Electron energy loss spectroscopy and high-resolution images of carbon nanotubes, obtained using a 2 nm probe and energy resolution of 0.5 eV, revealed a 15 eV surface plasmon for tubes containing ≤12 cylindrical graphitic sheets. As the number of sheets increases further the 24 eV bulk plasmon quickly dominates the low-loss spectra. It is concluded that carbon nanotubes should be semimetallic, as is graphite. The first few graphitic sheets of nanotube structures act as if they are effectively decoupled electronically. As the radius of curvature increases then the interlayer structures approach the structure of graphite, as do the electronic properties. The present results do not exclude the possibility that the electronic structure of carbon nanotubes is the same as that of graphite, due to the operation of a conformal invariance principle.

• Received 28 September 1993

DOI:https://doi.org/10.1103/PhysRevB.49.2882