Hopping conduction in disordered carbon nanotubes

We report electrical transport measurements on individual disordered multiwalled carbon nanotubes, grown catalytically in a nanoporous anodic aluminum oxide template. In both as-grown and annealed types of nanotubes, the low-field conductance shows an exp[-(T-0/T)(1/2)] dependence on temperature T, suggesting that hopping conduction is the dominant transport mechanism, albeit with different disorder-related coefficients T-0. The electric field dependence of low-temperature conductance behaves as exp[-(xi 0/xi)(1/2)] at high electric field at sufficiently low T. Finally, both annealed and unannealed nanotubes exhibit weak positive magnetoresistance at T = 1.7 K. Comparison with theory indicates that our data are best explained by Coulomb-gap variable-range hopping conduction and permits the extraction of disorder-dependent localization length and dielectric constant. (c) 2007 Elsevier Ltd. All rights reserved.