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 ni tride, alkali-metal fulleride polymers, and carbon nanotubes. In each case, the transport properties show some unusual features compared to convention al metals. In conjugated organic conducting polymers, electronic transport shows a sys tematic pattern involving both metallic and non-metallic character. We disc uss the physical conduction processes that can account for this behaviour. Key roles are played by the metal-semiconductor transition as the doping le vel is varied, and by the limited size of crystalline regions in the polyme rs, which gives rise to heterogeneous conduction. Transport data provide in direct evidence that the intrinsic conductivity of doped crystalline polyac etylene, in the absence of disordered regions, is higher than that of coppe r at room temperature; this high conductivity is consistent with the expect ed suppression of backscattering in highly anisotropic ('quasi-one-dimensio nal') metallic conduction. Bundles of single-wall carbon nanotubes have also been found to exhibit met allic behaviour. The temperature dependence of the conductivity of bulk sam ples is remarkably similar to the pattern characteristic of organic conduct ing polymers, typically showing a crossover from metallic to non-metallic b ehaviour as temperature decreases. Quantized one-dimensional conductance an d other quantum effects are seen in individual nanotubes.