LOW-TEMPERATURE ELECTRONIC TRANSPORT IN HYBRID THIN-FILM NANOSTRUCTURES BASED ON AN ELECTRICALLY CONDUCTIVE POLYMER

Abstract

Most polymers are usually poor conductors of electric current and, accordingly, they can be classified as organic dielectrics. However, there is a special class of polymers, usually characterized by presence of conjugated pbonds, which provide electron delocalization, leading to electrical conductivity in the ground state of the system. In addition to such materials, there have been found polymers that are insulators in the ground state, but under the influence of external factors demonstrate a finite electrical conductivity. Polydiphenylenephthalide (PDP) belongs to the class of carbocyclic organic electroactive polymers that exhibit electrically conductive properties when an external electric field and/or mechanical stress is applied. The effect is explained by the nonzero density of electronic states inside the band gap. The depth of such states increases if the system accepts an additional electron, which indirectly provides electrical conductivity along the polymer chain. Accordingly, the presence of free electrons in contact region with a metal is an important condition for the appearance of final electrical conductivity in PDP. The working hypothesis that stimulates the proposed study is the assumption that superconducting correlations may arise in the electrically conductive state of an electroactive polymer. In the present work, the transport properties of thin-film lead-PDP-lead structures in two configurations are experimentally studied in a wide temperature range: a layered “sandwich” type and a quasi-planar “field-effect transistor” type. At temperatures below ~8 K, in layered samples, features of the PDP electron transport are observed, which can be explained by the effect of induced superconductivity in a thin film of a conducting polymer enclosed between two massive superconductors (lead). In the planar configuration, the electrical conductivity of the polymer in the plane of the structure does not appear. The reason for this observation may be related to the mechanism of the electrically conductive state of the PDP, characterized by formation of quasi-one-dimensional channels of electric current in the direction perpendicular to the plane of contact with the "donor" of electrons: metal in the normal or superconducting state. The effect of appearance of superconductivity in PDP requires further stidies and is potentially of significant interest for various applications in the field of practical microand nanoelectronics.