Diameter dependence of the RamanD-band in isolated single-wall carbon nanotubes

Raman D-band spectra are reported for several different SWNTs using two different laser energies ${(E}_{\mathrm{laser}}=1.58\mathrm{}$ and 2.41 eV). At a fixed $E_{\mathrm{laser}},$ individual isolated SWNTs exhibit different diameter-dependent D-band frequencies ${\omega }_D$ around an average value. For both semiconducting and metallic tubes, ${\omega }_D$ decreases with decreasing nanotube diameter, though ${\omega }_D$ for isolated metallic SWNTs is higher than for isolated semiconducting SWNTs. The average D-band frequency depends linearly on $E_{\mathrm{laser}},$ as previously observed for SWNT bundles, suggesting that the D-band in SWNTs is activated by defects or by the finite size of the SWNTs.