Yahachi Saito

Field emission from carbon nanotubes and its application to electron sources

Abstract Carbon nanotubes possess the following properties favorable for field emitters: (1) high aspect ratio, (2) small radius of curvature at their tips, (3) high chemical stability and (4) high mechanical strength. A field emission microscopy (FEM) study was carried out for both multiwall nanotubes (MWNTs) and single-wall nanotubes (SWNTs) produced by arc discharge between carbon electrodes. Field emission patterns as well as current versus voltage characteristics and Fowler–Nordheim plots are discussed. As an application of nanotube field emitters, we manufactured cathode-ray tube (CRT) type lighting-elements and vacuum-fluorescence display (VFD) panels. In both display elements, conventional thermionic cathodes were replaced with MWNT field emitters. Stable electron emission, adequate luminance, and long life of the emitters was demonstrated.

Nanoparticles and filled nanocapsules

Encapsulation of foreign materials within a hollow graphitic cage was carried out for rare-earth and iron-group metals by using an electric arc discharge. The rare-earth metals with low vapor pressures, Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, and Lu, were encapsulated in the form of carbides, whereas volatile Sm, Eu, and Yb metals were not. For iron-group metals, particles in metallic phases (α-Fe, γ-Fe; hcp-Co, fcc-Co; fcc-Ni) and in a carbide phase (M3C, M = Fe, Co, Ni) were wrapped in graphitic carbon. The excellent protective nature of the outer graphitic cages against oxidation of the inner materials was demonstrated. In addition to the wrapped nanoparticles, exotic carbon materials with hollow structures, such as single-wall nanotubes, bamboo-shaped tubes, and nanochains, were produced by using transition metals as catalysts.

Cathode Ray Tube Lighting Elements with Carbon Nanotube Field Emitters

We have manufactured cathode ray tubes (CRTs) equipped with field emitters composed of multiwalled carbon nanotubes. The fabricated CRTs (20 mm in diameter by 74 mm in length) are of a triode type, consisting of a cathode (nanotube field emitter arrays), a grid and an anode (phosphor screen). The manufactured CRTs are lighting elements, which are assembled to form a giant outdoor display. Stable electron emission, adequate luminance and long life are demonstrated. The CRT lighting elements presented here are the first practical products utilizing carbon nanotubes on an industrial scale.

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.

Growth and structure of graphitic tubules and polyhedral particles in arc-discharge

Abstract Graphite tubules and polyhedral particles grown on an electrode surface of arc-discharge have been studied by electron microscopy. The observations of their structures and morphologies lead to a growth model in which ion bombardment and high electric field at the tip of a tubule play crucial roles for the tube growth.

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.

LaC2 encapsulated in graphite nano-particle

Arc discharging carbon rods containing 8.8 wt% La2O3 produces a carbonaceous deposit containing nano-particles (nm-size particles) consisting of graphite shells around core nano-crystals averaging 10-30 nm in diameter but sometimes 3-4 nm. Transmission electron microscopy, nm-size electron diffraction and electron energy-loss spectroscopy have revealed that the nano-crystals are La-carbide, LaC2, that the interfaces between the carbide and the graphite are flat, and that no intermediate layers are formed.

Bamboo-shaped carbon tube filled partially with nickel

Abstract An encapsulation experiment of nickel metal within hollow graphitic cages was carried out by using a carbon arc reactor. Among the carbonaceous materials formed on an electrode surface, carbon tubes with special features of bamboo-like structure, i.e., consisting of a series of long hollow compartments, were found by electron microscopy. The diameter of the tubes was about 30 nm and the length on the order of a few μm. The ends of the tubes were capped with needle-shaped nickel particles.

Exciton Effects of Optical Transitions in Single-Wall Carbon Nanotubes

Absorption spectra have been measured in the near-infrared region for a thin-film sample of single-wall carbon nanotubes of 1.2–1.5 nm diameter. Two absorption peaks are observed at 0.7 and 1.2 eV. The observed spectrum has been compared with the calculation based on the tight-binding model taking into account the distribution of diameter and the helical arrangement of tubes. We have found that a one-dimensional exciton effect plays an important role in the fundamental optical transition of semiconducting single-wall carbon nanotubes.