A. Châtelain

A Carbon Nanotube Field-Emission Electron Source

A high-intensity electron gun based on field emission from a film of aligned carbon nanotubes has been made. The gun consists of a nanotube film with a 1-millimeter-diameter grid about 20 micrometers above it. Field-emission current densities of about 0.1 milliampere per square centimeter were observed for applied voltages as low as 200 volts, and current densities greater than 100 milliamperes per square centimeter have been realized at 700 volts. The gun is air-stable, easy and inexpensive to fabricate, and functions stably and reliably for long times (short-term fluctuations are on the order of 10 percent). The entire gun is only about 0.2 millimeter thick and can be produced with virtually no restrictions on its area, from less than 1 square millimeter to hundreds of square centimeters, making it suitable for flat panel display applications.

Magnetism from the atom to the bulk in iron, cobalt, and nickel clusters

Molecular beam deflection measurements of small iron, cobalt, and nickel clusters show how magnetism develops as the cluster size is increased from several tens to several hundreds of atoms for temperatures between 80 and 1000 K. Ferromagnetism occurs even for the smallest sizes: for clusters with fewer than about 30 atoms the magnetic moments are atomlike; as the size is increased up to 700 atoms, the magnetic moments approach the bulk limit, with oscillations probably caused by surface-induced spin-density waves. The trends are explained in a magnetic shell model. A crystallographic phase transition from high moment to low moment in iron clusters has also been identified.

Field emission from single-wall carbon nanotube films

We report on the field emission properties of single-wall carbon nanotube films, with emphasis on current–versus–voltage (I–V) characteristics and current stability. The films are excellent field emitters, yielding current densities higher than 10 mA cm−2 with operating voltages that are far lower than for other film emitters, but show a significant degradation of their performances with time. The observed deviations from the Fowler-Nordheim behavior in the I–V characteristics point to the presence of a nonmetallic density of states at the tip of the nanotubes.

Aligned carbon nanotube films: production and optical and electronic properties.

Carbon nanotube material can now be produced in macroscopic quantities. However, the raw material has a disordered structure, which restricts investigations of both the properties and applications of the nanotubes. A method has been developed to produce thin films of aligned carbon nanotubes. The tubes can be aligned either parallel or perpendicular to the surface, as verified by scanning electron microscopy. The parallel aligned surfaces are birefringent, reflecting differences in the dielectric function along and normal to the tubes. The electrical resistivities are anisotropic as well, being smaller along the tubes than perpendicular to them, because of corresponding differences in the electronic transport properties.

Field emission properties of multiwalled carbon nanotubes

Field emission properties of multiwalled carbon nanotubes, in the form of both single tips and films, have been investigated. Nanotubes compare very favourably with other field emission sources, as they show low operating voltages and produce high current densities. Lifetimes in excess of 100 h in continuous operation at constant applied voltage were observed. Our measurements suggest that the good field emission properties are largely due to small tip size and to a non-metallic density of states at the tip.

Field emission from cylindrical carbon nanotube cathodes: Possibilities for luminescent tubes

We show that the field of application of cold electron film emitters can be extended to nonplanar geometries by demonstrating a cylindrical field emission diode. The cathode is a metallic wire on which multiwall carbon nanotubes are grown by the catalytic decomposition of acetylene over a Fe catalyst. The emitter shows excellent performances and can be used to realize a luminescent, mercury-free, tube.

Magnetism of Fe, Co and Ni clusters in molecular beams

The magnetic properties of molecular beams of Fe, Co and Ni clusters deflected with a Stern-Gerlach magnet are reviewed. Single-sided cluster deflections are observed indicating the occurrence of spin relaxation processes within the isolated clusters. We show that under strong supersonic expansion resonant spin-rotation coupling phenomena affect the spin dynamics. On the other hand, for weak supersonic expansion the cluster magnetization follows a superparamagnetic behavior. Cluster magnetic moments are determined as a function of size N (20 ⩽ N ⩽ 700) at low temperature. Measurements show that clusters are more magnetic than the bulk for sizes up to a few hundred atoms. Oscillations superimposed to the magnetic moment decrease are observed which cannot be entirely explained by geometrical structure effects. Cluster magnetic moments are also measured as a function of temperature T (78 K ⩽ T ⩽ 1000 K) for several cluster size ranges. Progressive convergence of the magnetic moment curve toward the bulk saturation magnetization curve is observed for Ni and Co clusters. However the magnetic moment curves are smeared out due to finite size effects. For Fe clusters, indications of a crystallographic phase transition from high to low moment are given.

Raman spectroscopy of closed-shell carbon particles

Abstract Raman spectra of annealed carbon soot reveal strong structural changes. Downshifts of the graphite-like phonon bands to lower energies after annealing are suggested to be related to strained or curved graphitic planes. The effect of curvature on the energy of the in-plane optical phonon mode is quantitatively estimated by applying the semi-empirical interatomic Tersoff potential. A weighted average curvature corresponding to a bond bending of 2.1° is deduced for spherical shells with 20.6 A radius. These findings are consistent with high-resolution electron microscopy images which reveal closed-shell carbon particles in the same size range.

Can we reliably estimate the emission field and field enhancement factor of carbon nanotube film field emitters

It is common practice in the literature to reduce the field emission properties of film field emitters to a few parameters to make possible a comparison between different samples. In particular, the macroscopic field needed to extract a given current density and the field enhancement factor are often used. The purpose of this contribution is to attract attention to the fact that such comparisons have to be done with utmost care, as the values of these parameters depend on the experimental configuration, and especially on the inter-electrode distance, used for the measurements. Current and future solutions are considered and discussed.

Varying the size and magnetic properties of carbon-encapsulated cobalt particles

Abstract Carbon-encapsulated cobalt particles were produced by a modified arc-discharge technique and subsequently purified with an acid treatment. The arc discharge was realized between a graphite cathode and a graphite crucible with a solid Co target, under partial He pressure with an additional quenching jet. The produced particles are spherical and covered by typically 3–5 graphitic carbon shells. The mean diameter could be varied between 5 and 45 nm by changing the deposition parameters (gas pressure and flow rate of the quenching jet). The magnetic properties were analyzed with a variable temperature SQUID magnetometer. The encapsulated particles showed ferromagnetic hysteresis loops with marked size-dependent properties.