We measure resonant Raman scattering of individual carbon nanotube bundles under axial strains of up to 17%. The main effect of this strain is to cause debundling of the nanotubes. The $G$ band Raman spectra of metallic and semiconducting nanotubes respond differently to strain and debundling, giving insight into the nature of the broad metallic $G_{-}$ band line shape. For metallic nanotubes, the $G_{-}$ band upshifts and becomes narrower with strain, making it appear more semiconductor-like. Surprisingly, this metal to semiconductor transition is not reversible with strain, indicating that nanotube-nanotube coupling plays a significant role in the observed $G_{-}$ band of unperturbed metallic nanotubes.

• Received 7 January 2007

DOI:https://doi.org/10.1103/PhysRevB.75.155421