Degassing of bundles of single-walled carbon nanotubes in vacuum at 500 K is found to drive the thermoelectricpower (TEP) strongly negative, indicating that the degassed metallic tubes in a bundle are $n$ type. The magnitude of the negative TEP indicates that important asymmetry in the electronic carbon $\pi$ bands exists near the Fermi energy. Easily measurable increases in the TEP $(\sim 5–10\mu \mathrm{V}/\mathrm{K})$ and resistivity ( $2\%–10\%$) are observed at 500 K upon exposure to ${\mathrm{N}}_2$ and He, suggesting that even gas collisions with the nanotube wall can contribute significantly to the transport properties.

• Received 2 March 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.1096