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By controlling the extent of disorder through electrochemical synthesis at reduced temperatures, conducting polypyrrole (PPy) can be obtained in the metallic regime, in the insulating regime, and in the critical regime of the disorder-induced metal-insulator (M-I) transition. We present the results of reflectance measurements (0.002 -6 eV) of PPy carried out at room temperature on the metallic side and on the insulating side of the M-I transition. While the reflectance spectra obtained from samples on both sides of the M-I transition exhibit spectral features expected for a partially filled conduction band, the electronic states near the Fermi energy (EF) are different in the two regimes. The data obtained from metallic samples indicate delocalized electronic wave functions in the conduction band, whereas the spectral features which characterize the insulating regime indicate that the states near E+ are localized. Consistent with theoretical predictions for the metallic and insulating regimes, the optical conductivity o. (co) and the real part of the dielectric function E&(co) each show different frequency dependences in the far infrared. In the metallic regime o(co)~c o' for Am&600 cm ' and c&(co) ( &0) increases rapidly as co~0, as described by the "localization-modified Drude model, " leading to the conclusion that metallic polypyrrole is a disordered meta/ near the M-I transition. In contrast, the insulating regime is characterized as a Fermi glass as confirmed by o (co)~co for A~& 600 cm

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Heavily doped polypyrrole-hexafluorophosphate, PPy(PF6), undergoes a metal-insulator (M-I) transition at resistivity ratio p, = p(1.4 K)/p"(300 K) = The effect of the partially screened Coulomb interaction is substantial at low temperatures for samples on both sides of the M-I transition. In the insulating regime, the crossover from Mott variable-range hopping (VRH) to Efros-Shklovskii hopping is observed. In the metallic regime, the sign of the temperature coeScient of 0 the resistivity changes at p, =2. At T=1. 4 K, the interaction length L&=(AD/k&T)' =30 A. Since this is smaller than the inelastic-scattering length, L;"=300 A, the contribution to p(T) from the electron-electron interaction is dominant. Application of high pressure decreases p" induces the transition into the metallic regime, and enables fine tuning of the M-I transition. For samples close to the M-I transition, the thermoelectric power is proportional to the temperature in both the metallic and insulating regimes. The correlation length (L, ) increases as the disorder, characterized by p", approaches the M-I transition from either side. The expected divergence in L, at the M-I transition is qualitatively consistent with the values for L, inferred from the extrapolated cr(0) in the metallic regime and from analysis of the VRH magnetoresistance in the insulating regime. Thus, by using p" to characterize the magnitude of the disorder, a complete and fully consistent picture of the M-I transition in PPy(PF6) is developed. I. INTRODUCirONRecent studies of doped polypyrrole (PPy), polymerized electrochemically at relatively low temperatures ( -20'C to -30'C), have shown that the roomtemperature conductivity oar=200 -500 S/cm, increasing to approximately 1000 S/cm after tensile drawing. 'A positive temperature coefficient of the resistivity (TCR) was' reported for temperatures below T =10-20 K for PFs-doped PPy, PPy(PFs). ' To date, however, the physical aspects of these phenomena as related to the metallic nature of heavily doped conjugated polymers have not been clearly understood. Models suggested earlier, ' such as the electron-hopping (or tunneling) conduction, small-polaron tunneling, local superconductivity, etc. seem to be either inappropriate or incapable of describing the entire range for the data (i.e. , in both metallic and insulating samples.Many of the properties that characterize heavily doped conducting polymers, such as relatively high electrical conductivity, temperature independent magnetic susceptibility, linear temperature dependence of thermoelectric power, ' '" absorption throughout the infrared with no energy gap, ' etc. , suggest that the electronic structure is that of a metal. However, the disorder generated during synthesis and during the doping process plays a critical role; microscopic disorder and/or structurally amorphous regions can dominate the transport.%'e present the results of a systematic study of the transport properties of PPy(PFs) near the disorderinduced metal-to-insulator (M I) transition. The ex-tent of di...

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, [2] 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. [3] 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 . [4] Transport studies [5] 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. [2,5] 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. [5] In spite of the evidence for the disorder-induced M-I transition as inferred from the transport [5] and optical measurements, [6] the metallic state of PPy-PF 6 remains a subject of controversy. Kohlman et al. [7] 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. [7]...

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Electrochemical impedance spectroscopy has been applied to investigate the formation of insulating layers at the surfaces of microscopic particles of mesocarbon microbeads (MCMB), graphite, and hard carbon during the first Li-intercalation into these materials at ambient temperature as well as at -20°C. Investigations were carried out in a three-electrode sandwich cell, designed for impedance measurements in the frequency range 64 kHz to 5 mHz. The impedance spectra, obtained in the potential range 1.5 and 0.02 V during the first charge, were analyzed by complex nonlinear least square fits. A new model, taking into account the porous structure of the intercalation material, electrochemical processes at the interface, as well as spherical diffusion of Li ions toward the centers of the particles, has been used for this analysis. The first intercalation at -20°C results in formation of an insulating layer, which is about 90 times thinner than in the room-temperature case, as concluded from an analysis of experimental results. The irreversible capacity loss, which is 1.3 times larger at -20°C than at room temperature, is ascribed to the formation of a porous precipitate of electrolyte decomposition products on the particle surface. Additional Li intercalation at room temperature results in an irreversible capacity loss of 26% from the initial value, and formation of a composite layer, including low-temperature and room-tempekature deposited components * Electrochemical Society Active Member.by Jacobsen and West.13 The impedance of the insulating layers on the surfaces of the particles has been included in the model as proposed by authors in Ref. 7. The results of the analysis of impedance spectra, measured during the first Li intercalation into several carbon-based materials, are compared for conventional and low-temperature cases.