Carrier Dynamics in Conducting Polymers: Case of PF6 Doped Polypyrrole (original) (raw)
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Metallic states of PF6-doped polypyrroles at low temperature
Synthetic Metals, 1993
The electrical resistivity of conducting PF6-doped polypyrroles exhibiting resistance maximum near 15 K was measured down to 1 mK. The thermoelectric power was almost proportional to temperature, while ESR measurements exhibited temperatureindependent paramagnetic spin concentration of the order of 1.1 × 1021 per mole of pyrrole rings. We propose a conduction model in relation to the spin concentration. The resistance maximum may be ascribed to crossover from hopping conduction to coherent motion. The logarithmic temperature dependence below 1 K is ascribed to scattering of conduction carriers by centers with internal degrees of freedom as in amorphous metal alloys.
Nature of the Metallic State in Conducting Polypyrrole
Advanced Materials, 1998
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.
Isotherms of the imaginary part of the permittivity from 10 −2 to 2 10 6 Hz from liquid nitrogen to room temperature for fresh and thermally aged specimens of conducting polypyrrole reveal a dielectric loss peak, which is affected by the reduction of conducting grains with aging. Charge trapping at the interfaces separating the conductive islands seems invalid. Thermal aging indicates that macroscopic conductivity and short range one have different aging evolution. The first dc conductivity is dominated by the tunneling of the carriers between neighboring grains through the intermediate insulating barriers, though the second ac conductivity is due to a backward-forward movement of the carriers and is controlled by the intrachain transport of them and their hopping between the chains.
Electrical characterization of conducting polypyrrole by THz time-domain spectroscopy
Applied Physics Letters, 2000
Using an optoelectronic THz beam system for THz time-domain spectroscopy ͑THz TDS͒, we have measured the absorption and index of refraction of a conducting polypyrrole film from low frequencies to 2.5 THz. From these measurements, the dc conductivity of 215/͑⍀ cm͒ and the complex conductance were obtained over this frequency range. All of the results were well fit by Drude theory, which gives a carrier scattering time of only 12.6 fs, less than 1/10 that of the semiconductors, thereby illustrating the disorder and low mobility of the polymer.
Metal-insulator transition inPF6doped polypyrrole: Failure of disorder-only models
Physical Review B, 2001
We performed dielectric spectroscopy on polypyrrole near the metal-insulator transition (MIT) down to 2 K. We evaluate the dependence of the plasma frequency (ωp) and the scattering time (τ) on the distance to the MIT, characterized by the Fermi-level relative to the band-edge of extended states, EF − Ec. Especially the strong decrease of τ with increasing EF − Ec is in conflict with the usually applied models for the MIT. Although morphology and disorder are important, the MIT is Mott-Hubbard-like being dominated by the competition of interchain charge transfer and electronic correlation.
Doping, density of states, and conductivity in polypyrrole and poly(p-phenylene vinylene)
Physical Review B, 2005
The evolution of the density of states ͑DOS͒ and conductivity as a function of well-controlled doping levels in OC 1 C 10-poly͑p-phenylene vinylene͒ ͓OC 1 C 10-PPV͔ doped by FeCl 3 and PF 6 , and PF 6-doped polypyrrole ͑PPy-PF 6 ͒, has been investigated. At a doping level as high as 0.2 holes per monomer, the former one remains nonmetallic while the latter crosses the metal-insulator transition. In both systems, a similar almost linear increase in DOS as a function of charges per unit volume ͑c * ͒ has been observed from the electrochemical gated transistor data. In PPy-PF 6 , when compared to doped OC 1 C 10-PPV, the energy states filled at low doping are closer to the vacuum level; by the higher c * at high doping, more energy states are available, which apparently enables the conduction to change to metallic. Although both systems on the insulating side show log ϰ T −1/4 as in variable range hopping, for highly doped PPy-PF 6 the usual interpretation of the hopping parameters leads to seemingly too high values for the density of states.
On the structure and transport properties of polypyrroles
1992
Conducting organic polymers have attracted much attention as electronic materials, since Shirakawa et al. reported [1] an increase of several orders of magnitude in the electrical conductivity of polyacetylene after reaction with oxidants. This oxidative process giving positively charged structures is known as ,,doping,, owing to its analogy with the doping process for inorganic semiconductors. However, from a chemical point of view, the two types of doping correspond to different types of chemical transformation [2]: the oxidative process upon the non-conducting neutral polymer results in an positively charged oxidized molecule which in its solid structure requires the incorporation of a counter anion. Obviously, the nature of this anion determines the physical properties of the material. In spite of the high electrical conductivities reported for polyacetylene [3] (see Scheme 1) it rapidly degenerates in air. Other it conjugated organic conductors are thus attracting more attentio...
Electronic Conduction in Polymers. I. The Chemical Structure of Polypyrrole
Australian Journal of Chemistry, 1963
The pyrolysis of tetraiodopyrrole in an inert atmosphere at temperatures from 120-700� yields black, infusible, amorphous polymers insoluble in solvents. Depending on the pyrolysis temperature, iodine may be present in the polymers as iodine of substitution and as chemisorbed molecular iodine, which is very tenaciously held. As a first approximation, the structure may be regarded as a three-dimensional network of pyrrole rings cross-linked in a nonplanar fashion by direct carbon to carbon linkages. The secondary nitrogen atoms form a hydroquine type system which may be oxidized by iodine or molecular oxygen under alkaline conditions. The extent of oxidation depends on the hydroxyl ion concentration. The nonplanarity of the oxidized quinonoid system renders it unstable but stability is enhanced, as in the triphenylmethane dyestuffs, by the formation of a carbinol. Despite their nonplanarity polypyrroles are relatively good conductors of electricity. The resistivity ranges from 1-200 ...
Electrical Transport and Magnetoresistance of Doped Polypyrrole
2015
56 www.ijeas.org Abstract— Different doped samples of conducting polypyrrole (PPy) are prepared electrochemically at different current densities. The electrical transport of conducting polypyrrole is investigated in a wide range of temperature starting from 4.2 K to 300 K. It is observed that, for samples prepared at high current densities, the electrical conduction is dominated mainly by the mechanism based on variable range hopping in a wide temperature range. On the other hand, the magnetoresistance of polypyrrole is investigated in the temperature range from 1.2 K to 4.2 K and magnetic field from H=0 T to H= 10.3 T. It is observed that, PPy sample is characterized by large positive magnetoresistance. This obtained large positive magnetoresistance in the insulating regime is typically expected for VRH conduction.
Structure–conductivity relationships in chemical polypyrroles of low, medium and high conductivity
Synthetic Metals, 2006
Chemically synthesized polypyrroles of low (σ < 75 S/cm), medium (75 < σ < 200 S/cm) and high (σ > 200 S/cm) electrical conductivity (σ) with the same dopant and degree of doping have been investigated by means of Wide Angle X-ray Scattering (WAXS), 13 C Cross Polarized Magic Angle Spinning Nuclear Magnetic Resonance ( 13 C CP/MAS NMR) spectroscopy and Fourier Transform Infrared (FTIR) Spectroscopy to establish structure-conductivity relationships useful for industrial applications. A similar amorphous structure was found by WAXS even for the higher conducting PPy (σ = 288 S/cm). WAXS spectra for polypyrroles of medium and high conductivity showed a weak peak at 2θ = 10-11 • due to improved order of the counterions in these materials. The effect of the counterion size in the asymmetry of the PPy main WAXS peak was elucidated by performing ion exchange of the Cl − dopant with counterions of larger size such as BF 4 − and ClO 4 − . From 13 C CP/MAS NMR measurements predominantly ␣-␣ bonding was found in these materials. The main 13 C CP/MAS NMR resonance peak of PPy located at 126-128 ppm was broadened upon increasing conductivity. Interestingly, a linear relationship was observed between the half-width at half-height (HWHH) of the 13 C CP/MAS NMR peak and conductivity where a doubling of the polypyrrole conductivity leads to an increase of HWHH by 6-7 ppm. FTIR data of these materials were analysed in the framework of the Baughman-Shacklette theory describing the dependence of conductivity on conjugation length. By comparison of model predictions and experimental results, the PPy samples were found to be in the regime of long conjugation lengths, L K 2 /k B T, where K 2 is a parameter related to the energy change on going from j − 1 to j charges on a conjugated segment of conjugation length L, k B the Boltzman constant and T is the absolute temperature.