Ferroelectric properties of ultrathin film of VDF/TrFE copolymers

Ferroelectric properties were examined for the ultra thin films of vinylidene fluoride/trifluoroethylene copolymers spin-coated onto Al-deposited glass substrate. As thickness is decreased, dielectric anomaly rapidly diminishes whereas the dielectric relaxation associated with glass transition remains. The polarization switching transient splits into fast and slow processes. The slow process shows a particular shift toward longer time regions with decreasing thickness. These results were analyzed on the basis of a double layer model consisting of ferroelectric bulk and metal-polymer interface. It is shown that the metal-polymer interface is a 2nm thick, non-ferroelectric layer with suppressed molecular motion. When polarization reversal starts to occur, the interface layer causes a production of the depolarization field in the ferroelectric bulk layer to impede polarization reversal until appropriate space charge rearrangement reduces the depolarization field.