Non-ohmic Conductivity: A Probe of the Localization Mechanism in Polyaniline Derivatives

Abstract

Electron localization in the methyl ring-substituted derivative of polyaniline, poly(o-toluidine), has been investigated by electric field dependence of conductivity u(E). We found /nu(,!?) = K P , with K varying as square root of temperature (K c* ,-I/*). This result is in accord with quasi-one-dimensional variable range hopping of charge carriers localized along a one-dimensional chain, with an effective Coulomb barrier between adjacent sites. The measured non-ohmic conductivity is modeled by the Poole-Frenkel effect where the Coulomb potential between localized sites is lowered by the electric field. Model analysis yields an estimated effective barrier height of -0.1 eV, corresponding to an effective charge carrier mass of the order of that of an electron. In contrast, the charging energy limited tunneling model predicts Inu E -I , which is not consistent with the experimental results.

Localization and Its Absence: A New Metallic State for Conducting Polymers

“…Such a window is narrow compared with the band width (2.6 eV) and hence is consistent with the narrow band of conducting states in the RDM. Our model is also consistent with the observed decrease in the conductivity of randomly substituted polyaniline derivatives (33). It is straightforward to show that methyl group substitutions on the benzene or quinoid rings, for example, change the location of the unscattered states relative to the Fermi level.…”

“…For example, the momentum dependence of the energy loss spectrum of the electron does not corroborate the granular metal picture (29). Recent calculations (33) suggest that the grain size required by the granular metal picture is at the least inconsistent with the experimental data on polyaniline derivatives.…”

“…This serious constraint on the applicability of the variable-range hopping model appears to have been overlooked in previous work (33,45). For example, Wang et al (33) and Nakhrnedov, Prigodin, and Samukhin (45) assumed that if only nearest neighbor connections between chains were considered, the quasi-1-D variable-range result would follow.…”

“…Theoretical interpretations (19,(30)(31)(32)(33)(34) of the insulator-metal transition in polyaniline have been shaped by the observation that the Pauli susceptibility appears to be proportional to the degree of protonation or oxidation of the polymer (22). Physically, this behavior signifies that on the metallic side, the material is inherently inhomogeneous and phase-segregates into oxidized or protonated metallic islands embedded in a sea of insulating islands.…”

“…For example, Wang et al (33) and Nakhrnedov, Prigodin, and Samukhin (45) assumed that if only nearest neighbor connections between chains were considered, the quasi-1-D variable-range result would follow. Such a system is, however, inherently 3-D.…”