Raman Spectroscopic Investigation of Individual Single-Walled Carbon Nanotubes Helically Wrapped by Ionic, Semiconducting Polymers

Raman-active vibrational modes of (6,5) chirality-enriched single-walled carbon nanotubes (SWNTs), helically wrapped by semiconducting poly[2,6-{1,5-bis(3-propoxysulfonic acid sodium salt)}naphthylene]ethynylene (PNES), are described in great detail. At an irradiation wavelength of 568.2 nm, the extent to which the environment impacts the nanotube vibrational signature can be probed; in particular, the absence of a G band shift for PNES-[(6,5) SWNT) samples relative to benchmark surfactant-coated nanotubes indicates the lack of any significant charge transfer between the PNES strand and the SWNT skeleton, but electronic spectra provide compelling evidence for polymer-to-SWNT energy transfer. At an irradiation wavelength of 457.9 nm, vibrational modes associated with PNES chains that wrap (6,5) SWNTs are conspicuously enhanced. Under 514.5 non irradiation, PNES-[(6,5) SWNTs] are not excited in resonance but G and G' bands associated with these nanohybrids are strongly enhanced, reflecting the excitation of a multiphonon-mediated vibronic transition of the (6,5) SWNT backbone. At a 488.0 nm irradiation wavelength, Raman spectral signatures of both the PNES polymer and the vibronically excited (6,5) SWNT skeleton through one-phonon-assisted processes are pronounced, demonstrating that a specific SWNT chirality and the corresponding semiconducting polymer helically wrapped about its surface can be probed using an excitation wavelength that does not resonantly excite the SWNT structure.