Atsushi Kubono

Remanent polarization of evaporated films of vinylidene fluoride oligomers

A remanent polarization of 130±3 mC/m2, large among the values reported for organic materials, and rectangular D–E hysteresis curves were realized in synthesized vinylidene fluoride (VDF) oligomer [CF3(CH2CF2)17I] film evaporated onto a platinum surface around liquid nitrogen temperature. The results suggested that the VDF oligomer film has an extremely high crystallinity, and the electric dipoles arrange almost perfectly perpendicular to the film surface, and that a Lorentz local field factor of ferroelectric VDF oligomer crystals is nearly zero. Moreover, the obtained value of the coercive field, which was larger than those of ferroelectric polymers, might be attributed to the steric hindrance arising from the existence of iodine atoms at the VDF oligomer chains.

Structures of vinylidene fluoride oligomer thin films on alkali halide substrate

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 (α phase) to form I (β 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 〈110〉 row of K+ or Cl− with the aid of electrostatic interaction under enough thermal movement. Moreover, in the ...

Molecular Ferroelectricity of Vinylidene Fluoride Oligomer Investigated by Atomic Force Microscopy

Nanometer-scale electrical properties of local ferroelectric domains formed in thin films of newly synthesized vinylidene fluoride (VDF) oligomer were investigated by atomic force microscopy (AFM). Local poling and observation of the piezoelectric response revealed that the polarized domains were reversibly formed and erased in a nanometer-thick film by applying dc or pulse voltages between the electrically conductive AFM tip and the bottom electrode. The formed domain size depends on the pulse poling conditions and increases when the magnitude and the duration of a pulse voltage are increased. A local domain with a diameter of 65 nm was successfully created. The results in this work are comparable to those of previous studies performed on ferroelectric polymer thin films, suggesting that this material is one of the promising candidates for ferroelectric applications such as high-density data storages, and for molecular controls of ferroelectric properties.

Structures and Ferroelectric Natures of Epitaxially Grown Vinylidene Fluoride Oligomer Thin Films

Structural and electrical properties of newly synthesized vinylidene fluoride (VDF) oligomer thin film have been investigated. The FTIR spectrum showed that the epitaxially grown film on KBr(001) substrate consists of form I (β phase) crystals and their c axes (molecular axes) and b axes (polar axes) are arranged parallel to the KBr substrate. To make electrical measurements possible, this film was transferred onto a gold bottom electrode without causing any changes in the crystalline structures. By using a modified atomic force microscope, we succeeded in the formation of local polarized domains as well as the clear observation of piezoresponse hysteresis curves in this sample. The coercive field and piezoelectric coefficient (d33) for the 37-nm-thick film were about 200 MV/m and -3 pm/V, respectively. It was suggested that the b axis in the as-grown film rotated from the parallel to the perpendicular direction to the film surface during the poling process. This study reveals the ferroelectric characteristics in the VDF oligomer thin films for the first time.

Second-Harmonic Generation in Poly(vinylidene fluoride) Films Prepared by Vapor Deposition under an Electric Field

Second-harmonic generation (SHG) revealed the dipole orientation of poly(vinylidene fluoride) (PVDF) thin films prepared by vapor deposition in the presence of an electric field along the substrate surface. The dipole alignment was confirmed for the substrate temperatures between -130°C and 100°C under an electric field of 16 MV/m. The degree of dipole orientation estimated from the ratio of nonlinear susceptibility components depended on the deposition conditions such as the substrate temperature and the field strength. Thermal stability of the poled structure was found to be much better than that obtained by postpoling.

Orientational mechanism for long-chain organic molecules during physical vapor deposition

Molecular orientations during physical vapor deposition of various linear long-chain compounds are interpreted on the basis of a simple model for the initial stage of anisotropic crystal growth. In this model, the initial stage involves adsorption from vapor, surface migration, re-evaporation from the substrate, capture into a cluster, re-evaporation from a cluster, and re-orientation, where the anisotropy of the molecules are considered. In addition, two fundamental orientations of initial small nuclei are assumed to be under thermal equilibrium. The set of differential equations for the individual elementary processes can be solved numerically. The calculated results agree well with actual orientational phenomena of long-chain molecules at various substrate temperatures, i.e., a higher substrate temperature results in a higher degree of normal orientation.

Effects of cell parameters on the properties of hybrid twisted nematic displays

This article presents the results of an investigation of the electro-optical response for a hybrid twisted nematic (HTN) liquid crystal display. The device used both a hybrid alignment and a twisted director configuration. In comparison with the conventional twisted nematic mode, a faster response and a smaller operation voltage were obtained by using an appropriate amount of chiral dopant. The characteristics of the HTN cells were dependent on cell parameters such as the chemical nature of the polymer alignment layer, the cell thickness, the set twist angle, and the concentration of chiral dopant. Response times of 1.0 ms for turn-on and 16 ms for turn-off were achieved by the regulation of cell parameters. Computer simulations of the director configuration demonstrated that twist deformation plays an important role in the determination of the response characteristics.

Classical nucleation theory applied to molecular orientations in vapor-deposited organic thin films

Molecular orientational mechanism during physical vapor deposition of various linear long-chain compounds has been interpreted on the basis of a classical nucleation theory modified for the anisotropic nucleus formation. In this model, the anisotropy of the surface energy is considered, and initial small nuclei with two fundamental orientations (normal and lateral) are assumed to be formed and dissociated repeatedly by the thermal energy. The critical energy of nucleus formation for each orientation is calculated as a function of the chemical-potential change of the system. The calculated results are similar to those observed for actual orientational phenomena of long-chain molecules at various chemical-potential changes; i.e., a higher substrate temperature and/or a lower deposition rate result/s in a higher degree of normal orientation.

High Piezoelectric Activity in Nonpoled Thin Films Prepared by Vapor Deposition Polymerization

The piezoelectric constant of 50 pm V-1 and pyroelectric constant of 20 µC m-2 K-1 were obtained for polyurea thin films (~300 nm) prepared by vapor deposition polymerization from methane diisocyanate and diaminofluorene. The pyroelectric signal and the infrared spectrum indicate that the dipoles of urea groups in the polymer were highly oriented normal to the surface of the aluminum substrate to induce remanent polarization without a conventional poling process. The piezoelectricity observed for the film is due to the coupling effect of the remanent polarization and electrostriction. No ferroelectric polarization reversal was observed for the film up to 390 K.

STRESS BEHAVIOR AND CONDUCTIVITY OF THE ELECTRO- RHEOLOGICAL SUSPENSIONS

The steady stress under static electric fields and the conduction current passing through the electro-rheological (ER) suspensions with cation exchange resin particles were investigated simultaneously as a function of the shear rate. The shear thinning behavior was found for a suspension with a higher viscosity of the continuous phase. Further, a remarkable dip behavior was found for a dilute ER suspension. The apparent conductivity, which was calculated from the conduction current, probed sensitively the corresponding behavior to the stress responses.