Far-infrared absorption of undoped and Br-doped carbon nanofiber powder in stacked-cup cone configuration

We carried out room-temperature far-infrared (40--650~cm$^{-1}$) transmission measurements on undoped and bromine-doped powder samples of carbon nanofibers in stacked-cup cone geometry. The transmission spectra of both doped and undoped samples were fit to a Drude-Lorentz model. A single Drude component and a small bandgap (around 8~meV) semiconducting Lorentzian component along with 3 other Lorentz components were essential to get a good fit in the entire measured frequency range. A decreased metallic conductivity along with a red-shift of the lowest semiconducting gap was found after bromine doping. A significant decrease in the scattering rate upon heavy doping has been qualitatively explained as partial ordering of intercalated dopant ions. Absorption spectra were derived from the transmission spectra under the assumption of non-dispersive reflectance. These spectra were compared to Drude-Lorentz model absorption spectra. The free-carrier density of the n-type powder and the electronic mean free path were estimated and compared with previously reported values for single-walled nanotubes and pyrolytic graphite.