Electronic transport in carbon nanotubes:: From individual nanotubes to thin and thick networks

We measure and compare the electronic transport properties of individual multiwall carbon nanotubes (MWNTs) and individual single-wall carbon nanotubes (SWNTs), and SWNT networks of varying thickness. The thinnest SWNT networks, like the individual semiconducting SWNTs, show nonlinear current-voltage (I-V) characteristics at low temperatures with a current that can be modulated by a gate-source voltage. The overall temperature dependence of conductance in the transparent networks changes systematically as the thickness of the network increases and is consistent with hopping conduction. On the other hand, the thickest SWNT networks (freestanding film) show more metallic behavior: their I-V characteristics are linear with no gate-voltage effect, and a large fraction of their conductivity is retained at very low temperatures, consistent with tunneling through thin barriers separating metallic regions. We make a comparison with individual MWNTs, which in some cases show even greater retention of conductance at very low temperatures, but (unlike the thickest SWNT networks) no changeover to metallic temperature dependence at higher temperatures. The temperature dependence of conductance in individual MWNTs is consistent with a model involving conduction in the two outer shells.