Percolation conductivity in Langmuir-Blodgett multilayer films containing a long-chain TTF derivative

The room-temperature DC in-plane electrical conductivity of multilayer Langmuir-Blodgett films of the long-chain tetrathiafulvalene (TTF) derivative octadecanoyl-TTF (ODTTF), mixed with either octadecanoic acid (OA) or pentacosa-10,12-diynoic acid (PA) and doped with iodine, has been measured with a standard two-probe technique. The conductivity was found to depend strongly on the proportion of fatty acid contained in the film, and this behaviour has been compared with that predicted by classical, two-site, two-dimensional (2D) and three-dimensional percolation theory. A percolation threshold corresponding to an ODTTF content of approximately 50% was observed in both cases, in agreement with the value obtained from the 2D model. For films diluted with OA, however, the conductivity transition was smeared out, suggesting a contribution by charge transport perpendicular to the plane of the substrate. Optical microscope studies of these multilayer films have provided further evidence of a two-phase structure.