Selectivity of Threefold Symmetry in Epitaxial Alignment of Liquid Crystal Molecules on Macroscale Single‐Crystal Graphene

Epitaxial alignment of organic liquid crystal (LC) molecules on single‐crystal graphene (SCG), an effective epitaxial molecular assembly template, can be used in alignment‐layer‐free liquid crystal displays. However, selectivity among the threefold symmetric easy axes of LCs on graphene is not well understood, which limits its application. Here, sixfold symmetric radial LC domains are demonstrated by dropping an LC droplet on clean SCG, which reveals that the graphene surface does not have an intrinsic preferential direction. Instead, the first contact geometry of the LC molecules determines the direction. Despite its strong anchoring energy on graphene, the LC alignment direction is readily erasable and rewritable, contrary to previous understanding. In addition, the quality of the threefold symmetric alignment is sensitive to alien residue and graphene imperfections, which can be used to detect infinitesimal impurities or structural defects on the graphene. Based on this unique epitaxial behavior of LCs on SCG, an alignment‐layer‐free electro‐optical LC device and LC alignment duplication, which can result in practical graphene‐based flexible LC devices, are realized.     

Improvement in device performance from a mixture of a liquid crystal and photosensitive acrylic prepolymer with the photoinduced vertical alignment method

In a multicomponent nematic liquid crystal (NLC) mixture of a liquid crystal (negative-type NLC) and a photosensitive acrylic prepolymer, photopolymerization upon UV irradiation induces the separation of the LC and photosensitive acrylic prepolymer layers, thereby leading to a vertical arrangement of LC molecules. In this study, we propose a simple vertical alignment method for LC molecules, by adding a chiral smectic A (SmA∗) liquid crystal having homeotropic texture characteristics to an NLC mixture solution. Measurements of electro-optical properties revealed that the addition of the SmA∗ LC not only strengthened the anchoring force of the copolymer alignment film surface, but also significantly enhanced the contrast ratio (∼73%), response time and grayscale switching performance of the device.     

Improvement in device performance from a mixture of a liquid crystal and photosensitive acrylic prepolymer with the photoinduced vertical alignment method

Abstract

In a multicomponent nematic liquid crystal (NLC) mixture of a liquid crystal (negative-type NLC) and a photosensitive acrylic prepolymer, photopolymerization upon UV irradiation induces the separation of the LC and photosensitive acrylic prepolymer layers, thereby leading to a vertical arrangement of LC molecules. In this study, we propose a simple vertical alignment method for LC molecules, by adding a chiral smectic A (SmA?) liquid crystal having homeotropic texture characteristics to an NLC mixture solution. Measurements of electro-optical properties revealed that the addition of the SmA? LC not only strengthened the anchoring force of the copolymer alignment film surface, but also significantly enhanced the contrast ratio (～73%), response time and grayscale switching performance of the device.     

Effect of boundary conditions on phase modulation of light in a nematic liquid crystal featuring the S-effect

Phase modulation of light has been studied under the S-effect conditions in a nematic liquid crystal (NLC) with a planar director alignment set by a layer of amorphous hydrogenated carbon (a-C:H). It is demonstrated for the first time that the formation of an anisotropic surface relief on the electrode surface favors a more homogeneous alignment of liquid crystal molecules by the a-C:H layer and allows the maximum phase shift to be obtained at a certain thickness of the NLC layer.     

Interplay of surface effects in a thin nematic liquid crystal layer

Manifestations of the competition of surface effects in a thin nematic liquid crystal (LC) layer are considered. It is shown that the LC director field lines remain straight irrespective of the preferred anchoring angles at the LC boundary surfaces. The angle between the director field lines and the plane of the LC layer depends on these preferred anchoring angles and on the Rapini-Papoular energy ratio at the boundary surfaces. There are two possible director orientations differing by 90°. Switching between these states can be induced by chemical factors or by external fields. In contrast to the well-known Fréedericksz transition, the work required for such switching is partly performed by the competing surfaces. Hence, the LC layer switching only requires overcoming the difference between the Rapini-Papoular energies, which can be rendered small by properly selecting LC-substrate pairs.     

Effect of the shape of imprinted alignment layer on the molecular orientation of liquid crystal

The molecular alignment of liquid crystals was studied with three different imprinted patterns. The three different shapes of molds (square-, V- and U-shaped) replicated the microstructures with electroplating process was fabricated. Surfaces with three different microgrooved patterns were used as the alignment layer of the liquid crystal cell. The effect of the shape of imprinted alignment layer on the molecular orientation of LC was investigated. Among these patterns, U-shaped groove pattern resulted in a high performance of liquid crystal orientation, pretilt angle and anchoring energy, which are quite comparable to the conventionally rubbed layer. It clearly shows that the molecular orientation of liquid crystals is influenced by the shape of microgrooved patterns.     

Weak anchoring of nematic liquid crystals on photo-induced surface relief gratings of organic polysilane

Surface relief gratings on organic polysilane thin films are fabricated by holographic exposure of ultra-violet light, and Au gratings are subsequently prepared on polysilane gratings by vapor deposition of Au. The anchoring energies of 4-pentyl-4′-cyanobiphenyl (5CB) nematic liquid crystal on the fabricated gratings are determined with a saturation voltage method. The anchoring energies of Au gratings are weaker than those of organic polysilane gratings because of suppression of π–π interaction between the liquid crystal and the alignment layer. The polar anchoring energies of Au gratings are also weaker than those reported in literature.     

Unwinding of a cholesteric liquid crystal and bidirectional surface anchoring

We examine the influence of bidirectional anchoring on the unwinding of a planar cholesteric liquid crystal induced by the application of a magnetic field. We consider a liquid crystal layer confined between two plates with the helical axis perpendicular to the substrates. We fix the director twist on one boundary and allow for bidirectional anchoring on the other by introducing a high-order surface potential. By minimizing the total free energy for the system, we investigate the untwisting of the cholesteric helix as the liquid crystal attempts to align with the magnetic field. The transitions between metastable states occur as a series of pitchjumps as the helix expels quarter- or half-turn twists, depending on the relative sizes of the strength of the surface potential and the bidirectional anchoring. We show that secondary easy axis directions can play a significant role in the unwinding of the cholesteric in its transition towards a nematic, especially when the surface anchoring strength is large.     

Anomalous light transmission by nematic liquid crystal cells

Light transmission through nematic liquid crystal (NLC) cells with a planar alignment and various boundary conditions has been studied as a function of the bias voltage in the regimes of maximum transmittance and complete extinction. The phenomenon of anomalous light leakage in the regime of complete extinction has been observed in the NLC sells with alignment layers of amorphous hydrogenated carbon (a-C:H) and poly(vinyl alcohol) (PVA), in which an anisotropy was induced by rubbing the electrode surface (for a-C:H) and the alignment layer (for PVA). The anomalous transmission is caused by the deformation of nematic molecules, which are situated near the surface, in the direction perpendicular to the direction of rubbing. The magnitude of anomalous transmission increases with the bias voltage. This behavior is explained by the anisotropy of the surface anchoring energy of nematic molecules at the interface.     

Screening effect of a-C:H alignment layer in nematic liquid crystal cell

The electrical and optical characteristics of nematic liquid crystal (NLC) cells and their dependence on the thickness of an amorphous hydrogenated carbon (a-C:H) alignment layer have been studied. An increase in the a-C:H layer thickness favors enhancement of the screening of a bias voltage applied to this layer, which is manifested by an increase in the threshold voltage of the electrooptical splay effect. This is accompanied by a decrease in the initial (pretilt) director angle, which is evidence for an increase in the anchoring energy due to the field of space charge localized at the interface.     

Influence of the alignment layer and the liquid crystal layer thickness on the characteristics of electrically controlled optical modulators

The screening effect of the amorphous hydrogenated carbon (a-C:H) alignment layer and its dependence on the thickness of a dual-frequency nematic liquid crystal (NLC) layer have been studied. Optimization of the a-C:H layer thickness allows a threshold voltage for the optical S-effect to be reduced and the characteristic switching time and relaxation time of 0.5 and 2.5 ms, respectively, to be obtained for a phase retardation of 2π at a wavelength of 0.86 μm.     

Effect of the dielectric layer with photo-controllable surface relief on liquid crystal devices

We present experimental results for the dielectric layer effect on the electro-optical properties of liquid crystal (LC) devices together with numerical simulations. The photo-controllable polymer is used as a dielectric material on which the alignment agent of the LC is prepared. The surface relief structure can be induced on the dielectric polymer layer by photo-irradiation for fabricating LC microlens arrays or wide-viewing LC displays. It is found that the operation voltage of the LC device decreases with increasing the dielectric constant or with decreasing the layer thickness of the polymer. The experimental data agree well with theoretical results predicted from a simple dielectric model in the continuum theory.     

Magnetic-field-assisted formation of alignment polymer coatings in liquid crystal cells

We describe a new method for obtaining liquid crystal (LC) layers with planar orientation in plane-parallel cells, which is based on the technology of LC-polymer interface formation in solution under the action of an applied magnetic field. The azimuthal anchoring energy of LC at the polymer surface has been determined by measuring the angle of orientation of the nematic LC director on the substrate surface as a function of the magnetic field. The LC orientation provided by the proposed method is stable, and the anisotropy of LC anchoring is comparable with that achieved using well-known methods of alignment polymer film preparation by rubbing.     

Unified surface anchoring strength from the splay elastic deformation in nematic slabs

Film thickness dependences of the unified surface anchoring strengths from the splay deformation for nematic cells were discussed. Thin nematic parallel (d = 2-50 μm) and wedge (d = 0.3-5 μm) cells with the same solid surface were prepared to evaluate the surface anchoring strength. The surface anchoring strengths from the splay deformations for two kinds of liquid crystals were measured by using the saturation method. The nematogens used were 4-pentyl-4′-cyanobiphenyl and mixture compound ZLI-4792 (Merck Japan Co., Ltd.) with positive dielectric anisotropy. Film thickness dependence of the surface anchoring strength was elucidated on the basis of the conventional ionic surface polarization. It was also discussed that a surface anchoring strength from the splay deformation is to be an intrinsic value defined at an interfacial region and not a value changing with the film thickness.     

Optical detection of the ligand–receptor binding by anchoring transitions of liquid crystals

An optical detection system of the specific binding events between ligands and bioreceptors was developed using the anchoring transitions of a liquid crystal (LC). The specific binding of ligand–receptor pairs on a modified surface disturbs the orientational order of the LC and induces the anchoring transition from the homeotropic to the homogeneous LC alignment. Since the initial homeotropic alignment is preserved in the absence of the binding process, the binding events are easily monitored by a simple optical system through the LC texture change. This concept of the optical detection would be useful for designing a variety of biosensors.     

Polyimide photo-alignment layers for inclined homeotropic alignment of liquid crystal molecules

We have succeeded in realizing an inclined homeotropic alignment of liquid crystal (LC) molecules by using photo-aligned films of a polyimide containing azobenzene in the backbone structure. To induce such an LC alignment, a side chain structure was introduced into the backbone structure. The LC pretilt angle, measured from the surface normal, could be controlled up to 1.75° by varying the light exposure in oblique angle irradiation with unpolarized light. Its thermal stability was examined by annealing the LC cell at 100 °C. No change was observed in the pretilt angle even after annealing for 36 h, indicative of its excellent thermal stability. Since photo-alignment has patterning capability, the photo-aligned polyimide film is expected as a promising alignment film for multi-domain vertical alignment mode LC displays.     

Investigation of the liquid crystal alignment layer: effect on electrical properties

Abstract

We investigate the electrical behavior of a symmetric liquid crystal (LC) cell: elecrode-silane-LC-silane-electrode. The silane (chlorodimethyloctadecyl-silane) layer induces a homeotropic orientation of the nematic liquid crystal (NLC) molecules. The wettability technique is used to detect the change of the surface energy of the electrode upon cleaning and silane layer deposition. We report on the dynamic impedance measurements of the nematic liquid crystal cell. It is found that the silane alignment layer has a blocking effect on the liquid crystal (LC) cell. We also study the relaxation behavior of the cell which is later assimilated as an electrical equivalent circuit.     

液晶垂直取向膜材料的研究

叙述了近几年来液晶垂直取向材料领域的发展。对目前常见的几种具有应用前景的垂直取向技术分别给予了关注,同时初步探讨了其各自的取向机理,并展望了该技术在未来几年可能获得的发展。     

Mixed alkylsilane functionalized surfaces for simultaneous wetting and homeotropic anchoring of liquid crystals

Surfaces functionalized with a self-assembled monolayer (SAM) formed from a mixture of two alkylsilanes with different chain lengths have been designed to simultaneously improve the liquid crystal (LC) wettability and promote homeotropic anchoring of the LC. Most chemically functionalized surfaces (e.g., long alkyl chain SAMs) that promote homeotropic alignment of LC possess low surface energy and result in poor LC wettability, inhibiting LC infiltration into microstructured surfaces and sometimes resulting in LC dewetting from the surface. However, a surface modified with a mixed SAM of octadecyltriethoxysilane (C18) and ethyltriethoxysilane (C2) exhibited very low LC contact angle while providing homeotropic anchoring. Ellipsometry was used to correlate the bulk concentration of C18 in the deposition solution to the surface coverage of C18 in the mixed monolayer; these bulk and surface concentrations were found to be equal within experimental uncertainty. The LC contact angle was found to depend nonmonotically with the surface coverage density, with a minimum (14.4 ± 0.1°) at a C18 surface coverage of 0.26 ± 0.08. Homeotropic LC anchoring was achieved at a C18 surface coverage of ≥0.11 ± 0.04, in the regime where a minimum in the LC contact angle was observed. The practical application of this approach to surface modification was demonstrated using a micropillar array sensor substrate. When the array was functionalized with a conventional C18 SAM, the LC did not infiltrate the array and exhibited a contact angle of 47.4 ± 0.5°. However, the LC material successfully infiltrated and wetted the same microstructured substrate when functionalized with a C18/C2 mixed SAM, while still exhibiting the desired homeotropic anchoring.     

Investigation of the liquid crystal alignment layer: effect on electrical properties

We investigate the electrical behavior of a symmetric liquid crystal (LC) cell: elecrode-silane-LC-silane-electrode. The silane (chlorodimethyloctadecyl-silane) layer induces a homeotropic orientation of the nematic liquid crystal (NLC) molecules. The wettability technique is used to detect the change of the surface energy of the electrode upon cleaning and silane layer deposition. We report on the dynamic impedance measurements of the nematic liquid crystal cell. It is found that the silane alignment layer has a blocking effect on the liquid crystal (LC) cell. We also study the relaxation behavior of the cell which is later assimilated as an electrical equivalent circuit.