Ultrafast exciton dynamics in isolated single-walled nanotubes

Ultrafast relaxation dynamics of photoexcitations in semiconducting single-walled carbon nanotubes (S-NT) isolated in D2O solution have been investigated using polarized pump-probe photomodulation (with 150 A time resolution), and cw polarized photoluminescence (PL). Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands were observed, which also showed photoinduced dichroism. After taking into account the PB spectral shift, then the PA and PB bands decay together in time following a power law of the form (t)(-a) with a in the range of 0.7-1. The PL emission shows polarization degree that agrees with that of the transient photoinduced dichroism. We therefore conclude that the primary photoexcitations in S-NT are excitons that are confined along the NTs. From the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime in isolated NTs in solution to be of the order of 500 ps. This relatively long PL lifetime is dominated by non-radiative decay processes, which when coupled with the minute PL emission quantum efficiency indicates a very small radiative recombination rate, in good agreement with recent theories that include electron correlation. (C) 2005 Elsevier B.V. All rights reserved.