Homogeneous Liquid Crystal Alignment Research Articles

Nanogroove-structured Indium Bismuth Oxide Film for Homogeneous Liquid Crystal Alignment Achieved by Brush Coating

Nanogroove-structured Indium Bismuth Oxide Film for Homogeneous Liquid Crystal Alignment Achieved by Brush Coating

Nano/microgroove zirconium‐doped lanthanum oxide film with self‐aligned molecules for liquid crystal device application

Herein, we propose a brush‐coated zirconium‐doped lanthanum oxide (ZrLaO) film as a liquid crystal (LC) alignment layer. The film‐curing temperature was adjusted to 70, 150, and 230°C. Polarized optical microscopy and pre‐tilt angle analysis confirmed that there was uniform and homogeneous LC alignment on the 230°C cured ZrLaO film. Atomic force microscopy revealed that the surface had a nano/microgroove structure, which was caused by the shear stress generated by the brush‐coating process. This structure induced the uniform LC alignment. X‐ray photoelectron spectroscopy verified that the ZrLaO film was well‐formed on a glass substrate. The ZrLaO film displayed hydrophilic characteristics, and its surface energy increased as the film‐curing temperature increased. The ZrLaO alignment layer displayed suitable optical transmittance for LC device applications. The ZrLaO layer‐based twisted‐nematic LC cell exhibited more stable switching properties and better threshold voltage characteristics than conventional polyimide layers. Therefore, we expect that this brush‐coated ZrLaO layer will be a suitable LC alignment layer for LC device applications.

A physicochemically reformed nanostructured graphene oxide-doped indium magnesium oxide film with enhanced electrical properties

Brush-coated graphene oxide (GO)-doped indium magnesium oxide (InMgO) films were produced in this study. The GO concentration was adjusted to 0, 5, and 15 wt%. The hybrid films were constructed using a one-step facile brush-coating process. The formed hybrid film had an oriented anisotropic morphology with a micro/nanogroove structure due to the shear stress induced by the brush hair movement. This anisotropic structure was used as a liquid crystal (LC) alignment layer. Polarized optical microscopy and pretilt angle analysis revealed the presence of a uniform and homogeneous LC alignment on the hybrid film. The film exhibited excellent light control with the LCs, as well as stable and high optical transmittance in the visible region. The GO doping of InMgO increased its polar anchoring energy and decreased its hysteresis characteristics for LC electronic device applications. These results demonstrate the potential of doping metal oxide films with GO and brush coating them for use in next-generation LC devices.

Spontaneously homogeneous alignment of liquid crystals on self-assembly organic rubrene

A novel spontaneously homogeneous liquid crystal (LC) alignment with self-assembled organic rubrene layer on the substrate surface has been reported. The spontaneous alignment technique without surface pre-treatment results in low cost, short processing time, and the room-temperature processing extends application to plastic device. The rubrene-nematic LC mixture is used to fabricate the LC cell. During the capillary filling, the LC molecules align in the flow direction. Meanwhile, the flow-induced LC alignment enforces the tetracene of the rubrene dopants along the flow direction because of the van der Waals and π–π interactions between the rubrene and LC molecules. Afterwards, the rubrene dopants spontaneously migrate onto substrate surface and self-assemble as the triclinic crystalline rubrene (TCR) with ribbon morphology. The LC alignment on TCR has a low pretilt angle, thereby providing a homogeneous LC alignment spontaneously. LC alignment on TCR shows a higher thermal stability compared to conventional polyimide due to the strong interactions of the rubrene and LC molecules. Moreover, the rubrene molecules remain in the bulk suppress the free-ions, decrease the rotational viscosity of LCs and hence shortening the response time of cell. The suppressed free-ions also decrease the screening effect on the substrate surfaces and hence the operation voltage of cell. The TCR structure on the substrate surface and the residual rubrene molecules dispersed in the bulk simultaneously contribute to the enhancement in electro-optical performance of the LC cell.

Bismuth-magnesium-oxide-based graphene oxide hybrid film for liquid crystal device application

Hybrid films containing bismuth magnesium oxide (BiMgO) and graphene oxide (GO) are investigated in this study. Solution-processed brush coating is used to fabricate the hybrid films, with the graphene oxide weight ratios adjusted to 0, 5, and 15%. The hybrid films show more stable and higher optical transmittances than the pure BiMgO film. A directional micro/nanogroove structure is observed on the hybrid film surface, which is derived from the shear stress during the brush-coating process; this anisotropic structure is used as a uniform liquid crystal (LC) alignment layer. Uniform and homogeneous LC alignment is then demonstrated via polarized optical microscopy and pretilt angle analyses, along with perfect light-control performance. Enhanced LC polar anchoring energy and hysteresis-free characteristics are observed for the brush-coated hybrid film as the GO-doping concentration is increased. Based on this perspective, the BiMgO-based GO hybrid film achieved via brush coating has potential for applications in next-generation LC systems.

Graphene-oxide-doped oriented nickel oxide film achieved through brush coating for liquid crystal system

Hybrid films containing nickel oxide and graphene oxide are introduced in this work. The hybrid films are fabricated through a one-step brush-coating process, and the weight ratios of graphene oxide are controlled to 0, 5, and 15 %. The fabricated hybrid films show oriented nano/microgroove structures on their surfaces by directional shear stress, which originates from the brush hair movements during the solution-processed brush-coating process. The hybrid film formation is confirmed using X-ray photoelectron spectroscopy. The films also demonstrate high optical transmittance with amorphous structures. The hybrid films are then used to form liquid crystal (LC) alignment layers, and uniform as well as homogeneous LC alignment is confirmed via polarized optical microscopy and pretilt angle analyses. Compared to the pure nickel-oxide film, the hybrid film with graphene oxide enhances LC alignment stability by increasing the polar anchoring energy of the surface LC molecules. From these observations, the hybrid film containing metal oxide and graphene oxide is expected to be a promising alternative for functional LC systems.

Alignment‐inducement property of physicochemically reconstructed EVA layer via plasma ion‐beam treatment for liquid crystal system

Uniform and homogeneous liquid crystal (LC) alignment on a poly(ethylene‐co‐vinyl acetate) (EVA) layer subject to plasma ion‐beam (IB) treatment is confirmed. The layer molecular concentrations are controlled to 2, 4, and 6 wt%. Scanning electron microscopy exhibits a wrinkle pattern on the EVA surface that hinders LC alignment after IB treatment. However, X‐ray photoelectron spectroscopy shows chemical reconstruction on the surface; this induces strong van der Waals interactions with the LCs, which contribute to uniform LC alignment. The layer with higher molecular concentration is more heavily modified by IB treatment than that with low concentration, thereby achieving uniform LC alignment and overcoming topological disturbance. Contact angle analysis is used to verify that the surface energy increases via IB treatment; the uniform and homogeneous LC alignment is also confirmed by polarized optical microscopy and pretilt angle analysis. The EVA layer shows good optical transmittance and fine electro‐optical performances in terms of the switching and operating properties. Therefore, the plasma IB treatment is an effective method of deriving the surface characteristics of various polymeric materials for LC alignment, and the IB‐treated EVA layer has potential for application to LC systems.

Physicochemically Constructed Unidirectional Aluminum Bismuth Gallium Zinc Oxide Film for Enhanced Nematic Liquid Crystal System Using a Brush‐Coating Method

We propose a convenient brush coating method for liquid crystal (LC) alignment. This method enables film deposition and an alignment layer treatment process simultaneously. Aluminum bismuth gallium zinc oxide (AlBiGaZnO) is used as the alignment layer. After the curing process, a unidirectional AlBiGaZnO film is formed; its surface morphology and chemical composition were verified using scanning electron microscopy and X-ray photoelectron spectroscopy. This oriented structure of the surface was produced by shear-stress which originated from brush movement. That structure induced a surface anisotropic characteristic and resulted in a uniform LC alignment. The uniform and homogeneous LC alignment state on the film was confirmed using polarized optical microscopy and pre-tilt angle analysis. The brush coated AlBiGaZnO film exhibited excellent thermal budget for advanced LC system. The film exhibited enhanced electro-optical performance with a low operating voltage. These results demonstrate the potential of LC alignment technology via the brush coating method.

Achievement of Unidirectional Aluminum Tin Oxide/UV-Curable Polymer Hybrid Film via UV Nanoimprinting Lithography for Uniform Liquid Crystal Alignment

A uniform unidirectional nanostructure composed of aluminum tin oxide and ultraviolet (UV)-curable polymer is introduced herein. The nanostructure was produced by UV-nanoimprint lithography (UV-NIL), and the fabricated hybrid film was used as a uniform liquid crystal (LC) alignment layer. Atomic force microscopy and line profile analysis were performed to confirm a well-ordered nanostructure with 760 nm periodicity and 30 nm height. X-ray photoelectron spectroscopy analysis was also conducted to examine the chemical modifications to the hybrid film surface during UV exposure. Optical transmittance investigation of the nanopatterned hybrid film revealed its compatibility for LC device application. Stable, uniform, and homogeneous LC alignment on the hybrid film was confirmed by polarized optical microscopy observance and analysis of LC pretilt angle. The unidirectional structure on the film surface enabled uniform LC orientation along with surface anisotropy property. Hence, we expect that the proposed UV-NIL process can be applied to fabricate high-resolution unidirectional nanostructures with various inorganic/organic hybrid materials and that these nanostructures have high potential for next-generation LC systems.

Well‐ordered nanostructured organic/inorganic hybrid thin film construction via UV nanoimprint lithography applicable to liquid crystal systems

AbstractIn this study, a well‐ordered unidirectional nanostructure was successfully imprinted on a UV curable polymer/indium oxide hybrid film using UV nanoimprint lithography (NIL). The surface morphologies of the hybrid films were analyzed by atomic force microscopy (AFM), and the uniform nanostructure with a period of 820 nm and height of 30 nm was observed on a 6 min UV‐irradiated film by line profile from AFM. The chemical modification of the films according to UV irradiation time was analyzed by X‐ray photoelectron spectroscopy. The nanopatterned hybrid film was applied to the liquid crystal (LC) alignment layer and the polarized optical microscopy and pretilt angle analysis demonstrated the highly uniform and homogeneous LC alignment state. The unidirectional nanostructure guided the LCs orientation and the aligned surface anisotropy which induces the uniform LC alignment. The hybrid film chemical affinity was analyzed by contact angle measurement, and the increased surface energy after UV irradiation contributed to the homogeneous LC alignment. Therefore, various organic/inorganic hybrid films can be used for nanostructure construction via UV NIL, and it has a high potential for application in diverse LC systems.

Surface modification of a poly(ethylene-co-vinyl acetate) layer by ion beam irradiation for the uniform alignment of liquid crystals

The post-processing of a poly(ethylene-co-vinyl acetate) (PEVA) film by ion-beam (IB) irradiation is described herein for use as a liquid crystal (LC) alignment layer. Various film curing temperatures are investigated in order to achieve a stable film prior to IB irradiation, and a uniform and homogeneous LC alignment with superior optical transparency is obtained at a curing temperature of 130 °C. The chemical and morphological effects of IB irradiation are further examined by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Although the surface morphology exhibits a random pattern, the IB irradiation is found to modify the chemical characteristics of the PEVA surface by simultaneously inducing van der Waals forces and strong dipole-dipole interactions with the LCs, thus leading to unidirectional LC alignment. In addition, LC cells are fabricated from the IB-irradiated PEVA films, and the LC alignment state is confirmed by polarized optical microscopy and pre-tilt angle measurements. Further, the high transparency of the deposited PEVA film compared to that of ITO-coated glass is demonstrated via transmittance measurement. In view of this superior performance, the IB-irradiated PEVA film is a promising alternative for advanced LC displays.

High-quality nano structures fabrication on organic/inorganic hybrid thin films by using UV nanoimprint lithography

In this study, UV nanoimprint lithography (NIL) was conducted with a hybrid mixture of tin oxide and a UV-curable polymer to fabricate 1-D nanostructures on a thin film. It was confirmed through chemical and physical surface analyses that a hybrid thin film with high-resolution nanostructures had been successfully fabricated. In particular, nanostructures with an amplitude of 49.354 nm and a wavelength of 0.768 μm, which are the most similar to the mold used, were achieved at 3 min UV exposure time. Afterward, they were applied to a liquid crystal (LC) device as an alignment layer, with which uniform and homogeneous LC alignment were achieved due to the anisotropic characteristics and geometric restrictions caused by fabricated nano structures. In addition, a high thermal budget was obtained due to the thermal endurance of the hybrid thin films with an LC alignment performance of a UV exposure time of 3 min. Furthermore, the electro-optical performance under this condition was superior to LC devices with conventional rubbed polyimide layers From the results, it was determined that using the UV-NIL process on a hybrid mixture of tin oxide and a UV-curable polymer created high-resolution 1-D nano structures on films that can be used this as LC alignment layers with strong potential for high-quality LC applications.

Using Dihydrazides as Thermal Latent Curing Agents in Epoxy-Based Sealing Materials for Liquid Crystal Displays.

In this study, highly adhesive epoxy-based sealing materials for liquid crystal (LC) displays were fabricated using different types of dihydrazides as thermal latent curing agents. Their curing characteristics, mechanical properties, LC contamination levels, and electro-optical characteristics were investigated depending on the chemical structure of dihydrazides. The epoxy-based sealing material containing a dihydrazide derivative with a bulky heterocyclic ring afforded a high heat curing conversion of 90.4%, high adhesion strength of 54.3 kgf cm−2, and a high elongation of 57.3% due to the relatively low melting characteristic under heat treatment compared to those involving dihydrazides with short aliphatic or aromatic spacers. In addition, the proposed sealing material exhibited an extremely low LC contamination level of 9 µm, which is essential to the successful operation of LC displays. With respect to electro-optical properties of the LC device, it was found that a dihydrazide derivative with a bulky heterocyclic ring afforded a normal voltage-dependent transmittance curve and fast response time due to the prevention of abnormal homogeneous LC alignment. This study developed highly adhesive and robust epoxy-based sealing materials based on the use of dihydrazides as thermal latent curing agents for advanced LC displays.

Manipulation of liquid crystal alignment through interfacial modification of photoresponsive polyimides remotely actuated by dual wavelength linearly polarized light

High-quality alignment of liquid crystals (LCs) for large area display has been highly demanding for display market. To achieve a uniform and stable LC alignment, a photoresponsive azo-polyimide with two different substitution ratio of azo-dye moiety in the side chain of the polyimide has been successfully synthesized and characterized. Depending on the ratio of azo-dye substitutions, the solubility and coating nature of polyimides vary greatly and initially show random planar or homeotropic alignment of LCs. In situ photoalignment process has been performed to achieve a homogeneous planar alignment of LCs from the initial random planar state and a uniform pretilt state from the initial homeotropic cells using linearly polarized visible light (LPVL) irradiation in the isotropic phase of LCs. Furthermore, obtained alignment states have been stabilized using unpolarized UV light for RM polymerization in the nematic phase of LCs. Hence, the dual wavelength photoalignment approach has been successfully implemented to obtain the different stable alignments of LCs. Also, the results conclude that the tilt angle varies by dose of LPVL irradiation and gets significantly altered due to change in reactive mesogen (RM) concentrations. The surface investigations reveal a contrasting nature for the planar and vertically aligned substrates before and after RM stabilization.

A new approach for directional homogeneous alignment of liquid crystals using electric field-guided orientation of nonionic amphiphiles

Conventional homogeneous alignment of liquid crystals (LCs) utilizes rubbed or photo-aligned polyimide surfaces that determine the molecular orientation of LCs along in-plane direction. However, this alignment method based on polymeric layer requires complicated layer forming processes and considerable synthetic effort for special functional polyimides. In this study, a simple and unprecedented directional alignment approach has been developed to horizontally align nematic LCs using electric field-guided orientation of nonionic amphiphiles, spontaneously forming nanoscale supramolecular thin layer on the indium tin oxide substrate via hydrogen bonding. The in-plane electric field between simple slit electrodes induced the horizontal orientation of the anchored amphiphilic molecules parallel with electric field. Furthermore, the subsequent immobilization of inclined amphiphiles along a directional electric field through photopolymerization of common mesogenic diacrylates within LC medium resulted in a new type of homogenous alignment layer with a well-defined orientation direction for LCs. The proposed facile alignment methodology using small quantities of nonionic amphiphiles and mesogenic diacrylates afforded uniform and stable planar alignment of LC molecules with the controlled director. This novel alignment approach is expected to find a wide range of potential applications involving LC displays, optically patterned retarders, biosensors, and electronic lenses.

Homogeneous Liquid Crystal Alignment on Anisotropic YSnO Surface by Imprinting Method

Homogeneous Liquid Crystal Alignment on Anisotropic YSnO Surface by Imprinting Method

Bulk-mediated in-situ homogeneous photoalignment induced by reactive mesogen containing diphenylacetylene moiety

ABSTRACTWe designed and synthesised a reactive mesogen containing diphenylacetylene moiety in the mesogenic core and two polymerisable acrylate groups at both ends. By irradiating linearly polarised UV light on the conventional host LC mixture containing a small amount of the synthesised reactive mesogen in a sandwiched cell without an alignment layer, we demonstrated an in-situ photo-induced homogeneous alignment of liquid crystals without a pre-treated alignment layer, which was achieved by an irreversible polarisation-selective [2 + 2] photodimerization of diphenylacetylene moiety with linearly polarised UV irradiation at above the isotropic temperature of LC mixture. The resulting homogeneous alignment showed a superior initial dark state, negligible pretilt angle and excellent stabilities. Furthermore, the in-plane switching (IPS) LC cell prepared by this method exhibited a better dark state and electro-optic performance compared to that with conventional-rubbed polyimide alignment layer. The single photoirradiation process automatically resulted in a perfect alignment matching of optical axes between the top and bottom substrates in the LC cell, giving rise to an excellent dark state overcoming an intrinsic misalignment issue and complex fabrication process. The proposed in-situ alignment method is a promising candidate for cost-effective, green-manufacturing, and high-quality alignment technique in the manufacturing of high-resolution liquid crystal displays.

A Study on the Parallel Patterning of Nickel Oxide thin Film for Liquid Crystal Display

I investigated the homogeneous liquid crystal alignment using parallel patterning on nickel oxide thin film. Nickel oxide thin film was prepared by sol-gel process, which is and cost effective method to form oxide thin film. Since the sol-gel process is solution driven method, the patterning on nickel oxide can be achieved by imprinting lithography. Nickel oxide with parallel pattern was used for liquid crystal alignment layer, which can be a alternative to conventional rubbing process to aligning a liquid crystal molecules. As a result, a high transmittance of 83.9% was confirmed for the NiO film, and a stable horizontal orientation pretilt angle of 0.2° occurred in the imprinted NiO film. These results were judged to be a positive level that can be commercialized in the horizontal alignment type liquid crystal display that is currently used universally. Anisotropic characteristics of nickel oxide induced by a parallel pattern leads to the alignment of liquid crystals.

Electrically Tunable-Focusing Liquid Crystal Microlens Array with Simple Electrode

An electrically tunable-focusing liquid crystal (LC) microlens array exhibiting a wide-range tunable focal length is proposed. The lower substrate has strip indium tin oxide (ITO) electrodes, the upper substrate has periodic ITO electrodes with a certain gap coated on the inner surface., and an LC microlens is generated between the two strip electrodes. For each LC microlens, the gap between the top planar electrodes is directly above the center of the microlens. Unlike the conventional LC lens, the individual LC microlens is not coated with ITO electrodes on the central part of its upper and lower substrates, which helps to maintain the LC’s horizontal orientation. In the voltage-off state, the focal length of the microlens array is infinity because of the homogeneous LC alignment. At a given operating voltage, an ideal gradient refractive index distribution is induced over the homogeneous LC layer, which leads to the focusing effect. The simulation result shows that the focal length of the LC microlens could be gradually drawn to 0.381 mm with a change of voltage.

Surface modified solution‐derived nickel oxide film via ion‐beam irradiation as a liquid crystal alignment layer

AbstractIn this article, we demonstrate the liquid crystal (LC) alignment characteristics of solution‐derived nickel oxide (NiO) film modified with ion‐beam (IB) irradiation. Cross‐polarized optical microscopy and pretilt angle measurements verified that uniform LC alignment was achieved using the NiO film as an alignment layer regardless of IB incidence angle. Contact angle measurements revealed that all of the NiO films had a deionized water contact angle below 90°, which indicates that they had hydrophilic surfaces that had an effect on the homogeneous LC alignment. Atomic force microscopy was conducted to determine the physical surface modification due to the IB irradiation, which showed that it reduced the size of the surface grains with agglomerations depending on the surface tilt from the IB incidence angle. Furthermore, microgroove structures strongly related to uniform LC alignment were observed after IB irradiation. Chemical surface modification was investigated via an X‐ray photoelectron spectroscopy analysis which revealed that IB irradiation modified the chemical bonds in the NiO film, and this affected the LC alignment state. Thus, these results indicate that using NiO film exposed to IB irradiation as an alignment layer is a suitable method for LC applications.