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.

Figure 1

Temperature dependence of the PL intensities deduced for specific nanotube species. The inset shows a typical PLE map measured at 5 K.

Figure 2

Values deduced for the energy gap between the lowest singlet dark exciton state and the bright, optically allowed state, together with values for the mixing parameter of the dark and light excitons, as a function of nanotube diameter.

Figure 3

Temperature-dependent PL intensities for selected nanotube species at zero and 19.5 T, showing experiment and fits using Eq. (1).

Figure 4

The magnetic field induced intensity enhancement $I(B)/I(0)$ produced by a magnetic field of 19.5 T