Defects Enable Dark Exciton Photoluminescence in Single-Walled Carbon Nanotubes

Abstract

Variable temperature photoluminescence excitation spectroscopy of three (n,m) species of single-walled carbon nanotubes revealed that at resonant S22 excitation, in addition to allowed excitonic optical transitions, several sidebands that should be forbidden based on selection rules were observed and appeared to have a strong temperature dependence. In particular, we found that a sideband located approximately 130 meV away from the bright S11 exciton peak relating to the K-momentum dark exciton state, called X1, decreased in intensity 5-fold as the samples were cooled. Direct optical excitation of this dark state is nominally forbidden, thus calling into question how the state is populated and why it is so prominent in the photoluminescence spectrum. Interestingly, the ratio of the integrated photoluminescence intensities of X1 to S11 scales with a Boltzmann factor unrelated to the phonon that is thought to be responsible for depopulating the K-momentum dark exciton state: an in-plane transverse optical p...