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, ε₁({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 ε₁({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 ε₁({ω}) in terms of the localization-modified Drude model. The analysis indicates that the mean free path is approximately 7 {}, comparable to the structural repeat unit along the PANI chain, implying that PANI-CSA is a disordered metal on the metal-insulator boundary.