  While addition of electrolyte to sodium dodecyl sulfate suspensions of
single-wall carbon nanotubes has been demonstrated to result in significant
brightening of the nanotube photoluminescence (PL), the brightening mechanism
has remained unresolved. Here, we probe this mechanism using time-resolved PL
decay measurements. We find that PL decay times increase by a factor of 2 on
addition of CsCl as the electrolyte. Such an increase directly parallels an
observed near-doubling of PL intensity, indicating the brightening results
primarily from changes in nonradiative decay rates associated with exciton
diffusion to quenching sites. Our findings indicate that a reduced number of
these sites results from electrolyte-induced reorientation of the surfactant
surface structure that partially removes pockets of water from the tube surface
where excitons can dissociate, and thus underscores the contribution of
interfacial water in exciton recombination processes.

Materials Science;Mesoscale and Nanoscale Physics