Layer-thinning effects on ferroelectricity and the ferroelectric-to-paraelectric phase transition of vinylidene fluoride-trifluoroethylene copolymer layers

Dielectric and electromechanical properties of vinylidene fluoride-trifluoroethylene copolymer layers with thickness ranging from 1300 nm down to 65 nm have been investigated by dielectric spectroscopy and electromechanical interferometry. The effects of layer thickness (h) on ferroelectricity and the ferroelectric-to-paraelectric phase transition are discussed on the basis of the temperature (T) dependence of the dielectric constant (epsilon) and electrostrictive and inverse-piezoelectric effects. In the region of h less than a few hundred nanometers, the layer-thinning effect on epsilon becomes prominent, and epsilon decreases with a decrease in h, but the phase transition temperature (T-c) and the Curie constants are not significantly influenced by layer thinning. The dependence of the electrostriction on the square of the applied electric field for unpoled films is nonlinear in the ferroelectric phase, while it is linear in the paraelectric phase. The degree of the nonlinearity decreases as the layer becomes thinner, and for a 65 nm thick film the nonlinearity almost vanishes at temperatures fairly below T-c. Remanent polarizations (P-r) achieved by poling are ca. 55 mC/m(2) for the films of h greater than or equal to 90 nm, while P-r for the 65 nm thick film (40 mC/m(2)) is definitely lower. Differential scanning calorimetry shows that the degree of crystallinity (fraction of ferroelectric crystalline phase) decreases with reduction in film thickness, and especially the crystallinity for the 65 nm thick film is much lower than those for the thicker ones. In comparison, between the 1300 and 65 nm thick films, the reduction in the degree of crystallinity is comparable to the decrease in P-r. The variation of the dielectric constant and the degree of crystallinity on h are reasonably well explained assuming the presence of a nonferroelectric amorphous-like surface near layer. Electron microscopic studies of the 65 nm thick layer suggest a preferred orientation of the chain axis of the crystallites parallel to the film surface. The presence of the preferential crystalline orientation might as well explain the appreciably different values of the surface near layer thickness (12 and 27 nm) evaluated from the h dependence of the dielectric constant and the degree of crystallinity. The layer thickness dependence of the dielectric and electromechanical properties is interpreted as a result of a combined effect of the reduction in degree of crystallinity and the specific crystallite orientation due to layer thinning.