Infrared reflectance of polypyrrole: ‘metal’ with a gap in the spectrum of charged excitations

We report reflectance (R) measurements of polypyrrole (PPy) films with d.c. conductivity of 350 S/cm at 300 K. The reflectance is metal-like in the IR at all temperatures, with R increasing in the far-IR to about 100% at temperatures below 50 K. The optical conductivity, σ(ω), and the real part of the dielectric function, ϵ1(ω), are not typical of a Drude metal. At 10 K, ε1(ω) = − A/ω2 for h̵ω < 200 cm−1 while σ(ω) decreases in the same spectral range, implying a narrow peak in σ(ω) at ħω well below 20 cm−1 with oscillator strength corresponding to about 2% of the total free-carrier oscillator strength. ‘Metallic’ PPy is, therefore, a conductor with a gap in the spectrum of charged excitations; the d.c. conductivity comes from a narrow peak in σ(ω) at or near ω = 0.