Mikka Nishitani-Gamo

Chemical modification of diamond surfaces using a chlorinated surface as an intermediate state

Abstract Chemical modification of diamond powder surfaces was performed using chlorine chemisorption as an intermediate. We found OH, NH, and CF groups on the diamond surface. Chemisorbed species on the diamond powder surface were characterized by diffuse reflectance IR Fourier-transform spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption spectroscopy. Hydrogen chemisorbed on the diamond surface is abstracted by chlorine above 250 °C, followed by the chemisorption of chlorine. Chlorine desorbs thermally from the diamond surface at approximately 300 °C. Hydroxyl groups are produced by treatment of the chlorinated diamond with water vapor at room temperature. Amino groups are produced by treatment of chlorinated diamond with ammonia at 425 °C. CF groups are produced by treatment of chlorinated diamond with CHF3 at 600 °C. Chlorine can easily chemisorb onto the diamond surface and easily desorb from the diamond surface. Using the chlorinated surface as an intermediate state, it is possible further to modify chemically the diamond surface.

Dehydrogenation of light alkanes over oxidized diamond-supported catalysts in the presence of carbon dioxide

Oxidized diamond demonstrated excellent support for the dehydrogenation of light alkanes to alkenes in the presence of CO 2 . Oxidized diamond-supported Cr 2 O 3 and V 2 O 5 catalysts exhibited comparatively higher catalytic activities in the dehydrogenation of lower alkanes in the presence of CO 2 . In the dehydrogenation of propane, the oxidized diamond-supported Cr 2 O 3 and V 2 O 5 catalysts in the presence of CO 2 afforded nearly twofold higher activities than that in the absence of CO 2 . The activity of the oxidized diamond-supported V 2 O 5 catalyst in the dehydrogenation of propane increased with increasing reaction temperatures. Furthermore, in the dehydrogenation of n-butane and iso-butane, a promoting effect of CO 2 on butane conversion and butenes yields was observed over the oxidized diamond-supported Cr 2 O 3 and V 2 O 5 catalysts, though the promotion effect was small. UV-Vis analyses of the fresh and the reacted catalysts in the presence and absence of CO 2 revealed that CO 2 kept the surface V 2 O 5 and Cr 2 O 3 in a state of oxidation slightly higher than that in the absence of CO 2 .

Homoepitaxial diamond growth with sulfur-doping by microwave plasma-assisted chemical vapor deposition

Abstract We have investigated sulfur incorporation and the effects of H 2 S addition on the crystal quality and electrical properties of sulfur-doped homoepitaxial (100) and (111) diamond grown by microwave plasma-assisted chemical vapor deposition. In the case of (100) homoepitaxial growth, adding a small amount of H 2 S effectively improved the crystal quality, whereas adding an excess of H 2 S degraded the crystal quality. The amount of incorporated sulfur increased with increasing amounts of H 2 S. At the same time, the amount of undesirable silicon impurities decreased. Adding H 2 S in the range of 50–100 ppm yielded a high-quality S-doped diamond. In a temperature range of 250–550 K, the high-quality S-doped homoepitaxial (100) diamond showed n -type conduction by Hall-effect measurements. The electrical properties of S-doped (100) diamond were sensitive to the quality of the crystal. The crystal quality of the S-doped (111) homoepitaxial diamond was poorer than that of the (100) diamond. The quality of the S-doped (111) diamond was insensitive to the amount of additional H 2 S. The resistivity of the (111) diamond was very high compared to that of the (100) diamond. The resistivity of the undoped (111) diamond was lower than that of the S-doped (111) diamond. The Hall effect measurement could not be performed on the (111) homoepitaxial diamonds.

Recent studies on diamond surfaces

Abstract The surface properties of diamond have been studied by ultra-violet photoemission spectroscopy (UPS), Kelvin probing and low energy electron diffraction (LEED). The atomic level structure of diamond surfaces was determined by LEED intensity vs. energy [I(E)] measurements in combination with Tensor LEED calculations. The LEED analysis of the C(100)-(2×1)-H surface revealed the formation of symmetrical dimers on the top carbon layer. For the C(100)-(1×1)-O surface, quantitative LEED analysis indicated a structural model where oxygen occupied the bridge site on the surface. Systematic investigations were carried out using UPS and a Kelvin probe measurement to reveal the effect of alkali metal fluoride overlayers on the work function of the diamond surfaces. LiF and RbF have been found to act as effective dipole layers to lower the surface work function and induce a negative electron affinity.

Synthesis of 3C–SiC nanowhiskers and emission of visible photoluminescence

Single-crystal 3C–SiC nanowhiskers with [111] axial orientation have been synthesized directly on an Si substrate with a large area and high surface density. The nanowhiskers were grown into the Si substrate like a foundation pile by using an Fe film as a catalyst in a microwave plasma chemical vapor deposition system. Most of the whiskers were cylinder shaped with a circular cross section, while some of the whiskers had cross sections in the shapes of squares, rectangles, triangles, and hexagons. The nanowhiskers possessed visible photoluminescence at room temperature and blueshift effect depending on the whisker diameter. These results offer interesting prospects for the fabrication of an Si-based light emitter through the visible wavelength regions. The synthesis process should allow for the development of nanowhisker devices integrated into Si technology.

THERMAL STABILITY OF THE NEGATIVE ELECTRON AFFINITY CONDITION ON CUBIC BORON NITRIDE

We have verified that the condition of negative electron affinity (NEA) exists on hydrogen-terminated polycrystalline cubic boron nitride (c-BN) grown by the high-pressure high-temperature method using ultraviolet photoelectron spectroscopy. The NEA condition is thermally stable to 950 °C. At higher temperatures, the surface reverts to a positive electron affinity condition due to the desorption of surface bound hydrogen. Repeated annealing at high temperatures results in the degradation of the surface crystallinity, which manifests in the growth of a π→π* feature attributable to sp2-type bonds. Complete regeneration of initial valence band features and NEA conditions along with the suppression of the π→π* features can be achieved by subjecting the surface to atomic hydrogen etching. It is discovered that the He (II)-excited valence band spectra of c-BN display significant differences between the hydrogen-terminated and hydrogen-free surface.

Electrochemical Fluorination of 1,4‐Difluorobenzene Using Boron‐Doped Diamond Thin‐Film Electrodes

Electrochemical behavior of boron‐doped diamond thin‐film electrodes has been studied by measuring cyclic voltammograms (CV) for anodic oxidation of 1,4‐difluorobenzene in the liquid electrolyte, neat , and electrochemical fluorination has been successfully carried out for the first time using diamond. While the CVs for and highly ordered pyrolytic graphite, respectively, contained waves associated with the formation of adsorbed oxygen or platinum oxide layer and the intercalation of fluoride ion, the CVs for diamond consisted essentially solely of the wave attributable to the fluorination of 1,4‐difluorobenzene in the same sweep range indicating a wide potential window and high chemical/electrochemical stability of diamond electrodes. ©1999 The Electrochemical Society

A Novel Synthesis Method for Aligned Carbon Nanotubes in Organic Liquids

Aligned carbon nanotube arrays have been grown on Si substrates in methanol by using a hot-substrate method. Samples were grown in a glass chamber equipped with a function in which methanol and ethanol vapors were condensed by means of water-cooling. Si substrates with a thin Fe film were electrically heated to 930°C in liquid methanol. Hollow mutiwalled carbon nanotubes standing well on the Si substrates were obtained with external diameters ranging from 13 to 26 nm and lengths up to 20 µm, with the ratio of tube radius to the thickness of tube shells ranging from 1.7 to 2.1. The top ends of the nanotubes were closed with nearly seamless caps. The mechanism of nanotube growth is a catalytic process at the substrate surface under thermal non-equilibrium.

Silicon incorporation into chemical vapor deposition diamond: A role of oxygen

The suppression effect of oxygen on Si incorporation in homoepitaxial diamond films grown by microwave assisted chemical vapor deposition is investigated by secondary ion mass spectrometry. The Si depth profile in the multilayered diamond films continuously synthesized with different oxygen addition shows that Si incorporation decreases with increasing oxygen addition into the plasma. A change in the interfacial composition at the quartz glass due to oxygen-promoted surface chemistry may be the origin of this suppression effect.

Suppression of surface cracks on (111) homoepitaxial diamond through impurity limitation by oxygen addition

The use of oxygen in improving diamond quality has been investigated by comparing two (111) homoepitaxial diamond films deposited with H2–CH4 and H2–CH4–O2 mixtures by microwave assisted chemical vapor deposition. The (111) diamond deposited using a H2–CH4 mixture showed surface cracks due to the presence of nondiamond phases as well as a significant amount of hydrogen and silicon impurities. The (111) diamond deposited using a H2–CH4–O2 mixture showed an absence of hydrogen and silicon impurities and nondiamond phases, and exhibited a flat surface. The addition of oxygen is one of the suitable methods to produce high-quality (111) homoepitaxial diamond.