Anisotropic electrical transport properties of aligned carbon nanotube films

We report a large scale synthesis of well-aligned carbon nanotube films with controllable diameter and length. A simple technique has been developed to measure anisotropic electrical transport properties of as-aligned carbon nanotube films. The temperature dependence of relative electrical resistances suggests that most of the well-aligned carbon nanotubes are semiconductive in both directions parallel and perpendicular to the tube axis. The anisotropy (R-perpendicular to/R-parallel to) of electrical resistances increases with decreasing temperature T, reflecting difference in the longitudinal and transverse hopping rates. The differences of the electrical properties in both directions could be explained by a difference in the degree of localization of charge carries. The plot of the logarithm of relative resistance against powers of the reciprocal temperature 1/T is closely fitted by three-dimensional variable range conduction. After annealing and Br-2-doping treatments, the resistivities of the aligned carbon nanotube films decreased by 2 orders of magnitude, which resulted from fewer defects and more carries density, respectively.