Resonant Raman spectroscopy of individual metallic and semiconducting single-wall carbon nanotubes under uniaxial strain

Uniaxial strain is induced by pushing single-wall carbon nanotubes (SWNTs) with an atomic force microscope tip. The vibrational and electronic energies of nanotubes are found to be very sensitive to strain. For both metallic and semiconducting SWNTs under strain, the D, G, and G^' band Raman modes are downshifted by up to 27, 15, and 400. 3em{0ex}cm^-1, respectively. The relative strain-induced shifts of the D, G, and G^' bands vary significantly from nanotube to nanotube, implying that there is a strong chirality dependence of the relative shifts. Semiconducting SWNTs remain strongly resonant under these large deformations, while metallic SWNTs appear to move in and out of resonance with strain, indicating a strain-induced shifting of the electronic subbands. Tight-binding calculations of the electronic band structure of semiconducting and metallic nanotubes under uniaxial strain predict significant shifting of the subband energies, leading to strain-induced changes in the Raman intensity. These theoretical predictions are consistent with what we observe experimentally for metallic nanotubes, but not for semiconducting nanotubes.