Raman Spectroscopy of Axially Strain Carbon Nanotubes

We investigate resonant Raman scattering of carbon nanotube bundles on an elastomer substrate under axial strains as high as 15{\%}. Over the applied strain range, the $G_{+}$ band Raman frequency decreases for both metallic and semiconducting nanotubes. The $G_{-}$ band Raman spectra, however, respond differently to strain for metallic and semiconducting nanotubes, giving insight into the nature of the broad metallic $G_{-}$ band lineshape. The $G_{- }$band frequency downshifts with applied strain for semiconducting nanotubes, while the $G_{-}$ band frequency increases with strain for metallic nanotubes. The $G_{-}$ band linewidth of metallic nanotubes also becomes narrower with strain, making it appear more semiconductor-like. Surprisingly, this metal to semiconductor transition is not reversible with strain, which indicates that nanotube-nanotube coupling plays a role in the observed broad $G_{-}$ band lineshape of metallic nanotubes.