Photophysical properties of single-walled carbon nanotubes

Thesis (Ph. D.)--University of Rochester. Department of Chemistry, 2018.Single-walled carbon nanotubes (SWCNTs) have unique electronic and optical properties due to the one-dimensional confinement, whereby strongly bound electron-hole pairs, i.e. excitons, play a very important role. Well-isolated SWCNTs display stable and robust photoluminescence (PL) in the near infrared (NIR), which depends on the SWCNT diameter and chirality. Previously, it has been discovered that the PL from DNA-wrapped SWCNTs can be enhanced by up to an order of magnitude upon addition of small molecular reducing agents to the SWCNT suspension. We report studies on the photophysics of isolated SWCNTs upon addition of dithiothreitol (DTT) for various aqueous surfactant-SWCNT suspensions. Similar to DNA-wrapped SWCNTs, sodium dodecyl sulfate (SDS) dispersed SWCNTs also display significant PL brightening upon addition of DTT, while the other surfactant-SWCNTs studied show PL quenching to different extents. Ensemble PL QY and PL lifetime measurements suggest that the addition of DTT has substantial effects on both radiative and non-radiative decay processes in SWCNTs. These results are consistent with changes caused by surfactant reorganization around SWCNTs. Additionally, PL imaging performed on individual long SDS-SWCNTs shows a single step, uniform PL brightening along the entire SWCNT, further supporting the hypothesis that surfactant reorganization is responsible for the PL enhancement. We also present studies on the excited-state decay dynamics of K-momentum dark excitons (KDE) in SWCNTs. These excitons have finite center-of-mass momenta, which are optically forbidden, and therefore should have substantially longer lifetimes than the bright S1 exciton. We have determined the PL lifetimes of both KDE and S1 states for chirality-enriched SWCNT samples using time-correlated single photon counting. Indeed, the average PL lifetime of the KDE state is more than three times longer than that of the S1 state. Somewhat unexpectedly, PL from the KDE state displays a biexponential decay for both SWCNT chiralities studied. The long time component most likely represents the PL decay from the KDE state, whereas the origin of the short time component is not fully understood yet and requires further investigation