Some problems in understanding the electronic transport properties of carbon nanotube ropes

The resistance of single-wall carbon nanotube (SWCN) ropes or mats, and some individual tubes, typically shows a crossover from non-metallic to metallic temperature dependence as temperature increases. This systematic pattern is consistent with a series heterogeneous model involving metallic resistance and tunnelling through barriers such as defects and inter-rope contacts. The metallic resistivity term increases linearly with temperature for the ropes or mats, but faster for the individual nanotubes. In contrast to the almost vanishing thermoelectric power expected from electronic band structure calculations, the measured values for mats or films (including recent measurements in a vacuum) are even larger than for typical metals. The thermopower increases with temperature as for metals, but has a characteristic non-linear shape. This temperature dependence can be modelled, for example, with parallel conduction in metallic and semiconducting tubes, but the size of the metallic thermopower required is anomalously large.