  We report that dark excitons can have a large contribution to the emission
intensity in carbon nanotubes due to an efficient exciton conversion from a
dark state to a bright state. Time-resolved photoluminescence measurements are
used to investigate decay dynamics and diffusion properties of excitons, and we
obtain intrinsic lifetimes and diffusion lengths of bright excitons as well as
diffusion coefficients for both bright and dark excitons. We find that the
dark-to-bright transition rates can be considerably high, and that more than
half of the dark excitons can be transformed into the bright excitons. The
state transition rates have a large chirality dependence with a family pattern,
and the conversion efficiency is found to be significantly enhanced by adsorbed
air molecules on the surface of the nanotubes. Our findings show the nontrivial
significance of the dark excitons on the emission kinetics in low dimensional
materials, and demonstrate the potential for engineering the dark-to-bright
conversion process by using surface interactions.
