Carbon nanotubes prepared by catalytic (CO) disproportionation were studied using TEM, XRD, UPS, XPS, and optical spectroscopies. In comparison with graphite, the carbon nanotubes show greater interplanar distance, smaller work function, steeper Fermi edge, negative core-level shift, and stronger plasma excitation. Their valence band is basically the same as that of graphite, with lower intensity in the binding energy region of 2–7 eV. The carbon nanotubes exhibit a strong optical limiting effect, superior to both carbon black and ${\mathrm{C}}_{60}$.

• Received 2 June 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.2548