Diffusion-Assisted Photoexcitation Transfer in Coupled Semiconducting Carbon Nanotube Thin Films

We utilize femtosecond transient absorption spectroscopy to study dynamics of photo­excitation migration in films of semi­conducting single-wall carbon nanotubes. Films of nanotubes in close contact enable energy migration such as needed in photovoltaic and electro­luminescent devices. Two types of films composed of nanotube fibers are utilized in this study: densely packed and very porous. By comparing exciton kinetics in these films, we characterize excitation transfer between carbon nanotubes inside fibers <i>versus</i> between fibers. We find that intrafiber transfer takes place in both types of films, whereas interfiber transfer is greatly suppressed in the porous one. Using films with different nanotube composition, we are able to test several models of exciton transfer. The data are inconsistent with models that rely on through-space interfiber energy transfer. A model that fits the experimental results postulates that interfiber transfer occurs only at intersections between fibers, and the excitons reach the intersections by diffusing along the long-axis of the tubes. We find that time constants for the inter- and intrafiber transfers are 0.2–0.4 and 7 ps, respectively. In total, hopping between fibers accounts for about 60% of all exciton downhill transfer prior to 4 ps in the dense film. The results are discussed with regards to transmission electron micrographs of the films. This study provides a rigorous analysis of the photophysics in this new class of promising materials for photovoltaics and other technologies

Polarization-Controlled Two-Dimensional White-Light Spectroscopy of Semiconducting Carbon Nanotube Thin Films

Polarized two-dimensional white-light (2D-WL) spectra are reported for thin films of semiconducting carbon nanotubes. The orientational responses for 4-point correlation functions are derived for samples that are isotropic in two dimensions. Spectra measured using ⟨−45°,+45°,0°,90°⟩ polarizations eliminate the diagonal peaks in the spectra arising from <i>S</i>₁ transitions to uncover cross peaks to a weaker transition that is assigned to radial breathing modes. In nanotubes purified by unwrapping PFO-BPY polymer using metal chelation, an absorption at 1160 nm is observed that is assigned to hole doping that forms trions. The trion peak may have a transition dipole nonparallel to the <i>S</i>₁ transitions, and so its cross peak is prominent in polarized 2D-WL spectra. Energy transfer of photoexcitons to the trion peak occurs within 1 ps. Identifying and understanding the effects of purification on the electronic structure of thin films of semiconducting carbon nanotubes is important for learning how the inherent photophysics of individual carbon nanotubes translates to coupled nanotube thin-film materials