Makoto Yoneya

Tristable nematic liquid-crystal device using micropatterned surface alignment

It has long been appreciated that liquid-crystal (LC) devices in which the LC molecules adopt multiple stable orientations could drastically reduce the power consumption required for high-information-content displays. But for the commonly used nematic LCs, which are intrinsically uniaxial in symmetry, no industrially feasible multi-stable LC device has been realized. Recently we demonstrated how bistability can be robustly engineered into a nematic LC device, by patterning a substrate with an orientational chequerboard pattern that enforces orthogonal LC alignment in neighbouring square domains. As a result of the four-fold symmetry of the pattern, the two diagonal axes of the chequerboard become equally stable macroscopic orientations. Here we extend this symmetry approach to obtain a tristable surface-aligned nematic LC. A microscopic pattern exhibiting six-fold symmetry is inscribed on a polyimide surface using the stylus of an atomic force microscope. The hexagonal symmetry of the microscopic orientational domains in turn gives rise to three stable macroscopic LC orientations, which are mutually switchable by an in-plane electric field. The resulting switching mode is surface driven, and hence should be compatible with demanding flexible display applications.

Surface alignment bistability of nematic liquid crystals by orientationally frustrated surface patterns

We demonstrate a robust in-plane bistability of liquid-crystal surface alignment based on tailored submicrometer-sized surface domains imposing a frustrated alignment. By a nanorubbing technique utilizing the atomic force microscope, we prepared an orientational checkerboard pattern on polyimide layer, consisting of square unit domains on which the alignment is locally constrained to be planar yet orthogonal between the neighboring domains. Due to the four-fold rotational symmetry of the pattern, the two diagonal axes of the square domain become equally stable directions for the macroscopic liquid crystal alignment. The alignment could be switched between these two states by an in-plane electric field above a certain threshold, determined by the local azimuthal anchoring.

In-plane bistable nematic liquid crystal devices based on nanoimprinted surface relief

The authors present a bistable nematic device, using a fourfold symmetrical bidirection nanometer-scale surface grating fabricated by the nanoimprinting lithography. The bistability is achieved by a composite action between two orthogonal surface undulations, which tend to stabilize the nematic director along either of the two diagonal axes. The switching between the bistable states is easily driven by orthogonal in-plane electric fields. A recent model of groove-induced surface anchoring due to Fukuda et al. [Phys. Rev. Lett. 98, 187803 (2007)] accounts for the azimuthal bistability in the present system.

SWITCHING OF NEGATIVE AND POSITIVE DIELECTRO-ANISOTROPIC LIQUID CRYSTALS BY IN-PLANE ELECTRIC FIELDS

This article compares switching behaviors between negative (Nn) and positive (Np) dielectro-anisotropic nematic liquid crystals driven by an in-plane electric field which is generated with interdigital electrodes. Even for Np type liquid crystals, excellent viewing angle characteristics were obtained as expected. Theoretical descriptions of the switching principle, i.e., threshold behavior and response mechanism, could be applied to both the Nn and Np type liquid crystals. However, the orientational deformation of the Np type liquid crystals caused by a distorted electric field which occurred near the edges of electrodes was not the same as that of the Nn type liquid crystals. The switching of the Np type liquid crystals near the edges of the electrodes by this field was followed by the in-plane switching of the liquid crystals between the electrodes. This remarkably distinctive dynamical behavior implied a difference in the response of the longitudinal axes for Nn and Np type liquid crystal molecules. The longitudinal axes of the latter seemed to be sensitive to the electric field component perpendicular to the substrates when applying the in-plane electric field. Furthermore, these experimentally obtained results were supported by computer simulations which analyzed the liquid crystal director distribution and transmittance pattern from edge-to-edge of a pair of electrodes when applying the in-plane electric field.

Nano-rubbing of a liquid crystal alignment layer by an atomic force microscope: a detailed characterization

Locally rubbing the surface of a polymer using the stylus of an atomic force microscope (AFM nano-rubbing) has recently found extensive applications in prototyping novel micro- and nano-structured liquid crystal (LC) electro-optic devices. We report here on the detailed characteristics of the AFM nano-rubbed polyimide in terms of the friction force and the LC alignment capability. A unidirectionally rubbed polyimide by the contact mode AFM showed anisotropic friction, along and against the rubbing direction, even at a rubbing load as small as 1 nN and yielded a finite pretilt angle reflecting the asymmetry. Rubbing at high loads generated conspicuous scratches on the polyimide surface; when annealed at temperatures well below the glass transition or soaked into an organic solvent, however, these scratches quickly relaxed and completely disappeared while maintaining the capability of aligning LCs. When the load was small, the LC alignment also disappeared altogether. These experimental observations suggest that although the top thin layer might be sufficient to initially orient LCs, persistent surface alignment entails deeper cultivation of anisotropic structures.

A NONITERATIVE MATRIX-METHOD FOR CONSTRAINT MOLECULAR-DYNAMICS SIMULATIONS

Abstract A non-iterative version of the matrix method which can be easily vectorised and parallelized is presented. The original matrix method, which is conceptually identical to the familiar SHAKE algorithm, is shown to be non-iterative when incorporated with the Verlet and leap-frog integration schemes with the same constraint error order as the (local) error order of the accompanying integration schemes. The method is checked by test simulations with n-butane and the liquid crystal moleculae 5CB (4-n-pentyl-4′-cyanobiphenyl) in which its effectiveness is demonstrated.

Molecular dynamics simulations of sliding friction of Langmuir–Blodgett monolayers

Molecular dynamics simulations have been used to study friction in Langmuir–Blodgett monolayers of perfluorocarboxylic acid and hydrocarboxylic acid on SiO2. The frictional force of perfluorocarboxylic acid is found to be about three times as large as that of hydrocarboxylic acid. The qualitative aspects of this simulation results are consistent with known experimental results. In order to interpret the difference in the frictional force, a series of simulations have been carried out by changing molecular potential parameters. The simulation results suggest that the 1–4 van der Waals interaction is the main cause of the larger frictional force for perfluorocarboxylic acid than that for hydrocarboxylic acid. The results also show that frictional force is roughly proportional to the excess root mean square fluctuation of the potential energy under shear from the equilibrium. The relation between the frictional force and the energy needed for molecular deformation under shear condition is also discussed.

Coordination-Directed Self-Assembly of M12L24 Nanocage: Effects of Kinetic Trapping on the Assembly Process

We demonstrate the spontaneous formation of spherical complex M12L24, which is composed of 12 palladium ions and 24 bidentate ligands, by molecular dynamics simulations. In contrast to our previous study on the smaller M6L8 cage, we found that the larger M12L24 self-assembly process involves noticeable kinetic trapping at lower nuclearity complexes, e.g., M6L12, M8L16, and M9L18. We also found that the kinetic trapping behaviors sensitively depend on the bend angle of ligands and the metal-ligand binding strength. Our results show that these kinetic effects, that have generally been neglected, are important factor in self-assembly structure determination of larger complexes as M12L24 in this study.

Depolarized light scattering from liquid crystals as a factor for black level light leakage in liquid-crystal displays

Depolarized light scattering due to thermally excited orientational fluctuations of liquid-crystal directors was investigated in the context of the black level light leakage in liquid-crystal displays. Particular attention was given on the comparison between homogeneous and homeotropic liquid-crystal alignments which are both used as black states of liquid-crystal displays. The homeotropic alignment was found to have lower light-scattering intensity and less light leakage than the homogeneous alignment in the small scattering angle regions. The dependency of the scattering intensity on the cell gap and the extrapolation length was also evaluated. The cell gap and the extrapolation length were found to have a large influence.

Surface nematic bistability at nanoimprinted topography

The azimuthal nematic bistability was realized by frustration between two azimuthally orthogonal anchoring axes induced by a nanoimprinted groove pattern and mechanical rubbing. The nematic bistability can be explained by the revised Berreman model of groove-induced surface anchoring, recently introduced by Fukuda et al. [Phys. Rev. Lett. 98, 187803 (2007)]. The azimuthal bistability can be tuned in arbitrary direction by changing the groove pitch and rubbing conditions. This simple combinatorial scheme may be considered as a practical candidate for bistable displays with tailored bistable directions required in various liquid crystal device modes.