J. Heremans

QUANTUM TRANSPORT IN A MULTIWALLED CARBON NANOTUBE

We report on electrical resistance measurements of an individual carbon nanotube down to a temperature T = 20 mK. The conductance exhibits a lnT dependence and saturates at low temperature. A magnetic field applied perpendicular to the tube axis increases the conductance and produces aperiodic fluctuations. The data find a global and coherent interpretation in terms of two-dimensional weak localization and universal conductance fluctuations in mesoscopic conductors. The dimensionality of the electronic system is discussed in terms of the peculiar structure of carbon nanotubes.

Electrical-resistance of a Carbon Nanotube Bundle

The first direct electrical resistance measurements performed on a single carbon nanotube bundle from room temperature down to 0.3 K and in magnetic fields up to 14 T are reported. From the temperature dependence of the resistance above 2 K, it is shown that some nanotubes exhibit a semimetallic behavior akin to rolled graphene sheets with a similar band structure, except that the band overlap, DELTA almost-equal-to 3.7 meV, is about 10 times smaller than for crystalline graphite. In contrast to graphite which shows a constant low-temperature resistivity, the nanotubes exhibit a striking increase of the resistance followed by a broad maximum at very low temperatures. A magnetic field applied perpendicular to the sample axis decreases the resistance. Above 1 K, this behavior is consistent with the formation of Landau levels. At lower temperatures, the resistance shows an unexpected drop at a critical temperature which increases linearly with magnetic field. These striking features could be related to the unique quasi-one-dimensional structure of the carbon nanotubes.

Electronic-properties of Carbon Nanotubes - Experimental Results

Band structure calculations show that carbon nanotubes exist as either metals or semiconductors, depending on diameter and degree of helicity. When the diameters of the nanotubes become comparable to the electron wavelength, the band structure becomes noticeably one-dimensional. Scanning tunneling microscopy and spectroscopy data on nanotubes with outer diameters from 2 to 10 nm show evidence of one-dimensional behavior: the current-voltage characteristics are consistent with the functional energy dependence of the density-of-states in 1D systems. The measured energy gap values vary linearly with the inverse nanotube diameter. Electrical resistivity acid magnetoresistance measurements have been reported for larger bundles, and the temperature dependence of the electrical resistance of a single micro-bundle was found to be similar to that of graphite and its magnetoresistance was consistent with the formation of Landau levels. Magnetic susceptibility data taken on bundles of similar tubes reveal a mostly diamagnetic behavior. The susceptibility al fields above the value at which the magnetic length equals the tube diameter has a graphite-like dependence on temperature and field. At low fields, where electrons sample the effect of the finite tube diameter, the susceptibility has a much more pronounced temperature dependence.

Optical and electronic properties of nitrogen-implanted diamond-like carbon films

Optical and electrical measurements on nitrogen ion-implanted diamond-like carbon films are presented. Raman scattering measurements, which probe the crystallinity of the film surface, indicate that nitrogen implantation reduces the finite crystallographic order in the pristine carbon films. The absence of molecular vibrations in the infrared absorption spectra of the films argues against a polymeric structure of the ion-implanted films. Spectroscopic ellipsometry experiments determine the change in the optical constants of the carbon film due to nitrogen implantation. Electrical de conductivity measurements are interpreted within the framework of a schematic density of states picture of graphitic τ-electrons in an amorphous carbon system. Taken collectively, the optical and electrical measurements suggest that nitrogen implantation increases the density of localized states within the 1.5 eV bandgap of the quasi-amorphous carbon film, thereby reducing the bandgap and increasing the conductivity of the nitrogen-implanted films.

Electrical measurements on submicronic synthetic conductors: carbon nanotubes

The synthesis of very small samples has raised the need for a drastic miniaturization of the classical four-probe technique in order to realize electrical resistance measurements. Two methods to realize electrical contacts on very small fibers are described here. Using classical photolithography the electrical resistivity of a submicronic catalytic chemical vapour deposited filament is estimated. Scanning tunneling microscopy (STM) lithography allowed to attach small gold contacts to a small bundle (diameter 50 nm) of carbon nanotubes. This bundle is found to exhibit a semimetallic behavior at higher temperature and an unexpected drop of the electrical resistivity at lower temperature.

ELECTRONIC PROPERTIES OF CARBON NANOTUBES: EXPERIMENTAL RESULTS

Band structure calculations show that carbon nanotubes exist as either metals or semiconductors, depending on diameter and degree of helicity. When the diameters of the nanotubes become comparable to the electron wavelength, the band structure becomes noticeably one-dimensional. Scanning tunneling microscopy and spectroscopy data on nanotubes with outer diameters from 2 to 10 nm show evidence of one-dimensional behavior: the current-voltage characteristics are consistent with the functional energy dependence of the density-of-states in 1D systems. The measured energy gap values vary linearly with the inverse nanotube diameter. Electrical resistivity and magnetoresistance measurements have been reported for larger bundles, and the temperature dependence of the electrical resistance of a single micro-bundle was found to be similar to that of graphite and its magnetoresistance was consistent with the formation of Landau levels. Magnetic susceptibility data taken on bundles of similar tubes reveal a mostly diamagnetic behavior. The susceptibility at fields above the value at which the magnetic length equals the tube diameter has a graphite-like dependence on temperature and field. At low fields, where electrons sample the effect of the finite tube diameter, the susceptibility has a much more pronounced temperature dependence.