Nature of the Metallic State in Conducting Polypyrrole (original) (raw)

The ªmetallicº state of conducting polymers continues to be a topic of interest and controversy. [1] Although disorder is generally recognized to play an important role in the physics of ªmetallicº polymers, the length scale of the disorder and the nature of the metal±insulator (M-I) transition are the central unresolved issues. [1±3] In particular, the question of whether disorder is present over a wide range of length scales or whether the properties are dominated by more macroscopic inhomogeneities has been a subject of considerable discussion. In the former case, the M-I transition would be described by conventional localization physics (e.g., the Anderson transition), while in the latter case, the M-I transition would be better described in terms of percolation between metallic islands. Recent progress in the processing of conducting polymers has significantly improved the quality of the materials with corresponding improvements in the electrical conductivity. An example is polypyrrole doped with PF 6 , PPy-PF 6 . Transport studies demonstrated that the improved material is more highly conducting and more homogeneous than that studied earlier. As is typical of conducting polymers, PPy-PF 6 is partially crystalline. The structural coherence length, x, is, however, only »20±50 , less than any length used to characterize the electronic properties near the M-I transition, i.e., less than the inelastic scattering length (L in » 300 ) in the metallic regime, and less than the localization length (L c » 200±300 ) in the insulating regime. The corresponding transport data in the critical regime and the crossover from metal to insulator have been successfully analyzed in terms of conventional disorderinduced localization. In spite of the evidence for the disorder-induced M-I transition as inferred from the transport and optical measurements, the metallic state of PPy-PF 6 remains a subject of controversy. Kohlman et al. reported infrared (IR) reflectance measurements, R(o), which they analyzed in terms of the frequency-(o-) dependent optical constants. They reported a zero-crossing in the dielectric function, e 1 (o), at o » 250 cm ±1 (well below the p-electron plasma frequency at 1.2 eV). At frequencies below the zero-crossing, they reported e 1 (o) becoming increasingly negative. This low-frequency zero-crossing is not consistent with a disordered metal near the M-I transition; Kohlman et al. attributed the zero-crossing to the plasma resonance of a low density of ªdelocalized carriersº with a long scattering time (t » 10 ±11 s). They concluded that metallic PPy-PF 6 is inhomogeneous, consisting of a composite of metallic islands (crystalline regions) embedded in an amorphous matrix and interpreted the M-I transition in terms of percolation between the metallic islands. The inference of a small fraction of carriers with long relaxation time was used to predict ultra-high conductivity polymers in which all the carriers were delocalized with similarly long scattering times. To clarify the nature of the metallic state, we have carried out high-precision reflectance measurements on a series of PPy-PF 6 samples in the insulating, critical, and metallic regimes near the M-I transition. Since the IR reflectance is sensitive to the dynamics of carriers near the Fermi energy (E F ), we expect that such a systematic reflectance study will provide information on how the electronic states near E F evolve as the system passes through the M-I transition. The results demonstrate that metallic PPy-PF 6 is a ªdisordered metalº and that the M-I transition is driven by disorder. We find no evidence of a zero-crossing in e 1 (o) at frequencies as low as o = 8 cm ±1 , even for the most metallic samples. The absence of the low-frequency zero-crossing implies that the small fraction of ªdelocalized carriersº with long scattering time does not exist.