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

Abstract

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, ${T}_{1}$ 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 \ensuremath{\Gamma} down to \ensuremath{\sim}4 ${\mathit{cm}}^{\ensuremath{-}1}$ at the charge neutrality point. Increasing disorder, on the other hand, leads to a decrease in ${T}_{1}$ along with an increase in \ensuremath{\Gamma}. We observe a decrease in ${T}_{1}$ of \ensuremath{\sim}0.4 ps at an estimated effective crystallite size ${L}_{a}$ \ensuremath{\sim} 130 nm based on the $D$-band to $G$-band peak intensity ratio. In the limit of zero doping and zero defects, the measured \ensuremath{\Gamma} of single semiconducting nanotubes coincides with a lifetime broadening of \ensuremath{\sim}4 ${\mathit{cm}}^{\ensuremath{-}1}$ based on a measured ${T}_{1}$ 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 ${T}_{1}$.