Structures of vinylidene fluoride oligomer thin films on alkali halide substrate

Abstract

Structures and crystal transformation of the newly synthesized vinylidene fluoride (VDF) oligomer with large electric dipoles evaporated on KCl (001) at various substrate temperatures have been investigated by an energy dispersive–grazing incidence x-ray diffraction system, Fourier transform infrared spectroscopy, and atomic force microscope (AFM). It was revealed that the molecules grow epitaxially and are influenced greatly by forces of the crystal surface field in terms of van der Waals or electrostatic potentials, and found that the phase transformation from form II ($\alpha$ phase) to form I ($\beta$ phase) is induced by raising the temperature of the substrate from 50 to 80 °C, accompanying the alternation in the crystal axes on the substrate from the $a$ axis of form II to the polar $b$ axis of the form I crystal. This fact suggests that the molecular chain of VDF oligomers aligns their $c$ axes along the $\text{〈110〉}$ row of ${\mathrm{K}}^{+}$ or ${\mathrm{Cl}}^{-}$ with the aid of electrostatic interaction under enough thermal movement. Moreover, in the transformation process, a pair of “rod-like” crystals, suggesting ferroelectric activity, were observed by AFM.