Determination of conductivity anisotropy and the role of doping in single walled carbon nanotube thin films with THz spectroscopic ellipsometry

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Wide spectral range spectroscopic ellipsometry (SE) measurements spanning from the THz to the UV (0.41 meV–5.9 eV) are taken of a single walled carbon nanotube thin film while in both undoped and nitric acid doped states. The complex dielectric function spectra extracted from the measurements are sensitive to the relative effects of doping on the free-carrier absorption which yields electrical properties when interpreted using the Drude model, metallic and semiconducting nanotube interband electronic transitions, and a π-plasmon resonance. The electrical resistivity (ρ) determined by the SE measurements is resolved into in- (||) and out-of-plane (⊥) components. While doped, ρ|| and ρ⊥ are found to be comparable at 0.516 ± 0.008 × 10−3 and 0.17 ± 0.04 × 10−3 Ω cm, respectively. By contrast, the undoped film state features a considerable difference between ρ|| and ρ⊥ of 2.21 ± 0.03 × 10−3 and 17 ± 1 × 10−3 Ω cm. The fact that nitric acid doping reduces ρ|| by a factor of ∼4 and ρ⊥ by a factor of ∼100 suggests that doping plays a significant role in reducing the energetic barrier to transport at tube-tube junctions.