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[−(T0/T)1/2]exp[−(T0/T)1/2] dependence on temperature TT, suggesting that hopping conduction is the dominant transport mechanism, albeit with different disorder-related coefficients T0T0. The electric field dependence of low-temperature conductance behaves as exp[−(ξ0/ξ)1/2]exp[−(ξ0/ξ)1/2] at high electric field ξξ at sufficiently low TT. Finally, both annealed and unannealed nanotubes exhibit weak positive magnetoresistance at T=1.7K. 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.