Room Temperature Ballistic Conduction in Carbon Nanotubes

Abstract

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 which contact with a liquid metal surface. These experiments follow and extend the original experiments by Frank et al (Science, 280 1744 1998) that 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 demonstrate ballistic conduction at room temperature up to macroscopic distances. The nanotube-metal contact resistances are in the range 0.1-1 kΩµm. Contact scattering can explain why the measured conductances are about half the expected theoretical value of 2 G 0 . For V>0.1V the conductance rises linearly (dG/dV~0.3 G 0 /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.

Growth of carbon nanotubes at temperatures compatible with integrated circuit technologies

“…The exceptional properties 2,3 of carbon nanotubes have made them very attractive in solving some of the key challenges faced at the individual device-component level in the semiconductor ITRS roadmap 1 . In the integration of carbon nanotubes into microelectronic applications, the synthesis temperature is critical among the various process parameters for CMOS integration compatibility.…”

Electronic mean free path in as-produced and purified single-wall carbon nanotubes

“…15 For the asproduced SWNTs, no rounded shape was observed, while the purified SWNTs showed a rounded shape (Fig. 3a).…”

“…The estimated R T-Hg for the purified SWNTs is ~10 Ωµm, which is less than one-tenth of that of multi-wall carbon nanotubes. 15 This low R T-Hg implies that the contribution of R T-Hg to the total resistance is negligible. We thus assume that the R T-Hg is independent of x in both the as-produced and purified samples.…”

Effect of scattering and contacts on current and electrostatics in carbon nanotubes

“…The non crossing subbands of small diameter nanotubes do not carry current due to their large band gap. In contrast, large diameter nanotubes experimentally show an increase in conductance with applied bias [1,2,12,13]. Reference [14] suggested that as the diameter increases, electrons may tunnel into non crossing subbands, thus causing an increase in differential conductance with applied bias.…”