Masahito Oh-e

Orientations of phenyl sidegroups and liquid crystal molecules on a rubbed polystyrene surface

Surface–specific sum–frequency vibrational spectroscopy and second-harmonic generation have been used to study the structure of a rubbed polystyrene (PS) surface and the orientation of 4′-n-pentyl-4-cyanobiphenyl (5CB) liquid crystal molecules on it. The results show that the phenyl sidegroups are well aligned by rubbing in the direction perpendicular to rubbing but tilt from the surface normal with a broad distribution. Although the PS surface is nonpolar, the 5CB molecules appear to adsorb on PS preferentially with the terminal cyano group facing the PS surface.

Sum-frequency generation (SFG) vibrational spectroscopy of side alkyl chain structures of polyimide surfaces

Sum-frequency generation (SFG) vibrational spectroscopy was used to study how side alkyl chains of a polyimide are oriented at the air–polymer interface and how they are affected by mechanical rubbing and adsorption of a liquid crystal monolayer. The spectra of polyimides with various alkyl chain lengths all indicate that the chains protrude out of the polyimide surface with a very broad distribution. Longer alkyl chains appear to contain more gauche defects. While the polyimide backbones are aligned by rubbing, the side chain orientation is hardly affected. Adsorption of a liquid crystal monolayer on the surface significantly reduces the gauche defects in the alkyl chains.

Dependence of viewing angle characteristics on pretilt angle in the in-plane switching mode

Viewing angle characteristics were systematically obtained when using in-plane switching (IPS) of liquid crystals. Although the IPS mode originally shows stable electro-optical performance regardless of viewing directions, the viewing angle characteristics are found to be strongly dependent on pretilt angle (slant angle of the liquid crystals from the substrate). Experimentally, the smaller the pretilt angle of the liquid crystal, the much wider the viewing angle characteristics, while larger pretilt angles of liquid crystals cause the characteristics to deteriorate. This deterioration occurs in a particular viewing direction, i.e. at right angles to the initial orientation direction of the liquid crystal when there is no in-plane electric field. The experimentally observed behaviour of the viewing angle dependence on the pretilt angle was also confirmed by computer simulations. Calculated iso-contrast contour lines, as a function of the pretilt angles, nearly coincide with the experimentally obtained vie...

High-frequency dielectric relaxation of liquid crystals: THz time-domain spectroscopy of liquid crystal colloids

Terahertz time-domain spectroscopy has been used to study the dielectric relaxation of pure 4’-n-pentyl-4-cyanobiphenyl (5CB) liquid crystal (LC) and its mixtures with 10 µm SiO2 particles in the frequency range 0.2–2 THz. For the pure sample, we find that spatial inhomogeneities consisting of oriented domains, comparable in size to our probe area (~1 mm2), cause a large scatter in the measured dielectric function, due to varying contributions from the ordinary and extraordinary components. In the LC/particle mixtures, ordering of the LC at the surface of the SiO2 particles results in a break-up of these domains, giving rise to a spatially much more homogeneous dielectric response. The inferred dielectric function can be interpreted using effective medium theory and the Debye relaxation model. We observe this stabilizing effect for interparticle distances < ~30 µm, setting a lower limit for the size of oriented domains in the bulk LC.

A functionally separated nanoimprinting material tailored for homeotropic liquid crystal alignment.

In order to homeotropically align liquid crystals (LCs) at the nanosized surface grooves processed by nanoimprint lithography technology (NIL), we propose to design a hybrid-type homeotropic polymer material consisting of two distinct moieties with largely different thermo-mechanical properties and surface activity. Surface contact angle measurements and sum-frequency vibrational spectroscopy allow us to conclude that the polymer film is a functionally separated composite suitable for the homeotropic LC alignment processed by NIL. As one of the potential applications using the hybrid-type homeotropic polymer, we demonstrate that the nanoimprinted grooves at the polymer surface can achieve a zenithal nematic LC bistability.

Imaging colloidal particle induced topological defects in a nematic liquid crystal using third harmonic generation microscopy

The nature of the third-harmonic generation (THG) process in a nematic liquid crystal is investigated for the case of tightly focused, low intensity, laser beams. Colloidal particle induced topological defects in a liquid crystal are visualized in three-dimensions using the dependence of the THG signal on both changes in non-linear susceptibility and the orientation of the liquid crystal director relative to the incident laser polarization state. We have found that the interpretation of THG images in a liquid crystal is complicated not only by the change in polarisation of the electric field as it propagates through the medium but also by anisotropic refractive index mismatch induced aberrations.

Surface anisotropy from photo-induced bond breaking at polymer surfaces: A sum-frequency vibrational spectroscopic study of polyimide

Sum-frequency vibrational spectroscopy was used to study how linearly polarized UV irradiation could cause bond breaking and alter the surface structure of a polyimide. The spectroscopic results allowed the determination of an approximate orientational distribution of the polymer backbones at a rubbed surface. They also permitted deduction of rates of bond breaking of the backbones at the surface by UV dosage. The surface anisotropy resulting from bond breaking by linearly polarized UV irradiation was found to be relatively small, suggesting a weak azimuthal anchoring energy for liquid crystal films deposited on such a surface.

Advanced nanoimprint lithography using a graded functional imprinting material tailored for liquid crystal alignment

Nanoimprint lithography technology, which is able to easily create nanometer-resolution two-dimensional surface grooves on substrates over a large area by a step and stamp process, opens up an intriguing opportunity for functional liquid crystal (LC) alignment. We describe here a hybrid polymer consisting of two distinct moieties with largely different thermomechanical properties and surface activity. We present the results of nanoimprint lithography experiment using the hybrid polymer to demonstrate the practical applicability as an LC alignment layer. This material shows excellent capability both as a nanoimprinting material, requiring softness at moderate temperatures, and as a LC alignment layer, requiring sufficient rigidity. LC devices using the alignment layer show fairly stable electro-optic characteristics even after thermal aging, due to its high thermal stability. The soft component of the hybrid polymer helps it provide the multiple imprinting capabilities at higher temperatures virtually with...

Anisotropy in amorphous films of cross-shaped molecules with an accompanying effect on carrier mobility: Ellipsometric and sum-frequency vibrational spectroscopic studies

Variable angle spectroscopic ellipsometry and sum-frequency vibrational spectroscopy have been used to study molecular orientations in thin films used in an organic light-emitting-diode. The films consist of sterically bulky and cross-shaped molecules that have small anisotropy in shape, 2-methyl-9,10-di(2-naphthyl)anthracene (MADN). As a result, anisotropic molecular orientation in the amorphous films has been observed with respect to the surface normal. The short axis of anthracene in MADN molecules, more or less, slightly tilts from the surface plane but preferentially close to the surface with a certain orientational distribution, while the long axis of anthracene is, on average, oriented close to the magic angle from the surface normal. This anisotropic molecular orientation gives rise to better carrier transportation properties than the isotropic orientation.

Advantageous voltage-holding ratio characteristics induced by in-plane electric fields, and the optimization concept of liquid crystals for an in-plane switching electro-optical effect

In-plane switching (IPS) of liquid crystals showed advantageous voltage-holding ratio (VHR) characteristics so that liquid crystals with low resistivity could provide higher VHRs compared with the twisted nematic effect. This experimental result was obtained when electric fields were applied approximately parallel to the substrate plane using the IPS electro-optical effect. We found that the in-plane electric field generates supplementary capacities which support retention of an externally applied voltage over the liquid crystal layer during non-selected periods of the active matrix driving scheme, because the liquid crystal layer can be connected with an insulating layer, an orientation layer and even a substrate in parallel. Based on these advantageous VHR characteristics, liquid crystal materials suitable for the IPS effect were appropriately optimized. We propose evaluation parameters, derived from the physical switching principles of the liquid crystals, to obtain lower driving voltage and faster res...