Structural and Electronic Characterization of Epitaxially-Grown Ferroelectric Vinylidene Fluoride Oligomer Thin Films
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ABSTRACT 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 direction of substrate surface, and were similar to polar Form Itype crystals of poly(VDF). While, the thin films evaporated on NaCl(001), SiO/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 topelectrode, and confirm their piezoelectricity of the VDF oligomers for the first time. 1. INTRODUCTION Ferroelectric ultra thin films have a potential in various application areas, such as sensors, memory and electroactive devices. Poly(vinylidene fluoride)(PVDF) and its copolymer with trifluoroethylene [P(VDF-TrFE)] are well known to be ferroelectic polymer with large electric dipole moment. These polymers have several crystal phases with different crystal parameter, molecular packing and conformation. Three of them, called as Form I(P), Form 111(y) and Form IV(6) -type crystals, possess ferroelectric property, the Form I crystals give the best performance. We have studied on a local polarized domain of P(VDF-TrFE) cast films by scanning probe technique, and discuss their piezoelectric behavior at nano scale[1-3]. A. V. Bune and coworker have reported piezoelectricity and pyroelectricity in ferroelectric Langumuir-Blodgett polymer films[4-6]. However, it is difficult to characterize the dipole switching and diffusion in the polymer due to their amorphous region and low crystallinity. In this study, we selected a newly synthesized vinylidene fluoride (VDF) oligomer as a simple ferroelectric organic molecule. The molecule with low molecular weight and its narrow distribution has the possibilities for fabricating the high crystalline thin films and for controlling their crystal 101
Mat. Res. Soc. Symp. Proc. Vol. 600 0 2000 Materials Research Society
phase. We attempt to fabricate the epitaxially grown ferroelectric thin films by using this sample, and also focus on ferroelectricity of VDF oligomer. In such film, it is interesting to evaluate the local piezoelectric properties and switching mechanism in microscale, in order to study more about the local ferroelectric domains in epitaxial organic crystals. 2. EXPERIMENTS 2-1. Sample and film fabrication The VDF oligomer [ CF3-(CH2CF 2 )n -I, (n=17)] was newly synthesized by the telomerization method at Daikin Kogyo Co., Ltd
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