 k Omega/mu m. The contact resistance at the junction between the CNT and the supporting W tip was estimated to be less than 0.7 k Omega. We confirmed that the PtIr thin layer remained at the CNT-W junction portion after excess current passed through, although the PtIr layer was peeled off on the CNT to aggregate into particles, which was likely due to electromigration or a thermally activated diffusion process. These results indicate that the CNT tips fabricated by our recipe possess high reliability and reproducibility sufficient for multiprobe STM measurements. (c) 2007 American Institute of Physics.

Pillar-shaped carbon nanotube (CNT) bundle emitter shows excellent emission. The field emission properties of the pillar-shaped CNT bundle emitter arrays were simulated using finite element method. The simulation results revealed that the decrease of threshold voltage for emission was caused by the electric field enhancement on the edge of the pillar-shape. The calculated optimum spacing to the height ratio for the CNT bundle emitter arrays to achieve the maximum field emission current was two.

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 degrees C, while horizontally lying SWNT networks with a low density were grown in the temperature range 800-950 degrees 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.

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 4x3-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 2ax3a 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.