Yuichiro Nishina

Conical beams from open nanotubes

Electron guns are indispensable devices that are widely used in household and industrial appliances. Field electron-emitting sources (which emit electrons by tunnelling effects in electric fields), with their small size, small energy spread, high current density and no requirement for heat, have distinct advantages over thermionic emitters. We have made a field electron emitter from hollow, open-ended carbon nanotubes.

Carbon nanocapsules encaging metals and carbides

Abstract Experiments aimed at the encapsulation of foreign materials within hollow graphitic cage were carried out for a series of lanthanide elements and for iron group metals (Fe, Co, Ni). A carbon arc reactor was used for the synthesis, and the products formed on the cathode were investigated by transmission electron microscopy. The lanthanides were encapsulated in the form of dicarbides within multilayered polyhedral carbon cages. For iron group metals, particles of both a carbide phase (M 3 C, M = Fe, Co, Ni) and also a metallic phase (α-Fe, γ-Fe; hcp-Co, fcc-Co; fcc-Ni) were encapsulated in graphitic carbon. Based on their morphology and structure, growth mechanism is proposed. Especially for Ni, exotic carbon materials with hollow structures, bamboo-shaped tubes and nanochains as well as single-layered nanotubes, were discovered.

Field Emission Patterns from Single-Walled Carbon Nanotubes

A bulk bundle (with a diameter of about 100 µ m) of single-walled nanotubes (SWNTs), which were obtained after a purification process of carbon soot containing SWNTs, was used as an electron emitter. Many spikes of micro-bundles (with a typical diameter of 20–30 nm) of SWNTs extruded from the surface of the bulk emitter. Field emission originating from individual micro-bundles was observed on a fluorescent screen 50 mm apart from the tip. Each micro-bundle of SWNTs gave a corresponding bright spot, but no fine structure was observed within the bright spots. A probe hole device was used to measure an electron current from a single micro-bundle. The Fowler-Nordheim plot of the current and the applied voltage yielded a straight line, confirming the current results from field emission.

Raman study of SiC fibres made from polycarbosilane

We have examined the evolution of Raman spectra of SiC fibres through structural and compositional transformations caused by heat treatment. The SiC fibre was made from polycarbosilane. Raman spectra of the SiC fibre indicate that it consists of (i) amorphous or microcrystalline SiC, (ii) carbon microcrystals, and (iii) silicon oxide. The amount of microcrystalline carbon in the fibre increases with heat treatment temperature up to 1400° C, and it decreases abruptly in those fibres heat treated above 1500° C. The tensile strength of the fibre drops virtually to zero after the heat treatment at 1500° C. Carbon microcrystals are precipitated from the Si-C random network with excess carbon, and they are distributed uniformly in the fibre. These carbon particles suppress the growth of SiC crystals. It is shown that the carbon microcrystals play an important role in maintaining the high mechanical strength of the SiC fibre.

Dynamics of laser‐ablated particles from high Tc superconductor YBa2Cu3Oy

The dynamics of light‐emitting particles produced by the excimer laser ablation of the high Tc superconductor YBa2Cu3Oy has been investigated by means of space/time resolved optical measurements near the surface region with a space resolution of 100 μm and a time resolution of 0.1 ns. Two distinct components of ablated particles were observed: one with high average velocities over 5×106 cm/s and the other with slow velocities, depending on laser energy density. The position of the maximum emission intensity in the slower component moved away from the surface and was further delayed from the time of maximum laser intensity as the laser energy density increased. If the incident laser was tilted from the normal of the target surface, the spatial distribution of the luminous plume inclined toward the incident laser beam. These results suggest that the slower component consists of light‐emitting particles resulting from the fragmentation of clusters ejected from the surface.

Iron particles nesting in carbon cages grown by arc discharge

Fine particles of iron and iron carbide wrapped in multilayered graphitic sheets, which were synthesized by arc discharge of carbon rods containing iron oxide (Fe2O3), were studied by transmission electron microscopy. The size of the wrapped particles was typically in the range 20–200 nm. Two kinds of nested materials were found; one was α-Fe and the other, Fe3C (cementite).

Field ion-scanning tunneling microscopy study of C60 on the Si(100) surface

Field ion-scanning tunneling microscopy was employed to study the monolayer and multilayer adsorption behaviors of the C60 fullerene on the Si(100)2×1 surface. The C60 molecules reside stably in the trough at room temperature without rotation, encompassing the 8 neighbouring dimer-forming surface Si atoms with the nearest neighbour distance of 12 A. For the first and second layers, only local ordering of square and quasi-hexagonal patterns was observed. The orderly Stranski-Krastanov mode island formation with the hexagonal packing was observed above the third layer with its lattice constant of 10.4 A.

High yield of single-wall carbon nanotubes by arc discharge using Rh–Pt mixed catalysts

Abstract Single-walled carbon nanotubes (SWNT) were produced using binary mixtures of the platinum-group metals as catalysts by arc evaporation in helium gas. Transmission electron microscopy and Raman scattering spectroscopy revealed that the production yield of SWNTs was remarkably enhanced when a Rh–Pt mixture was used as a catalyst. The density of SWNTs in raw soot was as high as that obtained from Fe–Ni and Y–Ni. The distribution of diameters of SWNTs was narrow (1.28±0.07 nm). The merit of this catalyst is that it is free from magnetic metals.

Cobalt particles wrapped in graphitic carbon prepared by an arc discharge method

Fine particles of cobalt and cobalt carbide nesting in multilayered graphitic sheets, which were synthesized by an electric arc discharge of carbon rods containing cobalt oxide (CoO), were studied by transmission electron microscopy, including microdiffraction and energy dispersive x‐ray analysis. The size of the wrapped particles was typically in a range from 50 to 200 nm. Three phases of nested materials, hcp(α)‐Co, fcc(β)‐Co, and Co3C, were identified.

Ag electrode reaction in NaOH solution studied by in-situ Raman spectroscopy

Abstract Ag electrode reaction in NaOH solution has been studied by in-situ measurement of surface enhanced Raman scattering. The Raman spectrum is measured while the electrode potential is swept. During and after the reduction process of oxide layers grown on the electrode surface, we observe Raman lines due to atomic oxygen and hydroxyl species adsorbed on the Ag surface. Atomically adsorbed oxygen which gives a vibrational frequency of 330 cm −1 exists in a limited potential range of the electrochemical reduction process. Two lines are observed at 470 and 860 cm −1 in a wider electrode potential range. These lines are due to the AgO stretching vibration (470 cm −1 ) and the AgOH bending vibration (860 cm −1 ) of the adsorbed hydroxyl species. The oxygen atoms migrate through the oxide and Ag film facing the electrolyte and then they become adsorbed on the rough Ag film which grows on the electrode surface during the reduction process. The oxygen atom in this adsorption state is rather unstable and it turns to hydroxyl species. These Raman lines are observed also in the anodic sweep, but the intensities are weaker than in the case of the cathodic sweep. Three additional lines appear at 500, 700 and 1615 cm −1 for an electrode potential more negative than −0.8 V (versus SCE). They are observed even in the potential range for hydrogen evolution. These Raman lines are assigned as vibrations of interfacial water molecules sandwiched between the electrode and Na + ions located in the outer Helmholtz plane. Photoelectrochemical oxidation of Ag 2 O grown on the electrode surface to AgO occurs if the thickness of Ag 2 O exceeds a couple of monolayers.