Abstract
Ultrafast relaxation of photoexcitations in semiconducting single-walled carbon nanotubes (S-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed thin NT films and D 2O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. The PA and PB decay dynamics as a function of time, t, follow a power law, (t) -α with α in the range of 0.7 to 1. Whereas the PA bands in S-NTs in solution uniformly decay, the PB bands, in contrast, have different decay dynamics across the spectrum, which originates from an ultrafast spectral shift. Nevertheless the dynamics of the PA and PB bands for NTs in solution are the same when the spectral shift is accounted for, indicating a common origin. In addition S-NTs in D 2O solution show polarized PL emission bands in the mid infrared spectral range that follow almost exactly the infrared absorption peaks of the isolated NTs, as well as their transient PB spectrum. The PL emission shows a degree of polarization that agrees with that of the transient photoinduced dichroism. We therefore conclude that the primary photoexcitations in S-NTs are not free carriers, rather they are excitons that are confined along the nanotubes. We found that the transient relaxation kinetics of the excitons depend on the NT form. The fastest exciton dynamics (with sub-picosecond lifetime) characterizes the annealed film, whereas the slowest dynamics (with lifetime of tens of ps) characterizes the isolated NTs in D 2O solution. From the polarization memory decay we could estimate the diffusion constant, D, and the diffusion length, L D, of the excitons along the nanotube. For the annealed films at room temperature we found D ≈ 100 cm 2 s -1 and L D ≈ 100 nm. From the average PL polarization degree, which remains constant across the PL spectrum, and the transient polarization memory decay, we estimate the PL lifetime in NT solution to be of the order of 500 ps. This relatively long PL lifetime is dominated by nonradiative decay processes, which when coupled with the minute PL emission quantum efficiency indicates a very small radiative recombination rate. The weak radiative transition strength is consistent with recent excited state calculations that include electron-hole interaction, which predict that excitons in NTs are basically dark. ©2005 The American Physical Society.