Effect of uniaxial strain deformation upon the Raman radial breathing modes of single-wall carbon nanotubes in composites

We report a Raman spectroscopic study on the strain-induced intensity variations of the radial breathing modes (RBM's) of single-wall carbon nanotubes (SWNT's) in epoxy/SWNT composites. Variations of between 30 and 150 % of the RBM intensities were observed over a range of strain between -0.3 and 0.7 %. The trend (increase or decrease) as well as the magnitude of the intensity variations depends on the nanotube diameter and its chirality. Using tight-binding calculations, we show that these intensity variations can be explained entirely by resonance theory. The electronic band structure of SWNT's is predicted to vary significantly with uniaxial strain. Electronic density of states calculations confirm that the energy separation between the Van Hove singularities shifts with strain, becoming closer or further away from the laser excitation energy depending on the nanotube structure. The nanotubes are thus moved closer or further away from resonance, causing the intensity variations. It is demonstrated that through the use of resonance theory, a tentative chirality can be assigned to each type of SWNT from knowledge of its RBM position and the effect of strain upon the RBM intensity, thus determining its entire structure.