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 helicit y. When the diameters of the nanotubes become comparable to the electr on 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 behav ior: the current-voltage characteristics are consistent with the funct ional energy dependence of the density-of-states in 1D systems. The me asured energy gap values vary linearly with the inverse nanotube diame ter. Electrical resistivity acid magnetoresistance measurements have b een reported for larger bundles, and the temperature dependence of the electrical resistance of a single micro-bundle was found to be simila r to that of graphite and its magnetoresistance was consistent with th e formation of Landau levels. Magnetic susceptibility data taken on bu ndles of similar tubes reveal a mostly diamagnetic behavior. The susce ptibility al fields above the value at which the magnetic length equal s 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 temperatu re dependence.