Uniform Liquid Crystal Alignment Research Articles

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.

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.

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.

Self-Aligned Liquid Crystals on Anisotropic Nano/Microstructured Lanthanum Yttrium Strontium Oxide Layer.

We propose an efficient alignment method for liquid crystals (LCs). A brush-coating method handles film deposition and LC alignment treatment simultaneously herein, meaning a reduction in the conventional alignment layer treatment process steps. A lanthanum yttrium strontium oxide (LaYSrO) film prepared by the sol–gel process was used for the alignment layer. Topographical details of the brush-coated LaYSrO films (compared with spin-coated films) were investigated by atomic force microscopy. Spin-coated LaYSrO meant that the film formation alone without orientation treatment represented an isotropic surface. On the other hand, the 270 °C-cured brush-coated LaYSrO showed nano/microstructure with directionality. It indicates that brush-hair sweeping induced shearing stress on the sol state of the LaYSrO, which results in surface anisotropy for LC alignment. The uniform LC alignment state was confirmed by polarized optical microscopy and pretilt analysis. The brush-coated LaYSrO shows fine optical transparency compared to plain and indium-tin-oxide coated glasses, and thermal stability up to 150 °C for LC alignment. Competitive electro-optical performances of the brush-coated LaYSrO were verified in a twisted-nematic LC system compared to those of the conventionally used polyimide layer. Consequently, we expect that the brush-coating process can be an innovative technology for LC alignment.

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.

Surface reformed anisotropic polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene‐graft‐maleic anhydride layer via ion‐beam irradiation for liquid crystals

AbstractThe surface modifications and liquid crystal (LC) alignment properties of polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene‐graft‐maleic anhydride (SEBS‐g‐MA) films induced by ion‐beam (IB) irradiation are reported herein. Changes to the surfaces of the SEBS‐g‐MA films from IB irradiation at various incidence angles and changes in the LC orientations upon LC cell fabrication were analyzed. The physical surface modifications due to IB irradiation were observed via atomic force microscopy. Additionally, changes in the molecular composition of the surface were observed through X‐ray photoelectron spectroscopy analysis. Polarized optical microscopy analysis shows that excellent LC alignment properties could be obtained after IB irradiation at angles exceeding 45°. Further, the anisotropy caused by IB irradiation was more responsible for uniform LC alignment than the corrugated pattern.

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.

Liquid crystals alignment and switching between surface reinforced poly(ethylene-co-vinyl acetate) thin layers

The alignment characteristics of liquid crystals (LCs) on poly(ethylene-co-vinyl acetate) (PEVA) films via ion-beam (IB) irradiation at incidence angles of 15°, 45°, or 75° were investigated. Uniform and homogeneous LC alignment were achieved with a PEVA film IB-irradiated at an incidence angle of 45°. Physicochemical modification progressed by IB irradiation were examined by using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The AFM results show that the plasma argon ions penetrated and thereby increased the surface roughness and decreased kurtosis of the PEVA. The XPS results suggest that chemical reconstruction of the PEVA film induced the van der Waals forces and dipole-dipole interactions with the LCs. These transformations resulted in uniform LC alignment at 45° IB incident angle, as confirmed by polarized optical microscopy and pre-tilt angle measurements. The 45° incidence-angle IB-irradiated PEVA film also showed high transmittance in the visible light region. A twisted nematic LC cell based-on the PEVA film showed successful electro-optical performance as stable switching behavior and fine operation characteristics. These results indicate that IB irradiation is an effective method for uniform LC alignment and the 45° incidence-angle IB-irradiated PEVA film can be adopted for LC alignment in future LC display applications.

Tunable alignment of liquid crystals between anisotropic polyacrylamide thin layer

We report uniform liquid crystal (LC) alignment on a polyacrylamide (PAM) film via ion beam (IB) treatment. The IB incidence angle is adjusted from 15° to 75°. Physicochemical modifications caused by the IB process are investigated by atomic force microscopy and x-ray photoelectron spectroscopy. Low IB incidence results in biased and bumpy surfaces, whereas high IB incidence forms isotropic and smooth surfaces; both of these are unsuitable for uniform LC alignment. The 45° IB incidence induces anisotropic surface chemical reformations, and these modifications induce van der Waals forces between the LCs and modified PAM, thereby leading to uniform LC alignment. The LC alignment state is verified by polarized optical microscopy and pretilt angle. The electro-optical characteristic of the modified PAM showed excellent switching performance in twisted-nematic LC system. Thus, the IB-treated PAM film is a good candidate for LC alignment layers and suitable for LC device applications.

A self-assembled polyimide layer based on graphene quantum dots for ultrafast switching of liquid crystals

Graphene quantum dots (GQDs) with a large specific surface area, wide emission spectrum, and low toxicity are advantageous for biomedical and optoelectronic applications. Here, we present a facile method to simultaneously provide ultrafast electro-optical switching and strong vertical anchoring of liquid crystals (LCs) using interfacial self-assembly between alkylated GQDs and polyimide (PI) layers in a closed LC device. The newly fabricated self-assembled PI layer with alkylated GQDs were simply fabricated by doping GQDs into the LC medium, and induced a stable and uniform homeotropic alignment of LCs. Moreover, this self-assembled layer provided a superfast switching speed for the device.

A solution-derived bismuth aluminum gallium tin oxide film constructed by a brush coating method for spontaneous liquid crystal alignment

The directional structure of a solution-derived bismuth aluminum gallium tin oxide film is achieved by a brush coating method. This structure is used for spontaneous and uniform liquid crystal alignment.

Physicochemically modified anisotropic polyacrylamide thin film via ion‐beam treatment for liquid crystal system

AbstractWe investigated ion‐beam (IB)‐treated polyacrylamide (PAM) film as a liquid crystal (LC) alignment layer. Polarized optical microscopy and pretilt angle analysis show the uniform LC alignment status. Atomic force microscopy shows the IB etching effect, and X‐ray photoelectron spectroscopy indicates the formation of carbon–oxygen bonds on the surface, which ensured the uniform alignment of LC molecules via van der Waals forces interaction. The reformed PAM films also had good thermal budgets and optical transparency for the LC system. The IB‐treated PAM films demonstrated stable electro‐optical performances in a twisted nematic LC system. Hence, IB treatment can be an effective method to apply several polymer materials to the LC alignment layer, and IB‐irradiated PAM films have remarkable potential for use in the LC system.

Orientation-induced properties of anisotropic polyacrylamide thin layer via plasma treatment in liquid crystal system

Uniform liquid crystal (LC) alignment was achieved on a polyacrylamide (PAM) film treated by a plasma ion beam (IB), and the alignment status of the LCs was confirmed by polarized optical microscopy and pretilt angle analysis using antiparallel LC cells assembled using the IB-treated PAM (IB-PAM) layer. Physically modified characteristics on the PAM surface caused by the IB treatment were observed by atomic force microscopy. Etching with the IB caused roughness reduction and smoothing on the surface with increased irradiation time. Chemical modifications on the surface were investigated by X-ray photoelectron spectroscopy, whose results demonstrated stoichiometric changes causing formation of CO bonding and polarity on the surface. The IB incident at 40° drives the surface polarity anisotropy and van der Waals interactions with the LC molecules, thereby achieving uniform LC alignment. Thermal endurance tests of the IB-PAM layer showed maintenance of the uniform LC alignment status up to 120 °C. Twisted-nematic cells based on the IB-PAM layer showed suitable electro-optical performance with low operating characteristics. The IB-PAM films also showed high and stable optical transparencies. Thus, IB treatment on polymer layer is deemed suitable for the LC alignment process, and IB-PAM film is an excellent choice for next-generation LC systems.

Nanopattern transfer on bismuth gallium oxide surface via sol-gel stamp process applied for uniform liquid crystal alignment

A convenient aligning method using nanoimprint lithography is introduced. Sol-gel-processed bismuth gallium oxide (BGO) thin films that are one-dimensionally nanopatterned by polydimethylsiloxane molds are used for the liquid crystal (LC) alignment layer. The one-dimensional nanopatterns are transferred to the films at a curing temperature of 250 °C. Atomic force microscopy data present clear line grooves on the surfaces, which allow the LCs to align parallel to this pattern in one direction. Polarized optical microscopy and pre-tilt angle data support uniformity of the LC alignment, indicating that the surface nanopatterns act as alignment guides for LC molecules. X-ray photoelectron spectroscopy data shows active thermal oxidation at a curing temperature of 250 °C, which contributes to nanopattern transfer on BGO film. Thermal endurance tests of the BGO films show good thermal stability until 180 °C. The twisted nematic cells in the BGO layer also show superior electro-optical performances compared with the conventional rubbed polyimide layer. From these characteristics of the nanopatterned BGO alignment layer and owing to its simple fabrication, we expect that this new alignment technique can be used for producing next-generation LC devices to replace the conventional rubbing process.

Uniformly aligned liquid crystal molecules on reformed poly(ethylene-co-vinyl acetate) layers driven by ion beam exposure

ABSTRACT The characteristics of a poly(ethylene-co-vinyl acetate) (PEVA) film formed by spin coating process and ion beam (IB) irradiation was investigated with irradiation times. A physicochemical analysis was used to analyse the suitability of the PEVA film as a uniform liquid crystal (LC) alignment layer. Atomic force microscopy (AFM) revealed an increased surface roughness after IB irradiation. X-ray photoelectron spectroscopy (XPS) analysis showed IB irradiation broke the polymer branches and increased the oxygen concentration and bonding on the surface. These derived the strong Van der Waals forces and dipole-dipole interactions between the PEVA surface and LCs. The IB irradiation generated surface anisotropy on the PEVA film and unidirectional LC alignment was achieved. Polarised optical microscopy (POM) and pre-tilt angle measurements were carried out to show the uniform LC alignment. The transmittance curve exhibited a high optical performance. These results show the promising capabilities of IB irradiated PEVA film as an alignment layer in LC displays.

Hierarchical self-constructed biomolecular nanolayers comprising cholesterol and cholesteryl hemisuccinate for automatic alignment of liquid crystals

Hierarchical supramolecular nanostructures are architecturally robust and functionally tunable, and therefore are widely utilized as functional building blocks for optoelectronic and biological applications. Herein, we report a simple and effective fabrication strategy to construct hierarchical biomolecular nanolayers comprising cholesterol and cholesteryl hemisuccinate for automatic alignment of liquid crystals (LCs) by using the programmed interfacial self-assembly of biomolecular amphiphiles on the polar electrode surface. The spontaneous formation of ultrathin biomolecular nanolayer was successfully achieved by simple doping and in situ self-organization of a small amount of steroid-based amphiphiles in the confined LC cell. The surface analysis demonstrated that the amphiphilic biomolecules of cholesterol and cholesteryl hemisuccinate formed hydrophobic self-constructed nanolayers with a dense surface coverage on the surface of indium tin oxide (ITO) electrode via hydrogen bonding. These nanolayers, bearing hydrophobic steroid groups and alkyl chains on their surface, induced uniform vertical alignment of LCs via van der Waals forces. Notably, the biomolecular nanolayer containing cholesteryl hemisuccinate exhibited more stably oriented LCs than cholesterol-based nanolayer at the same concentration owing to the strong interfacial hydrogen bonding with polar ITO electrode surface. The electro-optical properties of the LC cells containing the steroid-based amphiphiles revealed that hierarchical self-constructed nanolayers based on the amphiphilic biomolecules afforded low threshold voltage and fast response time comparable to those of conventional polyimide alignment layer.

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.