Evidence of Intertube Excitons Observed in the Raman Resonance Excitation Profiles of $(6,5)$-Enriched SWCNT Bundles

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 bundle size. Near-IR to UV absorption spectroscopy demonstrates a redshift and broadening of the main excitonic transitions with 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, (505 to 585)\,nm, covering the $(6,5)$-$E_{22}^S$ excitation. REPs of both the radial breathing mode (RBM) and G$_{\mathrm{LO}}^+$ reveal a redshifting and broadening of the $(6,5)\ E_{22}^S$ transition energy with increasing bundle size. Most interestingly, we observe an additional peak in both the RBM and G$_{\mathrm{LO}}^+$ REPs of bundled SWCNTs, which is shifted lower in energy than the main $E_{22}^S$ and is anomalously narrow. We attribute this additional peak to a transverse, intertube exciton.