Critical point in ferroelectric Langmuir-Blodgett polymer films

The ferroelectric critical point has been found in a ferroelectric polymer by exploring the influence of the electric field on the paraelectric-ferroelectric phase transition. Dielectric and pyroelectric measurements on 30-monolayer-thick films of the crystalline copolymer poly(vinylidene fluoride-trifluoroethylene) grown by Langmuir-Blodgett deposition show a single hysteresis loop below the zero-field phase transition temperature Tc0=80±10 °C, double hysteresis loops between Tc0 , and the critical temperature Tcr=145±5 °C, and no hysteresis above Tcr where the critical electric field is Ecr=0.93±0.1X109 V/m

Intrinsic Ferroelectric Coercive Field

The Landau-Ginzburg theory of ferroelectricity predicts the intrinsic coercive field for polarization reversal, but the observed extrinsic coercive field is always much smaller as a result of nucleation, dynamic processes not covered by the static theory. We have realized the intrinsic coercive field for the first time, in two-dimensional Langmuir-Blodgett polymer films as thin as 1 nm. The measured coercive field is in good agreement with the theoretical intrinsic value, exhibits the expected dependence on temperature, and does not depend on thickness below 15 nm

Changes in Metallicity and Electronic Structure Across the Surface Ferroelectric Transition of Ultrathin Crystalline Poly(vinylidene Fluoride-Trifluoroethylene) Copolymers

The surface ferroelectric-paraelectric phase transition in crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%), at about 20 °C, is accompanied by a dramatic change in both the electronic structure and metallicity. The temperature-dependent changes in the electronic structure occur primarily in the conduction band. This P(VDF-TrFE 70:30) material is observed to resemble an n-type semiconductor, and the change in electronic structure at the surface ferroelectric transition resembles a semiconductor-to-semimetal transition

Evidence of dynamic Jahn-Teller distortions in two-dimensional crystalline molecular films

The surface electronic structure in crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%) has been studied by photoemission and inverse photoemission as a function of alkali metal (sodium) doping. Sodium doping introduces at least two new states into the band (HOMO-LUMO) gap well away from the Fermi level. While the sodium-doped copolymer is observed to resemble an n-type semiconductor, the change in electronic structure with temperature suggests that dynamic distortions lead to a photoemission initial state splitting of the lower Hubbard-like bands. There is a decrease in the effective Debye temperature with sodium doping which may, in turn, enhance the dynamic Jahn-Teller distortion