Reflectance of polyaniline protonated with camphor sulfonic acid: Disordered metal on the metal-insulator boundary

Recent progress in the processing of conducting polyaniline (PANI) protonated with functionalized sulfonic acids [such as camphor sulfonic acid (CSA)] has enabled the fabrication of high-quality, homogeneous films with excellent surface quality. We present measurements of the reflectivity of such PANI-CSA films over a wide spectral range (0.006–6 eV) at temperatures from 10 to 300 K. The reflectance spectra of PANI-CSA are characterized by metal-like signatures in the infrared (IR), including high reflectance in far IR and a plasma resonance around 1.2 eV. However, the optical conductivity σ(ω) and the real part of the dielectric function, ${\mathrm{\varepsilon }}_1$(ω), are not typical of a Drude metal; σ(ω) is suppressed below the Drude extrapolation as ω→0 with a peak around 0.2 eV, and ${\mathrm{\varepsilon }}_1$(ω)>0 below 0.2 eV. These features arise from disorder-induced localization (Anderson localization) in PANI-CSA. We present a quantitative analysis of σ(ω) and ${\mathrm{\varepsilon }}_1$(ω) in terms of the localization-modified Drude model. The analysis indicates that the mean free path is approximately 7 Å, comparable to the structural repeat unit along the PANI chain, implying that PANI-CSA is a disordered metal on the metal-insulator boundary.