Single-Walled Carbon Nanotube Dark Exciton Photoluminescence Dynamics

Semiconducting single-walled carbon nanotubes (SWCNTs) often exhibit distinctive spectral features due to a complex dark exciton manifold. One of those features, the K-momentum dark exciton (KDE) state, has been of significant recent interest because of the unique photophysics required to brighten the nominally optically forbidden state. Although the energy of the KDE state relative to the bright singlet excitonic state (E11) is currently understood, how the KDE state is efficiently populated, and its resulting dynamics, is not. Time-correlated single photon counting (TCSPC) and kinetic modeling were used to study the dynamics of the KDE state as temperature and lattice defect concentration varied. Photoluminescence (PL) time decays corresponding to the KDE state exhibited biexponential character with average lifetime values roughly 6 times longer than those of the typical E11 state. As temperature was lowered or as the SWCNT lattice became more defective, the KDE state lifetime values increased by nearly 3 times, reaching values as high as ∼400 ps. This trend strongly suggested that the dark singlet excitonic state (D11) situated a few millielectronvolts below the bright plays a significant role in KDE dynamics. Transition times between the E11 and D11 states, as well as dark-to-bright excitonic conversion efficiencies, were extracted by using a kinetic analysis of the experimentally determined KDE state time decays. Together, the experimental results and kinetic modeling strongly suggest that mixing between the bright and dark singlet excitonic states is the driving force that dictates KDE state dynamics.