Resonance Raman Signature of Intertube Excitons in (6,5)-Enriched Carbon Nanotube Bundles

Electronic interactions in low-dimensional nanomaterial heterostructures can lead to novel optical response arising from exciton delocalization over the constituent materials. Similar phenomena have been suggested to arise between closely interacting semiconducting single wall carbon nanotubes (SWCNTs) of identical structure. We use resonance Raman spectroscopy (RRS) to probe intertube interactions in (6,5)-enriched SWCNTs, dispersed in aqueous solutions and separated using density gradient ultracentrifugation into fractions of increasing bundle size. A continuously tunable dye laser coupled to a triple-grating spectrometer affords measurement of Raman resonance excitation profiles (REPs) over a range of wavelengths, (505 to 585)nm, covering the (6,5)-E₂₂^S excitation. REPs of both the radial breathing mode (RBM) and G_LO⁺ display a previously unobserved sharp feature that is superimposed on the typical intrinsic exciton resonance response. This sharp feature provides evidence for the creation of intertube excitons, with the response being generated as the result of the interaction between the intratube and intertube excitons whose spectral responses overlap.