Strain-Induced D Band Observed in Carbon Nanotubes

We report the emergence of the D band Raman mode in single-walled carbon nanotubes under large axial strain. The D to G mode Raman intensity ratio (I_D/I_G) is observed to increase with strain quadratically by more than a factor of 100-fold. Up to 5% strain, all changes in the Raman spectra are reversible. The emergence of the D band, instead, arises from the reversible and elastic symmetry-lowering of the sp² bonds structure. Beyond 5%, we observe irreversible changes in the Raman spectra due to slippage of the nanotube from the underlying substrate, however, the D band intensity resumes its original pre-strain intensity, indicating that no permanent defects are formed.