Conduction properties of semiconductive multiwalled carbon nanotubes

We have undertaken low-temperature conduction measurements on arc-discharge synthesized, semiconducting multiwalled carbon nanotubes (MWNT). The diameters of these are in the range 2.5–10 nm, corresponding to the sizes just above single-walled carbon nanotubes (SWNT), up to middle-sized MWNTs. The energy gap, inversely related to the diameter, varies strongly in this range, and consequently there is a strong dependence of the transport on tube diameter. Certain transport characteristics are much alike those found in SWNTs, such as the ON-state resistance and Coulomb blockade. However, the transport gap has a more complex behavior than the corresponding one in semiconducting SWNTs, and a number of features, such as negative differential resistance are commonly observed. Different models for the small bias transport behavior are briefly discussed, and we consider especially the possibility of conduction via the second layer.