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

Raman D-band spectra are reported for several different SWNTs using two different laser energies (Eₗₐₛₑᵣ=1.58 and 2.41 eV). At a fixed Eₗₐₛₑᵣ, individual isolated SWNTs exhibit different diameter-dependent D-band frequencies ωD around an average value. For both semiconducting and metallic tubes, ωD decreases with decreasing nanotube diameter, though ωD for isolated metallic SWNTs is higher than for isolated semiconducting SWNTs. The average D-band frequency depends linearly on Eₗₐₛₑᵣ, 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.