Diameter dependence of Raman intensities for single-wall carbon nanotubes

The Raman-active radial breathing modes (RBM) and tangential modes (TM) of single wall carbon nanotubes (SWNT) are studied at fixed laser excitation energy 2.41 eV (514.5 nm). We focus on the striking diameter dependence of the relative intensity of the TM and RBM between 0.9 and 2.1 nm, which displays a series of plateaux separated by well-defined minima around 1.35 and 0.9 nm. This relates to the diameter dependence of allowed optical transitions (AOT) in SWNT. Diameters in the range 1–1.3 nm (above 1.4 nm) correspond to metallic (semiconducting) SWNT in resonance at 2.41 eV. The minima correspond to out-of-resonance conditions for TM. The measurement of the diameter dependence of the TM intensity for fixed laser energies is an alternative experimental way to plot the envelopes of the domains of AOT in SWNT.