Semiconducting single-walled carbon nanotubes (SWNTs) are fundamentally interesting and technologically relevant materials with size tunable absorption and emission across a range of visible and near infrared wavelengths. However, several important aspects of their photophysical properties are not known in enough depth to predict how SWNTs will behave as part of larger integrated nanosystems. In particular, simple concepts such as the fundamental factors that affect photoluminescence efficiency (PL QY) are not well understood for SWNTs.

We recently found that for DNA-wrapped SWNTS, the SWNT PL QY can be increased substantially in the presence of small amounts of mildly reducing molecules such as dithiothreitol (DTT) and Trolox. We will present comprehensive studies of the molecular-induced changes in PL from SWNTs suspended in several other commonly used surfactants. For example, upon addition of the reducing agents, sodium dodecyl sulfate (SDS) wrapped SWNTs showed a 4-fold PL intensity increase while sodium cholate (SC) wrapped SWNTs showed PL decrease by ~45%. Minimal change in PL was observed for SWNTs dispersed in sodium dodecylbenzene sulfonate (SDBS) and sodium deoxycholate (DOC). By using a state-of-the art NIR sensitive APD, we were able to acquire time resolved PL data from each sample. For SWNTs that were brightened (darkened), the non-radiative rate decreased (increased) and the radiative rate increased (decreased). Our preliminary conclusion is that adding the reductants causes a change in the molecular structure of the surfactant on the SWNT, which changes the local dielectric constant as well as the interaction of the exciton with the molecular orbitals of the surfactant.

We will also present recent data on the PL brightening of individual SDS wrapped SWNTs, whereby we found the PL efficiency enhancement varied substantially among individual nanotubes, with some brightening by up to 6.5 times while others brightened by only 16%. Studies of NTs longer than the diffraction limit reveal the surprising finding that upon adding the reductants, the NTs brightened uniformly, as determined from the PL image. Since non-uniform brightening would result from consecutive interactions with individual point defects, the observation of uniform PL brightening is consistent with the interpretation that the reductants are causing changes in the local SWNT environment. Further, single step brightening across the entire NT followed by a constant, nonblinking PL level suggests the brightening effect could be due to as little as a single molecule interacting with the NT.