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.     

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.     

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.     

Fullerene-containing liquid crystal spatiotemporal light modulators with surface-electromagnetic-wave-treated conducting layers

We have studied the switching characteristics of multilayer electrooptical structures comprising quartz substrates, transparent conducting layers, and an oriented nematic liquid crystal (NLC) film doped with photosensitive charge-transfer complexes based on electrooptically active organic monomer or polymer molecules and fullerenes. The transparent conducting layers treated with surface electromagnetic waves are used for the orientation of NLC molecules. The surface relief with variable permittivity, which is created by this treatment, not only ensures the alignment of NLC molecules, but also significantly influences the relaxation times of the electrooptical mesophase, thus making possible effective control over the switching times of the electrooptical light modulator within 1–1.5 ms.     

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.     

Nematic liquid crystal alignment on the ion beam-exposed ZnO film

This paper investigates the nematic liquid crystal (NLC) alignment on ion beam-exposed zinc oxide (ZnO) films. The ZnO films are deposited by a radio frequency magnetron sputtering. During the deposition of ZnO film, we supplied sufficient oxygen gas for high resistivity and transmittance. The deposited films show a high transmittance of over 90% and high resistivity of over 10¹⁰ Ω cm. The ZnO films show a high deposition rate of 26.7 Å/min. Images obtained via scanning electron microscopy of the ZnO film surfaces, before and after the ion beam exposure, show that groove patterns are formed being to be parallel to the ion beam exposure direction. LC cells are fabricated with the ion beam-exposed ZnO films. The NLC molecules align parallel to the ion beam exposure direction. The electro-optic and response characteristics of fabricated cells show the possibility of application to liquid crystal displays.     

Prospects of the use of fullerenes for the orientation of liquid-crystalline compositions

The dynamic characteristics of liquid crystal (LC) structures containing fullerenes C₆₀ and C₇₀ were studied, and the effect of fullerenes on the reorientation capacity of liquid crystal molecules was assessed. It is established that fullerenes C₆₀ and C₇₀ influence the operation time of polymer-dispersed LC cells containing photosensitive 2-cyclooctylamine-5-nitropyridine molecules. A possible mechanism of reorientation in the system is considered, and it is shown that fullerenes can provide for effective switching of the electrooptical response in such LC cells. Fullerenes have good prospects for use in orienting coatings of various types employed in the LC technology.     

On the possibility of a homeotropic alignment in nematic liquid crystal elements using carbon nanotubes

The possibility of obtaining homeotropic orientation in thin-film nematic liquid crystal (NLC) cells using carbon nanotubes is briefly considered. The results of this investigation can be used to develop optical elements for displays with vertical orientations of NLC molecules (MVA-display technology).     

水显影光固化材料的制备及其各组分对感光性能的影响

利用热塑性酚醛环氧树脂的环氧基，依次用丙烯酸及叔胺盐进行开环反应，合成了水溶性丙烯酸酯类感光高分子，将其配制成水显影光固化材料，并研究了叔胺盐的种类，不同光引发剂和光谱效剂以及稀释性单体对其感光性能的影响。     

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.     

Surface alignment of liquid crystal multilayers evaporated on photoaligned polyimide film observed by surface profiler

The investigation of the surface alignment of liquid crystal (LC) multilayers evaporated on photoaligned polyimide vertical alignment (PI-VA) film was carried out by means of the novel three-dimensional (3D) surface profiler. We report the first use of the surface profiler to visualize a microscopic image of the monolayer arrangement of LC molecules in contact with the surface of photo-treated PI-VA film. The photoinduced anisotropy of partially UV-exposed PI-VA film can be visualized as a topological image of LC multilayers. It seems that the topology of LC multilayers is indicating the orientational distribution of LC molecules on the treated film. It was found that the periodically photoaligned PI-VA film surface can align an adsorbed LC monolayer and the LC molecular alignment can be extended to the bulk via the epitaxylike LC–LC interaction, i.e. a short-range molecular interaction. With regard to the unexposed PI-VA film surface, noticeable anisotropy in the monolayer alignment was not observed, indicating that the long-range elastic interaction may be responsible for the bulk alignment. The appearance of small droplets in the masked region may be presumably related to the dewetting phenomena.     

Asymmetric Fullerene Nanosurfactant: Interface Engineering for Automatic Molecular Alignments

Since the molecular self‐assembly of nanomaterials is sensitive to their surface properties, the molecular packing structure on the surface is essential to build the desired chemical and physical properties of nanomaterials. Here, a new nanosurfactant is proposed for the automatic construction of macroscopic surface alignment layer for liquid crystal (LC) molecules. An asymmetric nanosurfactant (C₆₀NS) consisted of mesogenic cyanobiphenyl moieties with flexible alkyl chains and a [60]fullerene nanoatom is newly designed and precisely synthesized. The C₆₀NS directly introduced in the anisotropic LC medium is self‐assembled into the monolayered protrusions on the surface because of its amphiphilic nature originated by asymmetrically programmed structural motif of LC‐favoring moieties and LC‐repelling groups. The monolayered protrusions constructed by the phase‐separation and self‐assembly of asymmetric C₆₀NS nanosurfactant in the anisotropic LC media amplify and transfer the molecular orientational order from surface to bulk, and finally create the automatic vertical molecular alignment on the macroscopic length scale. The asymmetric C₆₀NS nanosurfactant and its self‐assembly described herein can offer the direct guideline of interface engineering for the automatic molecular alignments.     

Surface alignment of liquid crystal multilayers evaporated on photoaligned polyimide film observed by surface profiler

The investigation of the surface alignment of liquid crystal (LC) multilayers evaporated on photoaligned polyimide vertical alignment (PI-VA) film was carried out by means of the novel three-dimensional (3D) surface profiler. We report the first use of the surface profiler to visualize a microscopic image of the monolayer arrangement of LC molecules in contact with the surface of photo-treated PI-VA film. The photoinduced anisotropy of partially UV-exposed PI-VA film can be visualized as a topological image of LC multilayers. It seems that the topology of LC multilayers is indicating the orientational distribution of LC molecules on the treated film. It was found that the periodically photoaligned PI-VA film surface can align an adsorbed LC monolayer and the LC molecular alignment can be extended to the bulk via the epitaxylike LC–LC interaction, i.e. a short-range molecular interaction. With regard to the unexposed PI-VA film surface, noticeable anisotropy in the monolayer alignment was not observed, indicating that the long-range elastic interaction may be responsible for the bulk alignment. The appearance of small droplets in the masked region may be presumably related to the dewetting phenomena.     

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.     

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.     

Variation of the orientational order parameter in a nematic liquid crystal-COANP-C<Subscript>70</Subscript> composite structure

Behavior of the orientational order parameter in structures of the nematic liquid crystal (NLC)-organic donor-fullerene type was studied by pulsed ¹H NMR techniques. In the NLC sensitized by a fullerenecontaining charge transfer complex, the order parameter at temperatures 10 K below the nematic-isotropic phase transition exhibits an increase and exceeds the value for the initial liquid crystal. This effect is not typical of nonmesogenic additives.     

Blue phase liquid crystal: strategies for phase stabilization and device development

Abstract

The blue phase liquid crystal (BPLC) is a highly ordered liquid crystal (LC) phase found very close to the LC–isotropic transition. The BPLC has demonstrated potential in next-generation display and photonic technology due to its exceptional properties such as sub-millisecond response time and wide viewing angle. However, BPLC is stable in a very small temperature range (0.5–1 °C) and its driving voltage is very high (～100 V). To overcome these challenges recent research has focused on solutions which incorporate polymers or nanoparticles into the blue phase to widen the temperature range from around few °C to potentially more than 60 °C. In order to reduce the driving voltage, strategies have been attempted by modifying the device structure by introducing protrusion or corrugated electrodes and vertical field switching mechanism has been proposed. In this paper the effectiveness of the proposed solution will be discussed, in order to assess the potential of BPLC in display technology and beyond.     

Specific features of attenuated light transmission by liquid-crystal twist cells in constant and alternating electric fields

Optical transmission characteristics of dual-frequency nematic liquid crystal (NLC) twist cells with different alignment layers (rubbed polyimide and obliquely deposited cerium dioxide) have been studied in constant and alternating electric fields. It has been established that a change in the optical (twist effect) threshold and dynamic range of attenuated transmission depend both on the boundary conditions (that influence the screening of applied voltage) and on the parameters of the applied electric field. The maximum dynamic range (49.5 dB) has been obtained in the cell with a CeO₂ alignment layer controlled by a constant potential. In the case of an alternating electric field, the dynamic range decreases because of reduced effective voltage.     

Double sine-Gordon solitons in nematic liquid crystals under applied electric and magnetic fields

In this paper, we derive the dynamic equation of molecular motion for a twisted nematic liquid crystal (NLC) under applied electric and magnetic fields, and show that it takes the form of a double sine-Gordon (DSG) equation. Two kink and anti-kink solitary solutions of the liquid crystal molecules are obtained by using the F-expansion method to solve the DSG equation. Finally, we confirm that the twist of the NLC molecules can propagate in the form of solitary waves. The propagation velocity and amplitude of the kink and anti-kink solitons induced by the electric and magnetic fields are discussed.     

Multidimensional calculation of ray path in a twisted nematic liquid crystal cell

In general, light propagating an inhomogeneous liquid crystal (LC) cell can be modeled as ‘bundle rays’ because the LC cell consists of many birefringence layers. In order to calculate the optical path of the propagating light in the inhomogeneous LC cell, we multidimensionally calculated the wavevector, k, and the Poynting vector, S, of an ordinary and an extraordinary ray at LC grid interfaces, which are isotropic to a uniaxial medium and a uniaxial-to-uniaxial medium, by using the phase matching method. Furthermore, we also investigated the transmission coefficients and transmittance of the ordinary and the extraordinary rays as a function of difference of the optical axes of the facing birefringence medium at the interface to obtain the significant rays in the LC cell. Finally, we could calculate the exact path of the significant rays in the inhomogeneous LC cell, and compared the ray path in an electrically controlled birefringence (ECB) mode and a twisted nematic (TN) LC mode.