The Resonance Structure of Raman Scattering for Emitted and Absorbed Phonons in Chirality-Pure Carbon Nanotube Films

Chirality-pure single-wall carbon nanotubes (SWCNTs) are ideal samples for resonant Raman scattering (RS) as they have sharp optical transitions and are strong Raman scatterers, having diameter and chirality specific resonances in the visible and in the near-infrared. Recently, it has become possible to rapidly obtain broadband maps of many vibrational modes of SWCNTs and their excitation resonances. Here, we use a full spectrum Raman excitation mapping technique to obtain experimental Raman excitation maps for several species of SWCNT [(7,6), (7,5), and (6,5)] purified by two different methods, mapping both phonon emission (Stokes) RS and phonon absorption (anti-Stokes) RS. These experimental maps show intricate patterns which match well with some of the predictions of quantum models of RS, and strong signals enhanced ≈3000× over graphite G bands near incident photon resonance. All RS bands necessarily have different excitation structure, but Stokes and anti-Stokes pairs are closely symmetrical, yielding an overall butterfly shaped pattern interpretable in terms of excitonic resonances with incident photons and scattered photons. Temperature provides the phonon population for anti-Stokes RS and is critical to interpreting relative feature intensity compared to Stokes RS. SWCNTs are robust and strong RS, both Stokes and anti-Stokes and their ratio, can be obtained over orders of magnitude in laser power, which we show is a critical variable affecting relative response based on sample preparation factors.