Pure optical dephasing dynamics in semiconducting single-walled carbon nanotubes

Matthew W. Graham, Ying Zhong Ma, Alexander A. Green, Mark C. Hersam, Graham R. Fleming

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Abstract

We report a detailed study of ultrafast exciton dephasing processes in semiconducting single-walled carbon nanotubes employing a sample highly enriched in a single tube species, the (6,5) tube. Systematic measurements of femtosecond pump-probe, two-pulse photon echo, and three-pulse photon echo peak shift over a broad range of excitation intensities and lattice temperature (from 4.4 to 292 K) enable us to quantify the timescales of pure optical dephasing (T 2*), along with exciton-exciton and exciton-phonon scattering, environmental effects as well as spectral diffusion. While the exciton dephasing time (T 2) increases from 205 fs at room temperature to 320 fs at 70 K, we found that further decrease of the lattice temperature leads to a shortening of the T 2 times. This complex temperature dependence was found to arise from an enhanced relaxation of exciton population at lattice temperatures below 80 K. By quantitatively accounting the contribution from the population relaxation, the corresponding pure optical dephasing times increase monotonically from 225 fs at room temperature to 508 fs at 4.4 K. We further found that below 180 K, the pure dephasing rate (1/T 2*) scales linearly with temperature with a slope of 6.7 0.6 eV/K, which suggests dephasing arising from one-phonon scattering (i.e., acoustic phonons). In view of the large dynamic disorder of the surrounding environment, the origin of the long room temperature pure dephasing time is proposed to result from reduced strength of exciton-phonon coupling by motional narrowing over nuclear fluctuations. This consideration further suggests the occurrence of remarkable initial exciton delocalization and makes nanotubes ideal to study many-body effects in spatially confined systems.

Original languageEnglish
Article number034504
JournalJournal of Chemical Physics
Volume134
Issue number3
DOIs
StatePublished - Jan 21 2011

Funding

This work is supported by NSF. The steady-state fluorescence spectra reported in this work were measured at the Molecular Foundry, Lawrence Berkeley National Laboratory, which is supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. M.W.G. and A.A.G. thank the Natural Sciences and Engineering Research Council of Canada for postgraduate scholarship. Density gradient processing was supported by the National Science Foundation, the Office of Naval Research, and the Nanoelectronics Research Initiative. Y.-Z.M. also acknowledges the support by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy. We thank L. V. Valkunas, D. Abramavicius, and Y.-C. Cheng for their helpful contributions.

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