Optical Properties of Single-Walled Carbon Nanotubes Separated in a Density Gradient: Length, Bundling, and Aromatic Stacking Effects

Single-walled carbon nanotubes (SWNTs) are promising materials for in vitro and in vivo biological applications due to their high surface area and inherent near-infrared photoluminescence and Raman scattering properties. Here, we use density gradient centrifugation to separate SWNTs by length and degree of bundling. Following separation, we observe a peak in photoluminescence quantum yield (PL QY) and Raman scattering intensity where the SWNT length is maximized and bundling is minimized. Individualized SWNTs are found to exhibit a high PL QY and high resonance-enhanced Raman scattering intensity. Fractions containing long, individual SWNTs exhibit the highest PL QY and Raman scattering intensities compared with fractions containing single, short SWNTs or SWNT bundles. Intensity gains of approximately similar to 1.7- and 4-fold, respectively, are obtained compared with the starting material. Spectroscopic analysis reveals that SWNT fractions at higher displacement contain increasing proportions of SWNT bundles, which causes reduced optical transition energies and broadening of absorption features in the UV-vis-NIR spectra and reduced PL QYs and Raman scattering intensities. Finally, we adsorb small aromatic species on bright, individualized SWNT sidewalls and compare the resulting absorption, PL, and Raman scattering effects to that of SWNT bundles. We observe similar effects in both cases, suggesting that aromatic stacking affects the optical properties of SWNTs in an analogous way to SWNT bundles, likely due to electronic structure perturbations, charge transfer, and dielectric screening effects, resulting in reduction of the excitonic optical transition energies and exciton lifetimes.