Raman Scattering Study of Coalesced Single Walled Carbon Nanotubes
Journal of Materials Research
High temperature heat treatment of single wall carbon nanotube bundles in flowing H 2 was used to produce a significant fraction (ϳ40%) of diameter-doubled, or coalesced tubes with a mean diameter corresponding to that of ϳ(20, 20) tubes. At three laser excitation wavelengths (514.5, 647, and 1064 nm), a reduction in the Raman scattering intensity of the strong radial and tangential modes was observed in the H 2 -treated sample, consistent with the reduced fraction of tubes in the sample after
... n the sample after coalescence. However, using 488 nm excitation, little or no change is observed in the Raman spectrum after the H 2 treatment, suggesting that this excitation wavelength couples only to chiral symmetry tubes. Using the 647 nm excitation, the effect of H 2 treatment on the tangential band is quite unique, and a significant change in the shape of the tangential band was observed. Our lineshape analysis, and other results reported in this issue, suggest that this unique change of shape is due to lost scattering intensity from metallic tubes partially compensated by tangential mode scattering from the coalesced tubes. The normally prominent radial breathing mode band, which would be expected at ϳ90 cm 21 for ϳ(20, 20) tubes was not observed, indicating that these larger diameter tubes do not exhibit strong resonant scattering, at least at any of the wavelengths used in this study.