  The electronic properties of multi-wall carbon nanotubes (MWNTs) have
attracted much attention because they could lead to nano-sized electronic
devices. Using chemical vapor deposition (CVD), MWNTs are produced in large
quantities at a low cost. CVD-produced MWNTs, however, generally contain a
considerable amount of structural defects due to their low temperature
synthesis. Since these defects act as scattering centers in electron transport
and thus limit the electronic mean free path (EMFP), high-temperature annealing
should lengthen the EMFP. This is based on the previous experiments in which
annealing eliminated structural defects in various carbon materials including
nanotubes. In this study, we investigate how high-temperature annealing affects
the transport properties, especially EMFP, of CVD-grown MWNTs. To measure the
EMFP of a nanotube, we submerged samples into liquid mercury (Hg) and measured
the variation in conductance. We found that the EMFP is much longer in
nanotubes annealed at 2700oC compared with that of as-produced ones. The
annealed nanotubes show quasi-ballistic electronic transport with the EMFP
reaching a few microns, even at room temperature.
