Ultrafast Transient Absorption Spectroscopy Investigations of Excited State Dynamics in SWNT/Polymer Composites
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Ultrafast Transient Absorption Spectroscopy Investigations of Excited State Dynamics in SWNT/Polymer Composites David J. Styers-Barnett†, Steven P. Ellison†, Cheol Park‡, Kristopher E. Wise‡, and John M. Papanikolas† † Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599-3290 ‡ National Institute of Aerospace, NASA Langley Research Center, MS 226, Hampton, VA 23681-2199 ABSTRACT Wavelength-resolved femtosecond transient absorption spectroscopy is used to follow the electronic dynamics of single-walled carbon nanotubes in polymers following visible and near IR photoexcitation. Electron-hole (e-h) pairs give rise to sharp features in the transient spectra that decay in amplitude and exhibit rapid spectral shifts. The decay reflects (e-h) recombination on both short (1.3 ps) and long (35 ps) time scales. Transient spectra also exhibit a broad photobleach at early times that arises from the cooling of a hot electron gas created via excitation at the red edge of a π-plasmon band. INTRODUCTION While there has been a great deal of work focusing on the mechanical, electronic, and steady-state optical properties of SWNTs,1-12 the efforts13-24 performed on ultrafast time scales have not provided a definitive picture of the excited state relaxation dynamics. Because these systems were probed using single-wavelength detection in most cases, assignment of the kinetic components to a specific dynamic process was difficult. Single-wavelength methods provide a limited view of the excited state spectroscopy, and in general it is impossible to determine whether a multiexponential decay is the result of a decrease in the amplitude of an absorption band, or a shift in its position. Using single wavelength techniques, most ultrafast studies of SWNT bundles17,18,22-24 have assigned excited state kinetics to intraband scattering processes (electron-electron, electronphonon, etc.) occurring on time scales ranging from 0.2 to 1 ps. This designation is counter to recent publications20,21 that link the observed kinetics to interband electronhole recombination processes, showing that a definitive assignment from singlewavelength studies has remained elusive Our experimental methodology differs from these previous studies in one major respect: we are able to examine the evolution of the transient spectrum, not just single wavelength kinetics. The use of spectrally-resolved transient absorption (TA) spectroscopy allows us to identify of both interband and intraband dynamics. . Two recent publications21, 25 have also employed probe detection using white light. Both provide additional insight into the dynamics of SWNT, however, Tatsuura et al. investigate only the first nanotube absorption band. Korovyanko, et al. while more complete in their investigation, are studying tubes dispersed in surfactant solution, and their results are not directly comparable to ours (and in fact show different dynamic components). By measuring the transient spectrum of nanotube bundles imbedded in
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polymer films, we are abl
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