Room Temperature Ballistic Conduction in Carbon Nanotubes

Multiwalled carbon nanotubes are shown to be ballistic conductors at room temperature, with mean free paths of the order of tens of microns. The measurements are performed both in air and in high vacuum in the transmission electron microscope on nanotubes that protrude from unprocessed arc-produced nanotube-containing fibers that contact with a liquid metal surface. These experiments follow and extend the original experiments by Frank et al. (Science 1998, 280 1744), which demonstrated for the first time the large current carrying capability, very low intrinsic resistivities, and evidence for quantized conductance. This indicated 1D transport, that only the surface layer contributes to the transport, and ballistic conduction at room temperature. Here, we follow up on the original experiment including in-situ electron microscopy experiments and a detailed analysis of the length dependence of the resistance. The per unit length resistance ρ < 100 Ω/μm, indicating free paths l > 65 μm, unambiguously demonstrates ballistic conduction at room temperature up to macroscopic distances. The nanotube−metal contact resistances are in the range from 0.1 to 1 kΩμm. Contact scattering can explain why the measured conductances are about half of the expected theoretical value of 2 G0. For V > 0.1 V, the conductance rises linearly (dG/dV∼0.3 G0/V) reflecting the linear increase in the density-of-states in a metallic nanotube above the energy gap. Increased resistances (ρ = 2−10 kΩ/μm) and anomalous I−V dependences result from impurities and surfactants on the tubes. Evidence is presented that ballistic transport occurs in undoped and undamaged tubes for which the top layer is metallic and the next layer is semiconducting. The diffusive properties of lithographically contacted multiwalled nanotubes most likely result from purification and other processing steps that damage and dope the nanotubes, thereby making them structurally and electronically different than the pristine nanotubes investigated here.