Influence of defects and doping on optical phonon lifetime and Raman linewidth in carbon nanotubes

How doping and defects alter linewidths and lifetimes of G-band optical phonons, in carbon nanotubes is examined. Optical phonon lifetimes, T ₁, in thin films of nanotubes are measured by time-resolved incoherent anti-Stokes Raman spectroscopy and considered along with Raman linewidths of isolated individual nanotubes. Within the doping range achievable in nanotube films in this study, T₁ does not appear to change. A varying degree of doping in individual nanotubes by means of electrostatic gating reveals decreasing full-width at half-maximum Γ down to ∼4 cm^-1 at the charge neutrality point. Increasing disorder, on the other hand, leads to a decrease in T₁ along with an increase in Γ. We observe a decrease in T₁ of ∼0.4 ps at an estimated effective crystallite size L_a ∼ 130 nm based on the D-band to G-band peak intensity ratio. In the limit of zero doping and zero defects, the measured Γ of single semiconducting nanotubes coincides with a lifetime broadening of ∼4 cm^-1 based on a measured T₁ of 1.2 ps. Samples displaying different degree of metallic or semiconducting contributions in their static Raman spectrum are also compared and are shown to exhibit similar values of T₁.