New Feature Observed in the Raman Resonance Excitation Profiles of $(6,5)$-Enriched, Selectively Bundled SWCNTs

Understanding the photophysics of exciton behavior in single wall carbon nanotube (SWCNT) bundles remains important for opto-electronic device applications. We report resonance Raman spectroscopy (RRS) measurements on $(6,5)$-enriched SWCNTs, dispersed in aqueous solutions and separated using density gradient ultracentrifugation into fractions of increasing bundling. Near-IR to UV absorption spectroscopy shows a redshift and broadening of the main excitonic transitions with increasing bundling. A continuously tunable dye laser coupled to a triple-grating spectrometer affords measurement of Raman resonance excitation profiles (REPs) over a range of wavelengths covering the $(6,5)$-$E_{22}$ range (505 to 585)\,nm. REPs of both the radial breathing mode (RBM) and G-band reveal a redshifting and broadening of the $(6,5)\ E_{22}$ transition energy with increasing bundling. Additionally, we observe an unexpected peak in the REP of bundled SWCNTs, which is shifted lower in energy than the main $E_{22}$ and is anomalously narrow. We compare these observations to a theoretical model that examines the origin of this peak in relation to bundle polarization-enhanced exciton response.