Crystal and Layer Structures of Ferroelectric Oligomer Thin Films

The crystal structure of ferroelectric vinylidene fluoride (VDF) oligomer [CF3(CH2CF2) n I] (n = 12) thin films has been investigated in detail. Films with molecular orientations parallel and perpendicular to the silicon substrate were prepared by deposition at substrate temperatures of 120 K (low-T s) and 310 K (high-T s), respectively. X-ray diffraction revealed a base-centered monoclinic structure with space group Cm and lattice parameters a = 0.488 nm, b = 0.865 nm, and c = 3.137/sin β nm (the monoclinic angle β could not be evaluated). The average density of both films determined from X-ray reflectivity (2.0−2.1 g/cm3) was lower than that calculated from the lattice parameters (2.42 g/cm3) due to the surface topology. Small crystal grains are loosely packed in the low-T s film, while large hexagonal grains increase the surface roughness in the high-T s film. In both films, furthermore, the even lower density (1.5−1.7 g/cm3) near the film−substrate interface suggests that the molecules are aligned vertically in this region.