Raman scattering of carbon nanotube bundles under axial strain and strain-induced debundling

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.