Investigation of Ferroelectric Properties of Vinylidene Fluoride Oligomer Evaporated Films
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Investigation of Ferroelectric Properties of Vinylidene Fluoride Oligomer Evaporated Films K. Noda1, K. Ishida1, 2, K. Mochizuki1, A. Kubono3, T. Horiuchi1, H. Yamada1 and K. Matsushige1 1 Department of Electronic Science and Engineering, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, JAPAN 2 PRESTO, Japan Science and Technology Corporation, JAPAN 3 Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, JAPAN ABSTRACT Structures and electrical properties of newly synthesized vinylidene fluoride (VDF) oligomer [CF3(CH2CF2)17I] films evaporated onto various substrates around liquid nitrogen temperature were investigated. As a result, the VDF oligomer films were mainly formed with ferroelectric phase (form I) crystals and the molecular chains were oriented parallel to the substrate surfaces regardless of both the kind of the substrates and the thickness of the VDF oligomer films. In addition to these properties, the VDF oligomer films showed polarization reversal due to 180° rotation of the polar VDF oligomer molecules according to the applied voltages. This ferroelectric behavior was verified by local poling and piezoresponse measurements with an atomic force microscope, and by measurements of D-E hysteresis curves.
INTRODUCTION Vinylidene fluoride (VDF) oligomers are substances with low molecular weights of poly(vinylidene fluoride) [PVDF] which is well known as a ferroelectric polymer. A number of studies on VDF oligomers have demonstrated that VDF oligomers show complicated behaviors on their crystalline structures and molecular orientations due to the differences in molecular chain lengths, terminal groups of molecular chains, conditions for film preparation and so on [1-8]. So far, only CF3(CH2CF2)17I (hereafter we called this molecule as the VDF oligomer) films, which were epitaxially grown on alkali halides and crystallized in ferroelectric phase (form I) crystals, was proved to be ferroelectric experimentally among all VDF oligomers [4,5]. It is still difficult to fabricate ferroelectric crystals of the VDF oligomer directly onto metal surfaces because the VDF oligomer tends to crystallize in paraelectric phase (form II) crystals with normal molecular orientation to the metal surfaces during the evaporation process, as described in latter section. However, there exists a report on PVDF that form I phase is likely to appear during the crystallization from the melting PVDF by ultraquenching [9]. Therefore, we attempted to realize the formation of the VDF oligomer form I crystals on metal surfaces by employing substrates cooled with liquid nitrogen during evaporation. After that, electrical properties of the films were evaluated by using scanning probe microscope (SPM) technique and the measuring method of D-E hysteresis curves. From these results, the ferroelectricity of the VDF oligomer was discussed in terms of both microscopic and macroscopic viewpoints.
U3.16.2
EXPERIMENTAL DETAILS The VDF oligomer [CF3(CH
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