Taketoshi Matsumoto

Reduction of Pt usage in fuel cell electrocatalysts with carbon nanotube electrodes

The 12 wt% Pt-deposited carbon nanotube electrode gives 10% higher voltages than 29 wt% Pt-deposited carbon black and reduces the Pt usage by 60% in polymer electrolyte fuel cells with hydrogen and oxygen.

Observation of single- and double-stranded DNA using non-contact atomic force microscopy

Non-contact atomic force microscopy (NC-AFM) has been applied to observe single- and double-stranded DNA. For the wet processes used to prepare the sample, a strong adhesion force at the surface is observed even in vacuum conditions. Despite the presence of this adhesion force, single- and double-stranded DNA images can be obtained by NC-AFM. Because of the high sensitivity of the tip-sample interaction, NC-AFM images provide stronger contrast than tapping mode (TM)-AFM images. NC-AFM images reveal detailed structures of single- and double-stranded DNA which are not revealed by TM-AFM. In addition, several NC-AFM images show contrast artifacts, which might provide information on the detailed structure of DNA.

A scanning tunneling microscopy study of laser molecular beam epitaxy on SrTiO3(100) surface

We have constructed a new combined system consisting of an ultra-high vacuum scanning tunneling microscope (UHV-STM) and a laser molecular beam epitaxy (laser-MBE) growth chamber with reflection high-energy electron diffraction (RHEED). One of the capabilities of this system was demonstrated by STM imaging of a laser-ablated strontium layer on an atomically flat surface of reduced SrTiO3(100). The surface structure was investigated in real space by atomic-resolution STM and the results were compared with reciprocal space information obtained by RHEED. At room temperature, the STM image showed small Sr islands on the surface which have lateral sizes of 2 to 3 nm with monatomic height. At 320°C, the Sr atoms were in row structure along the √5×√5−R26.6° reconstructed surface structure of reduced SrTiO3(100).

Fuel Cell Anode Composed of Mo2C Catalyst and Carbon Nanotube Electrodes

Replacement of the normal Pt/C anode of a polymer electrolyte fuel cell (PEFC) with a Mo 2 C/CNT (carbon nanotube) electrode gave about half the voltage (below 600 mA/cm 2 ). 2-20 nm Mo 2 C nanoparticles were deposited on multiwalled carbon nanotubes (MWCNTs) by adsorption of MoO 2 (acac) 2 and carburization in methane. Although the Mo 2 C/CNT electrodes studied herein worked at a much higher overvoltage compared to Pt-based electrodes, nevertheless, their activity thus proven may be further improved. Hence, the combination of Mo 2 C and CNT presented herein is a step on the way to develop Pt-free electrodes.

Ultrathin SiO2 layer with an extremely low leakage current density formed in high concentration nitric acid

An ultrathin silicon dioxide (SiO2) layer of 1.2–1.4 nm thickness has been formed by immersion of Si wafers in nitric acid (HNO3) aqueous solutions, and its electrical characteristics and physical properties are investigated as a function of the HNO3 concentration. Measurements of transverse optical and longitudinal optical phonons of Si–O–Si asymmetric stretching vibrational mode for SiO2 indicate that the atomic density of the SiO2 layer increases with the HNO3 concentration. X-ray photoelectron spectroscopy measurements show that the valence band discontinuity energy at the SiO2/Si interface also increases and the concentration of suboxide species decreases with the HNO3 concentration. The leakage current density of the ⟨Al/SiO2/Si(100)⟩ metal-oxide-semiconductor (MOS) diodes with the SiO2 layer formed in HNO3 aqueous solutions decreases with the HNO3 concentration and also decreases by postmetallization annealing (PMA) treatment at 250 °C in 5 vol % hydrogen atmosphere. For the MOS diodes with the SiO...

Ultrathin SiO2 layer on atomically flat Si(111) surfaces with excellent electrical characteristics formed by nitric acid oxidation method

We have developed a method of formation of atomically smooth Si∕SiO2 interfaces by oxidation of atomically flat Si(111) surfaces by use of azeotropic nitric acid (HNO3) aqueous solutions (i.e., 68wt% HNO3 at 121°C). For the SiO2 layer on the atomically smooth Si substrates, the concentration of suboxide species, Si2+, is ∼50% of that on the rough Si substrates, and the valence band discontinuity is higher by ∼0.1eV. In this case, the leakage current flowing through the ∼1.2nm SiO2 is low, and further decreased by postmetallization annealing at 250°C in hydrogen (e.g., 0.5A∕cm2 at VG=1V).

Ultra-low reflectivity polycrystalline silicon surfaces formed by surface structure chemical transfer method

A nanocrystalline Si layer can be formed by the surface structure chemical transfer (SSCT) method in which a platinum mesh is instantaneously contacted with polycrystalline Si wafers immersed in hydrogen peroxide plus hydrofluoric acid solutions. The polycrystalline Si surface after the SSCT method possesses an ultra-low reflectivity. The nanocrystalline Si layer possesses a 100–150 nm thickness, and gives a photoluminescence with a peak maximum at ∼670 nm, indicating band-gap widening. The minority carrier lifetime of as-sliced Si wafers greatly increases after the SSCT method most probably due to the enlargement of the nanocrystalline Si band-gap.

Spin-polarized scanning tunneling microscopy on half-metallic manganite thin film with half-metallic manganite tip

Spin-polarized tunneling was demonstrated on La0.7Sr0.3MnO3 epitaxial thin film with a La0.7Sr0.3MnO3-coated PtIr tip at room temperature. A magnetic-field-dependent gap structure was found in the differential tunneling spectra due to the spin-valve effect. The spatial mapping of the differential tunneling conductance shows strongly contrasting variations, which demonstrates the presence of magnetic domains with different spin alignments.

Effects of pressure on the electrical resistivities of PrBa2Cu4O8 and Pr2Ba4Cu7O15−δ

Abstract We have investigated the effects of pressure on the electrical resistivities of PrBa 2 Cu 4 O 8 (Pr124) and Pr 2 Ba 4 Cu 7 O 15−δ (Pr247). The temperature coefficient of the resistivity is positive below 150–190 K ( T m ) while that above T m is negative for both systems. We found that the resistivity below T m is insensitive to pressure in the pressure range 0–1 GPa and by contrast the resistivity above T m remarkably increases with increasing pressure. At higher pressures the resistivity below T m is also affected by pressure. We attribute the positive temperature coefficient of the resistivity below T m to the metallic conduction along the double chains and the negative one above T m to the semiconducting conduction through the CuO 2 planes.

Ultrathin SiO2 layer with a low leakage current density formed with ~ 100% nitric acid vapor

An ultrathin silicon dioxide (SiO(2)) layer with 0.65-1.5 nm thickness has been formed by approximately 100% nitric acid (HNO(3)) vapor oxidation, and its electrical characteristics and physical properties are investigated. The oxidation kinetics follows a parabolic law except for the ultrathin (<or=0.8 nm) region, indicating that diffusion of oxidizing species (i.e. oxygen atoms generated by decomposition of approximately 100% HNO(3) vapor) through a growing SiO(2) layer is the rate-determining step. The diffusion activation energy for HNO(3) vapor oxidation is 0.14 eV, much lower than that of thermal oxidation of 1.24 eV. The leakage current density for the 0.65 nm SiO(2) layer formed by HNO(3) vapor oxidation is lower by approximately one order of magnitude than that for a thermal oxide layer with the same thickness. The low leakage current density is attributed to (i) the atomically flat SiO(2)/Si interface and uniform thickness of the ultrathin SiO(2) layer, (ii) the low concentration of suboxide species and the low interface state density and (iii) the high atomic density of the SiO(2) layer, which leads to a high band discontinuity energy at the SiO(2)/Si interface. The leakage current density is further decreased by PMA at 250 degrees C in 5 vol% H(2) atmosphere.