Photoinduced Spontaneous Free-Carrier Generation of (7,5)-Chirality-Enriched Single-Walled Carbon Nanotubes in a Low Dielectric Solvent

Abstract

We demonstrate photoinduced spontaneous free-carrier generation in highly individualized (7,5)- chirality-enriched SWCNT samples ((7,5)-SWCNTs) suspended in a non-polar organic solvent. Frequency-resolved solution-phase flash photolysis time-resolved microwave conductivity (fp- TRMC), provides a contactless and quantitative measurement of the real and imaginary photoconductance of these individually suspended nanotubes. We use substantially lower excitation fluences than have been possible in previous studies, corresponding to absorbed photon densities of ~0.05 photons per micron length of (7,5)-SWCNTs, allowing us to avoid complications from dielectric screening by nearby excitons or exciton-exciton collisions. Even under these mild conditions, we unambiguously measure a photoconductance due to free carriers that are generated following excitation of both the first and second exciton transitions (S11 and S22, respectively). The low-fluence carrier generation quantum yield (Φ) with S22 excitation appears c.a. 2.4 times higher than that with S11 excitation, suggesting that the autoionization of S22 excitons clearly enhances the carrier generation efficiency. In contrast, the transient decay dynamics are independent of excitation wavelength, suggesting that the mobile carriers generated from either S11 or S22 excitation undergo identical decay pathways (i.e. trapping, recombination, etc.).