Role of bright and dark excitons in the temperature-dependent photoluminescence of carbon nanotubes.

We report studies of the temperature dependence of the photoluminescence efficiency of single walled carbon nanotubes which demonstrate the role of bright and dark excitons. This is determined by the energy splitting of the excitons combined with 1-D excitonic properties. The splitting of the bright and dark singlet exciton states is found to be only a few meV and is very strongly diameter dependent for diameters in the range 0.8-1.2 nm. The luminescence intensities are also found to be strongly enhanced by magnetic fields at low temperatures due to mixing of the exciton states.