Near-infrared resonance Raman excitation profile studies of single-walled carbon nanotube intertube interactions: A direct comparison of bundled and individually dispersed HiPco nanotubes

Complete Raman excitation profiles for single-walled carbon nanotube radial breathing modes were obtained for bundled HiPco carbon nanotube samples in the region from 700 to 985 nm excitation. Results are compared to similar profiles generated from individual carbon nanotubes dispersed in aqueous solution, allowing a direct determination of intertube interaction effects on electronic properties for 12 specific semiconducting nanotube chiralities. Redshifts in the excitation profiles (ranging from 54 to 157 meV) are observed on going from isolated individual to bundled nanotubes. Additionally, bundling is found to broaden the electronic transitions by an average factor of 2.4 compared to individualized nanotube bandwidths. These results compare well with recent theoretical predictions for bundling effects. An investigation of bundling effects on radial breathing mode frequencies for 17 different nanotube chiralities finds no evidence for significant perturbation of these frequencies resulting from intertube interactions. Our results demonstrate that previously reported radial breathing mode frequency shifts are apparent shifts only, resulting from redshifting of the resonant electronic transitions for bundled nanotubes. Bundle inhomogeneity, packing efficiency, orientational disorder, and symmetry reduction are indicated as important factors in determining the degree of intertube interaction.