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-trifluor oethylene copolymer layers with thickness ranging from 1300 nm down to 65 n m have been investigated by dielectric spectroscopy and electromechanical i nterferometry. The effects of layer thickness (h) on ferroelectricity and t he ferroelectric-to-paraelectric phase transition are discussed on the basi s of the temperature (T) dependence of the dielectric constant (epsilon) an d electrostrictive and inverse-piezoelectric effects. In the region of h le ss than a few hundred nanometers, the layer-thinning effect on epsilon beco mes prominent, and epsilon decreases with a decrease in h, but the phase tr ansition temperature (T-c) and the Curie constants are not significantly in fluenced by layer thinning. The dependence of the electrostriction on the s quare of the applied electric field for unpoled films is nonlinear in the f erroelectric phase, while it is linear in the paraelectric phase. The degre e 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) fo r the films of h greater than or equal to 90 nm, while P-r for the 65 nm th ick film (40 mC/m(2)) is definitely lower. Differential scanning calorimetr y shows that the degree of crystallinity (fraction of ferroelectric crystal line phase) decreases with reduction in film thickness, and especially the crystallinity for the 65 nm thick film is much lower than those for the thi cker ones. In comparison, between the 1300 and 65 nm thick films, the reduc tion in the degree of crystallinity is comparable to the decrease in P-r. T he 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 cry stallites parallel to the film surface. The presence of the preferential cr ystalline orientation might as well explain the appreciably different value s of the surface near layer thickness (12 and 27 nm) evaluated from the h d ependence of the dielectric constant and the degree of crystallinity. The l ayer thickness dependence of the dielectric and electromechanical propertie s is interpreted as a result of a combined effect of the reduction in degre e of crystallinity and the specific crystallite orientation due to layer th inning.