Revealing new electronic behaviours in the Raman spectra of chirality-enriched carbon nanotube ensembles

We present Raman spectroscopy of single-walled carbon nanotubes (SWNTs) that are enriched in metallic species by density gradient ultracentrifugation (DGU) and enriched in single semiconducting chiralities through DNA-based separations. Radial breathing mode (RBM) spectra demonstrate that DGU samples are highly enriched in armchair chiralities. The enrichment allows acquisition of pure G-band spectra of the armchair SWNTs and reveals that the LO mode is absent in these structures. Raman excitation profiles for the G-band in nearly pure (10,2) samples reveals a strong asymmetry in the intensities of the resonance coupling to incident and scattered photons. The experimental data may be fit using a four-level molecular model for Raman scattering and the strong asymmetry can be understood as a consequence of the presence of non-Condon effects. The result requires a reassessment of the assumption that the incident and scattered resonances are equivalent. The consequences of such non-Condon effects on other SWNT electronic and optical processes will be an important topic for future study.