Fano Resonances in Mid-Infrared Spectra of Single-Walled Carbon Nanotubes

We show that optical phonon modes in single-walled carbon nanotubes (SWNTs) become observable in mid-infrared (MIR) spectroscopy by the means of Fano resonances. The scattering of a low energy electronic continuum over phonon discrete states yields anti-resonances that are recognizable in the spectra by their characteristic asymmetric line shape. Experimentally, we control the charge carrier density in SWNTs by \emph{p}~doping with different molecular oxidizers at saturation and compare the spectra of doped and intrinsic samples. The only measurable feature in the intrinsic state is a kink at $\sim865$~cm$^{-1}$. Kinks at $\sim1600$ and $\sim1250$~cm$^{-1}$ appear upon doping. We find no significant differences between the dopants; hence the bands belong to the SWNTs. Fitting of the band at $\sim1600$~cm$^{-1}$ yields good agreement with a phenomenological Fano resonance model. Finally, SWNTs mats are functionalized with bromophenyls, which are known to increase the number of defects. We find that upon \emph{p}~doping, the Fano resonances' cross sections of damaged SWNTs increase compared to that of \emph{p}~doped pristine SWNTs. Hence, we conclude that defects lower the symmetry of the lattice and activate optical phonon modes in MIR spectroscopy.