Properties of thin ferroelectric polymer films

The polymer polyvinylidene fluoride and its copolymers with trifluoroethylene 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 dielectric constant is virtually unchanged down to 100 nm. Hysteresis loops are readily 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 pyroelectric and piezoelectric coefficients are also similar in magnitude to those 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 polarisation dropping to half its normal value after a few thousand cycles, The benign chemical nature of the polymers, coupled with their low poling voltage, makes such thin polymer films interesting candidates for applications in various types of integrated transducer.