Crystalline structures and ferroelectric properties of ultrathin films of vinylidene fluoride and trifluoroethylene copolymer

Thin and ultrathin films of copolymer of 83% mole fraction vinylidene fluoride and 17% mole fraction trifluoroethylene were prepared using a spin-coating technique. Atomic force microscopy results showed that spun films formed tiny grains on the surface. A micro-thermal analyzer showed that tiny grains had a ferroelectric Curie transition at 128.8 °C and melted at 140.7 °C. The crystallinity of the film was measured to be ca. 40% by an X-ray diffraction for the film with thickness above 170 nm. Crystallinity decreased quickly with decreasing thickness. The crystallinity was at most 5% for the film with thickness below 75 nm. We found the ferroelectric polarization reversal effect strongly depended on the film thickness. Clear polarization reversal due to a switching of ferroelectric dipoles was observed for the film with thickness above 90 nm and broad ferroelectric reversal was obtained for the film with thickness below 90 nm. For the films with thickness above 100 nm, the remnant polarizations were almost constant and the coercive fields were significantly decreased with decreasing film thickness.