Structural and electronic characterization of epitaxially-grown ferroelectric vinylidene fluoride oligomer thin films

The structural and electric properties of newly synthesized vinylidene fluoride (VDF) oligomer thin films evaporated on various substrate were investigated. The structural behavior of the VDF oligomer thin films strongly depended on the kinds of substrate and substrate temperature during film preparation. In particular, the VDF oligomers epitaxially grew on KBr(001), aligned their molecular chain along the 〈110〉 direction of substrate surface, and were similar to polar Form I-type crystals of poly(VDF). While, the thin films evaporated on NaCl(001), SiO₂/Si were non-polar Form II-type or mixture of both Form I- and Form II-type crystals with their molecular chain normal to the surface. These facts indicated that crystal field of substrate, based on van der Waals and electrostatic interactions, strongly influenced to ferroelectric phase transition of the VDF oligomer. In addition, we demonstrate the formation of the local polarized domains in the epitaxial crystals by applying electric pulses from a conducting AFM probe used as a positionable top-electrode, and confirm their piezoelectricity of the VDF oligomers for the first time.