Electronic transport properties of conducting polymers and carbon nanotubes

We review and compare electronic transport in different types of conducting polymer: conjugated organic polymers, the inorganic polymer polysulphur nitride, alkali-metal fulleride polymers, and carbon nanotubes. In each case, the transport properties show some unusual features compared to conventional metals. In conjugated organic conducting polymers, electronic transport shows a systematic pattern involving both metallic and non-metallic character. We discuss the physical conduction processes that can account for this behaviour. Key roles are played by the metal-semiconductor transition as the doping level is varied, and by the limited size of crystalline regions in the polymers, which gives rise to heterogeneous conduction. Transport data provide indirect evidence that the intrinsic conductivity of doped crystalline polyacetylene, in the absence of disordered regions, is higher than that of copper at room temperature; this high conductivity is consistent with the expected suppression of backscattering in highly anisotropic ('quasi-one-dimensional') metallic conduction. Bundles of single-wall carbon nanotubes have also been found to exhibit metallic behaviour. The temperature dependence of the conductivity of bulk samples is remarkably similar to the pattern characteristic of organic conducting polymers, typically showing a crossover from metallic to non-metallic behaviour as temperature decreases. Quantized one-dimensional conductance and other quantum effects are seen in individual nanotubes.