Properties of thin ferroelectric polymer films

The polymer polyvinylidene fluoride and its copolymers with trifluoroethyle ne are ferroelectric; they have switchable polarisation and display a Curie transition. These polymers are readily produced in the form of films with thickness down to 50 nm by conventional spin-casting. The properties of these very thin films are remarkably similar to those of commercial films with thickness of 10 (mu)over dot m or more. The dielectri c constant is virtually unchanged down to 100 nm. Hysteresis loops are read ily obtained, which indicate a coercive field of around 50 MVm(-1), meaning that the films can be poled with voltages of the order of 10 V. The pyroel ectric and piezoelectric coefficients are also similar in magnitude to thos e in thicker films and, when measured with a varying bias field, display a field dependence which reflects the underlying ferroelectric switching. The films show signs of fatigue after repetitive switching, with the polari sation dropping to half its normal value after a few thousand cycles, The benign chemical nature of the polymers, coupled with their low poling v oltage, makes such thin polymer films interesting candidates for applicatio ns in various types of integrated transducer.