Yukio Ouchi

Antiferroelectric Chiral Smectic Phases Responsible for the Trislable Switching in MHPOBC

At least two antiferroelectric liquid crystalline phases were discovered in MHPOBC. These phases appear below the usual ferroelectric Sm C* phase. Because of the alternation of the molecular tilt directions as well as the dipole orientations in successive layers, the optic axis is along the layer normal. This strong stabilization along the layer normal brings about the so-called third stable state responsible for the tristable switching. The antiferroelectric structure was strongly supported by selective reflections in oblique incidence; a full-pitch band does not appear in the antiferroelectric phases, while it does appear in the ferroelectric phase.

Dependence of indium–tin–oxide work function on surface cleaning method as studied by ultraviolet and x-ray photoemission spectroscopies

The effect of the method used to clean indium–tin–oxide (ITO) on its work function was investigated by ultraviolet photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy. With only ultrasonic cleaning in the organic solvent, considerable carbon contamination remained on the ITO surface and the work function was low (4.5 eV). In contrast, ultraviolet (UV)–ozone treatment removed significant carbon contamination, with an increase in the work function to 4.75 eV, which improves the hole-injection efficiency into the organic hole-transport layer in organic electroluminescent devices. Although carbon contamination on the ITO surface was also removed by Ar+ sputtering, it was accompanied by the removal of oxygen from ITO, and the work function was reduced (4.3 eV). Three factors, i.e.,: (i) C-containing contaminants, (ii) the O/In ratio, and (iii) the In/Sn ratio on the ITO surface affect the work function. The present results and those of other workers suggest that these three factors affect the...

Tristable Switching in Surface Stabilized Ferroelectric Liquid Crystals with a Large Spontaneous Polarization

A new switching process was observed in surface stabilized ferroelectric liquid crystals. The switching is associated with a third stable state in addition to the well-known bistable states. The appearance of the third state is characteristic of materials with a large spontaneous polarization and is caused by minimizing induced polarization charges. The switching between each bistable state and the third state exhibits a sharp dc-threshold and hysteresis, suggesting a possible application for a switching device.

Novel Phases Exhibiting Tristable Switching

The phase, designated as Sm CA*, which shows tristable switching was investigated in C8H17O–--COO–-COO*CH(CH3)C6H13 (MHPOBC) by means of thermal analyses, a miscibility test and microscope observation. The phase transition from Sm C* to Sm CA* is not observable by AC calorimetry but small peaks comparable to the Sm A-Sm C* transition are discernible in DSC. According to the miscibility test of racemic MHPOBC with a standard compound, this new phase was found to exist between Sm C* and Sm I* and not to be miscible with Sm Bhex. Therefore, the assignment of Sm CA* to one of the known phases is ruled out, which means that Sm CA* is a totally new phase. The difference between the pure enantiomer and the racemate is also pointed out.

Structures of Ionic Liquids with Different Anions Studied by Infrared Vibration Spectroscopy

We investigated the structures of ionic liquids (1-butyl-3-methylimidazolium iodide [BMIM][I] and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4]) and their aqueous mixtures using attenuated total reflection (ATR) infrared absorption and Raman spectroscopy. The ATR spectrum in the CHx (x = 1, 2, 3) vibration region from 2800 to 3200 cm-1 was very different between [BMIM][BF4] and [BMIM][I] even though all the spectral features in this region were from the butyl chain and the imidazolium ring of the same cation. The spectrum did not change appreciably irrespective of the water concentration for [BMIM][BF4], whereas the spectrum from [BMIM][I] showed significant changes as the water concentration was increased, especially in CH-vibration modes from the imidazolium ring. For very diluted solutions both aqueous mixtures of [BMIM][I] and [BMIM][BF4] showed very similar spectra. Mixing of [BMIM][I] with heavy water (D2O) facilitated the isotopic exchange of the proton attached to the most acidic carbon of the imidazolium ring into deuterium from D2O, whereas even prolonged exposure to D2O did not induce any isotopic exchange for [BMIM][BF4]. Raman spectra around 600 cm(-1) indicative of the butyl chain conformation also changed differently as the water concentration was increased between [BMIM][I] and [BMIM][BF4]. These differences are considered to come from the variation in the position of the anion, where I- is expected to be closer to the C(2) hydrogen of the imidazolium cation and interacting more specifically as compared to BF(4-).

Molecular Orientational Structures in Ferroelectric, Ferrielectric and Antiferroelectric Smectic Liquid Crystal Phases as Studied by Conoscope Observation

Molecular orientational structures in MHPOBC were studied by means of conoscope observation. Contrary to a ferroelectric response of the conoscope to an electric field in the smectic C* phase, the conoscopic figure in the antiferroelectric smectic CA* phase does not shift its center and is biaxial with its optic plane perpendicular to the field direction. This conoscope change is only due to a dielectric contribution, indicating the existence of the inherent threshold in the electric field induced transition to the ferroelectric phase. In between the ferroelectric and antiferroelectric phases, a kind of ferrielectric phase was clearly distinguished in the field response of the conoscopic figure; the shift of the conoscope center perpendicular to the applied field and the biaxial optic plane parallel to the field suggest a novel molecular orientational structure.

Structural Change of 1-Butyl-3-methylimidazolium Tetrafluoroborate + Water Mixtures Studied by Infrared Vibrational Spectroscopy

Mixtures of ionic liquid (IL, 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4]) and water with varying concentrations were studied by attenuated total reflection infrared absorption and Raman spectroscopy. Changes in the peak intensities and peak positions of CHx (x = 1, 2, 3) vibration modes of the cation of the IL and OH vibration modes of the water molecules were investigated. Peaks from normal-mode stretch vibrations of CH bonds belonging to the imidazolium ring of the cation did not change their positions, while those from the terminal methyl group of the butyl chain blueshifted by approximately 10 cm-1 with the addition of water. On the other hand, change in the spectral shape in the OH stretch vibration region shows hydrogen-bonding network of water molecules breaking down rapidly as the IL is added. Trends in the change of the peak positions and the peak intensities suggested qualitative change of the intermolecular structure in the [BMIM][BF4] + H2O mixture at 32 +/- 2 and 45 +/- 2 mol/L of water concentration.

Examination of band bending at buckminsterfullerene (C60)/metal interfaces by the Kelvin probe method

This study addresses the question of band bending at the C60/metal interface. The change in the energy of the vacuum level upon the deposition of C60 on various metal substrates (Au, Cu, and Ag) was examined by the Kelvin probe method under ultrahigh vacuum as a function of C60 thickness d. We observed (1) an abrupt shift of the energy of the vacuum level relative to the Fermi level of the metal substrate evacF at d⩽1 nm, to a uniform value of about 4.65 eV at all of the interfaces examined and (2) a slower shift with further deposition of C60, which stopped at d∼500 nm at another common value. These abrupt and gradual shifts can be ascribed to the formation of an interfacial dipole layer and to band bending leading to Fermi level alignment, respectively. The value of evacF for the thick region is ascribed to the bulk work function of the specific specimen studied, and it is noted that the values reported for a few monolayers in the literature should not actually be regarded as the bulk work function. The...

Electronic structures of organic molecular materials for organic electroluminescent devices studied by ultraviolet photoemission spectroscopy

Electronic structures of evaporated films of five organic light-emitting and carrier-injecting materials for organic electroluminescent devices were studied by ultraviolet photoemission spectroscopy. The compounds examined were (i) light-emitting materials tris(8-hydroxyquinolino) aluminum (Alq3) and 1,2,3,4,5-pentaphenylcyclopentadiene, (ii) a hole-injecting material N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine, and (iii) electron-injecting materials N,N′-diphenyl-1,4,5,8-naphthyletracarboxyldiimide and 1,3,5-tris(5-phenyl-1,3,4-oxadiazol-2-yl)benzene. The spectral features corresponding to the top parts of the valence states, which dominate the electric properties of the materials, were assigned by the comparison with the simulated density of states obtained from PM3 molecular orbital calculations. Using these calculations, the evolution of the electronic structure of each molecule from those of constituent parts was discussed. The characters of the unoccupied states obtained by t...

Core hole effect in NEXAFS spectroscopy of polycyclic aromatic hydrocarbons: Benzene, chrysene, perylene, and coronene

Carbon K-edge NEXAFS spectra of chrysene, perylene, and coronene were measured using synchrotron radiation, and the results were analyzed by ab initio molecular orbital (MO) calculations. The spectra do not agree well with the calculated density of unoccupied states (DOUS), indicating significant core-hole effect leading to deviation of the NEXAFS spectra from the DOUS. On the other hand, the observed spectra were well simulated by theoretical calculations taking this effect into account by the improved virtual orbital method. This allowed a detailed analysis of the core-hole effect, which affects both transition energy and intensity. During the course of this analysis, the core-hole effect in benzene was also analyzed. It was found that the magnitude of the core-hole effect is strongly dependent on the combination of the excited site and final vacant orbital. This dependence could be semiquantitatively explained in terms of the combination of the excitation site and the orbital patterns of the final stat...