  ultiwalled carbon nanotubes, prepared by both electric arc discharge and
chemical vapor deposition methods, show a strong visible light emission in
photoluminescence experiments. All the samples employed in the experiments
exhibit nearly same super-linear intensity dependence of the emission bands on
the excitation intensity, and negligible temperature dependence of the central
position and the line shapes of the emission bands. Based upon theoretical
analysis of the electronic band structures and optical transition, it is
suggested that besides the electronic transitions across the fundamental gap,
the transitions between pi and sigma conduction bands are the major source of
the light emissions. A two-step transition mechanism is proposed.
