Optical absorption and transient photobleaching in solutions of surfactant encapsulated and DNA wrapped single-walled carbon nanotubes (SWNTs) are studied. Optical transitions between van Hove singularities are red shifted in solutions of DNA wrapped SWNTs compared with transitions in solutions of sodium dodecyl sulfate (SDS) encapsulated SWNTs. This red shift may be due to changes in the local surrounding dielectric constant and corresponding changes in charge screening. Transient photobleaching at the E<sub>11</sub> transition of semiconducting SWNTs is observed in both solutions of SDS encapsulated SWNTs and DNA wrapped SWNTs in response to optical excitation at corresponding E<sub>22</sub> transitions, and the saturation of photobleaching at high excitation intensities greater than 500 W cm<sup>-2</sup> is studied. It is found that the photobleaching intensity does not saturate as significantly in solutions of DNA wrapped SWNTs as in solutions of SDS isolated SWNTs. Lastly, using degenerate, delayed pump-probe characterization, the temporal relaxation of excited charge carriers is investigated. Measured decays are characterized by both fast and slow processes. The slow decay time constant across the band gap of semiconducting SWNTs is fit to 120 ps for SDS encapsulated SWNTs and 73 ps for DNA wrapped SWNTs.
Recently it has been shown that aqueous solutions of sodium dodecyl sulfate (SDS) encapsulated and polymer wrapped single-walled carbon nanotubes (SWNTs) fluoresce in the near infrared (NIR) in the regime of the E<sub>11</sub> van Hove transitions for semiconducting SWNTs. For bundled SWNTs, fluorescence is observed to be quenched along with a shift and broadening of the absorbance spectrum. Here, we study two other commercially available surfactants, BRIJ-97 and Triton-X-100, by analysis of carbon nanotube fluorescence and absorptivity in the NIR. It is found that changing the surfactant alters the corresponding optical properties of the solubilized carbon nanotubes. The NIR absorbance spectra of BRIJ-97 and Triton-X-100 carbon nanotube solutions are also compared with the absorbance spectrum of NaCl destabilized SDS-SWNT solutions. By controlling the amount of NaCl added to an aqueous solution of SDS-SWNTs, the optical absorbance spectrum can be made to match that of BRIJ-97 and Triton-X-100 solutions. Lastly, a correlation is drawn between the amount of shift in the absorbance spectrum and the fluorescence intensity, independent of surfactant used. This shift and decrease in fluorescence intensity may be due to carbon nanotube bundling.