Diameter dependence of the Raman<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>D</mml:mi></mml:math>-band in isolated single-wall carbon nanotubes

Raman D-band spectra are reported for several different SWNTs using two different laser energies ${(E}_{\mathrm{laser}}=1.58$ and 2.41 eV). At a fixed ${E}_{\mathrm{laser}},$ individual isolated SWNTs exhibit different diameter-dependent D-band frequencies ${\ensuremath{\omega}}_{D}$ around an average value. For both semiconducting and metallic tubes, ${\ensuremath{\omega}}_{D}$ decreases with decreasing nanotube diameter, though ${\ensuremath{\omega}}_{D}$ 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.