Published papers

A new micro-electrode structure for Hall-effect measurements of organic semiconductor layers was proposed. The equations for the Hall-effect measurements based on the van der Pauw method indicate that narrower electrode gaps can decrease the resistance between contact points and enable Hall-effect measurement even for higher resistance materials such as organic semiconductors. According to this “scaling rule”, we fabricated a new electrode chip which consists of four small finger electrodes with a gap of micrometer-scale between diagonally opposite electrodes. In order to demonstrate the validity of this concept, Hall-effect measurements for FeCl3-doped poly(3,4-ethylenedioxythiophene) (PEDOT) was performed using this new electrode structure, revealing that a 10-µm-gap electrode chip is effective for evaluating carrier concentration of down to around 1019 cm-3. These Hall electrode chips are quite useful for quantitative analysis of carrier concentration and ca rier mobility in organic semiconductor materials.

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.

The pyroelectricity of ferroelectric vinylidene fluoride (VDF)-oligomer [CF3(CH2CF2)17I]-evaporated film was investigated utilizing a low-frequency sinusoidal heat source. A current profile appeared as the summation of a fundamental sine wave with a frequency of the temperature change and its harmonic responses. The pyroelectric coefficient (p) of the 600-nm-thick VDF oligomer film was estimated to be -68 μC/m2K at 37°C, whose absolute value is larger than those reported for ferroelectric polymers. Various characteristics such as a large |p|, a small dielectric constant and suitability for preparation of thinner films suggest that the VDF oligomer is a promising material for application in pyroelectric thin-film infrared detectors.

Remanent polarization of evaporated films of vinylidene fluoride was investigated. Hysteresis curves were also realized. The film was found to have an extremely high crystallinity, and the electric dipoles arrange almost perpendicular to the film surface. The obtained value of coercive field was attributed to the steric hindrance.

Epitaxially grown vinylidene fluoride oligomer thin films formed on KCl(0 0 1) surfaces were investigated by ultrahigh vacuum non-contact AFM (UHV-NC-AFM). Molecularly resolved NC-AFM images of organic thin layers on insulating surfaces were successfully obtained for the first time, which revealed the epitaxial growth process on a molecular-scale. It was found that the first monolayer formed on KCl(0 0 1) surface was composed of characteristic honeycomb-like structures while the following upper molecular layers consist of molecular-scale stripe structures. In addition, NC-AFM images of the first monolayer suggest a strong electrostatic interaction between the molecule in the layer and the AFM probe tip.

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.

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.

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.

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 transformation process, a pair of "rod-like" crystals, suggesting ferroelectric activity, were observed by AFM.