Structural and electrical characterisation of betaine-type, organic, molecular, thin evaporated films and LB multilayers

Abstract

The structure and electrical properties of highly polar indandione-1,3 pyridinium betaine (IPB) derivatives have been studied in vacuum-evaporated thin films and Langmuir-Blodgett (LB) multilayer assemblies. Phase transitions induced by temperature and/or electric field have been observed in LB films of an amphiphilic derivative of IPB. The LB films of IPB, obtained at room temperature, form a Y-like structure which melts at about 50 °C to produce spherical domains, having Z-like structure, which remain stable up to 110 °C. Similar phase transitions can be induced by an electric field with ε ≥ 2 × 10 5 V cm −1 at room temperature. In the new Z-like phase of the IPB LB films, the electrical conductivity increases by some five or six orders of magnitude and the activation energy of dark conductivity decreases from 0.18 ± 0.03 eV to practically zero. The vacuum-evaporated IPB films yield low electrical conductivity (σ = 10 −15–10 −16S cm −1), whereas in the LB multilayers a notable anisotropy of conductivity is observed. In case of coplanar cells the conductivity increases to σ = 10 −8S cm −1. In sandwich-type LB samples the conductivity value is similar to that of the vacuum-evaporated polycrystalline thin films.