Vibrational mode assignments for bundled single-wall carbon nanotubes using Raman spectroscopy at different excitation energies

This study establishes a generic fitting approach to assignment of nanotube chiralities based on radial breathing mode frequencies (omega (RBM)) of SWCNTs in as-produced bundles. Four laser lines with energies of 2.62 eV, 2.33 eV, 1.88 eV and 1.58 eV were employed. The observed RBM frequencies, omega (RBM), were plotted as a function of the possible diameters, d, as identified from the so-called Kataura plot and reported values of the parameters A and B, where omega (RBM)=A/d+B, assuming that SWCNTs resonant at the respective laser frequencies dominate the spectrum. The refined values of A and B, obtained by the best fit of a linear regression between omega (RBM) and 1/d, were found to vary significantly for different laser frequencies. This variation is interpreted in terms of the differences in electronic properties of SWCNTs resonant at different frequencies. The assigned nanotubes match well with those identified in the Kataura plot, falling within a resonant line width of +/- 0.2 eV of the respective laser lines.