Electron localization and charge transport in poly(<i>o</i>-toluidine): A model polyaniline derivative

Abstract

The electron localization is increased with increasing one dimensionality of a quasi-one-dimensional disordered system (quasi-1D-DS). This concept is tested by studying the electron localization in the methyl ring-substituted derivative of polyaniline (PAN), poly(o-toluidine) (POT). The studies include the measurements of temperature dependence of the dc conductivity ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{dc}}$(T), thermoelectric power S(T), microwave conductivity ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{mw}}$(T), and dielectric constant \ensuremath{\epsilon}(T) at 6.5 GHz, dc susceptibility \ensuremath{\chi}(T), electron paramagnetic resonance, and electric-field dependence of conductivity \ensuremath{\sigma}(F). The experimental results showed greater electron localization in the HCl salt of POT than that of PAN, reflected in much smaller ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{dc}}$, ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{mw}}$, and \ensuremath{\epsilon}, increased Curie susceptibility, and decreased Pauli-like susceptibility. The localization is attributed to the reduced interchain diffusion rate caused by decreased interchain coherence and increased interchain separation, both of which result from the presence of ${\mathrm{CH}}_{3}$ on the ${\mathrm{C}}_{6}$ rings. The T dependences of ln\ensuremath{\sigma}\ensuremath{\sim}-${\mathit{T}}^{\mathrm{\ensuremath{-}}1/2}$ and S(T)\ensuremath{\sim}${\mathit{S}}_{0}$+B/T are interpreted as quasi-1D variable-range hopping (VRH) between the nearest-neighboring chains. Within the model, \ensuremath{\sigma}(F)\ensuremath{\sim}${\mathit{scrKF}}^{1/2}$ with scrK\ensuremath{\sim}${\mathit{T}}^{\mathrm{\ensuremath{-}}1/2}$ can be understood. The charging energy limited tunneling model for granular metals and the three-dimensional VRH model with a Coulomb gap are not consistent with the experiments. Other possible mechanisms for electron localization and the general implications for control of dimensionality and conductivity are discussed.