Ratchet effect, which has been observed in many systems ranging from a living organism to an artificially designed device, is a manifestation of motion in asymmetric potentials. Here we report results of a conductivity study of Polypyrrole nanowires, which have been prepared by a simple method that generates a variation of doping concentration along the length of the wires. This variation gives rise to a ratchet effect that hinders the symmetry of the hopping process of charges and hence the value of measured resistance of these nanowires becomes sensitive to the direction of current flow. The asymmetry in resistance was found to increase with decreasing nanowire diameter and increasing temperature. The observed phenomena could be explained with the assumption that the spatial extension of the localized state involved in the hopping process is reduced as the doping concentration is reduced along the length of the nanowires. The temperature dependence of the static dielectric constant $(ϵ\propto T^{-\delta })$ gave a value of $\delta$ as 1.4 instead of 1.0 perhaps due to dipole rotation being hindered in nanowires, and this would explain the observed reduction of ratchet effect with lowering temperature.

• Received 30 November 2005

DOI:https://doi.org/10.1103/PhysRevB.73.125313