Curvature-induced symmetry lowering and anomalous dispersion of the G-band in carbon nanotubes

Here, we report rich and new resonant Raman and infrared (IR) spectral features for several sub-nanometer diameter single wall carbon nanotubes (sub-nm SWCNTs) samples grown using chemical vapor deposition technique operating at different temperatures. We find that the high curvature in sub-nm SWCNTs leads to (i) an unusual S-like dispersion of the G-band frequency due to perturbations caused by the strong electron–phonon coupling, (ii) an activation of diameter-selective intermediate frequency modes that are as intense as the radial breathing modes (RBMs), and (iii) a clear observation of the IR modes. Furthermore, an analytical approach which includes the effects of curvature into the overlap integral and the energy gap between the van Hove singularities is discussed. Lastly, we show that the phonon spectra for sub-nm SWCNTs obtained from the molecular dynamic simulations which employs a curvature-dependent force field concur with our experimental observations.

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► We find that the high curvature in sub-nm SWCNTs leads to the following interesting features in the Raman and IR spectra:
► an unusual S-like dispersion of the G-band frequency due to perturbations caused by the strong electron-phonon coupling,
► an activation of diameter-selective intermediate frequency modes that are as intense as the radial breathing modes (RBMs), and
► a clear observation of the infra-red modes.