Liquid Crystal Alignment Layer Research Articles

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.

High electrical characteristics through graphene oxide doping process on physicochemically reformed inorganic thin films.

This study investigated the improvement of the electro-optical properties of a liquid crystal (LC) cell fabricated through brush coating using graphene oxide (GO) doping. The physical deformation of the surface was analyzed using atomic force microscopy. The size of the groove increased as the GO dopant concentration increased, but the direction of the groove along the brush direction was maintained. X-ray photoelectron spectroscopy analysis confirmed that the number of C-C and O-Sn bonds increased as the GO concentration increased. Since the van der Waals force on the surface increases as the number of O-metal bonds increases, we were able to determine why the anchoring energy of the LC alignment layer increased. This was confirmed by residual DC voltage and anchoring energy measurements that were later performed. As the GO concentration increased, the width of the hysteresis curve decreased, indicating that the residual DC voltage decreased. Additionally, the 15% GO-doped sample exhibited a significant increase in its anchoring energy up to 1.34 × 10-3J/m2, which is similar to that of rubbed polyimide. It also secured a high level of electro-optical properties and demonstrated potential as a next-generation thin-film display despite being produced via a simple brush-coating process.

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.

Multifunctional Al-doped ZnO thin films for vertically aligned liquid crystal devices

The integration of ITO-free transparent conductive layers remains a big challenge for development of next generation technologies. Here, we demonstrate the feasibility of transparent and conductive Aluminum-doped Zinc Oxide (AZO) thin films to operate simultaneously as electrode and alignment layer in Liquid Crystal (LC) device configuration. Controlling the growth process of AZO thin films by Atomic Layer Deposition (ALD) technique results of predominantly (100) crystallographic oriented AZO with very high transparency and excellent conductivity. The exceptionally low surface free energy of (100) oriented AZO, along with the topological modification following the mechanical rubbing treatment were used to elucidate the mechanism of LC molecules alignment on the surface of AZO film. The uniform vertical orientation of the nematic LC director was confirmed by polarized optical microscopy and pretilt angle measurements. The competitive electro-optical performance in terms of phase modulation, threshold and saturation voltages and contrast ratio of the assembled LC device reveals the great potential of AZO thin films as transparent electrode and alignment layer for future LC display applications with versatile functionality.

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.

Temperature-switched orientation properties through physicochemically constructed aluminum zirconium oxide films

In general, it has one of the characteristics of homeotropic alignment and homogeneous alignment depending on the material and manufacturing process of the liquid crystal alignment layer. However, in this study, we propose a new type of alignment film that has the characteristics of changing the liquid crystal alignment direction according to the curing temperature. The AlZrO alignment film produced by the solution process and brush coating method showed homeotropic alignment characteristics when cured at low temperatures and homogeneous alignment characteristics when cured at high temperatures. These switching characteristics according to the curing temperature showed stable LC alignment, respectively, and the threshold voltages were 0.81 V and 0.857 V in vertical alignment (VA) mode and twisted nematic (TN) mode, respectively, demonstrating excellent electro-optical properties. The mechanism of these switching characteristics was analyzed through surface modification analysis. This study introduces new insights into liquid crystal alignment films that satisfy all of the high-efficiency fabrication process, various application possibilities due to their switching properties, and excellent electro-optical properties.

Inducement of an Anisotropic One‐dimensional Surface of Solution‐derived Indium Gallium Oxide Film via Imprint Lithography for Liquid Crystal Alignment

AbstractThis study uses imprint lithography to produce an anisotropic unidirectional indium gallium oxide (IGO) film for the uniform liquid crystal (LC) alignment layer. The curing temperature was controlled to 110, 180, and 250 °C during the process, and the 250 °C cured film presented a distinct one‐dimensional pattern transferred from the polydimethylsiloxane (PDMS) mold during scanning electron microscopy (SEM). X‐ray photoelectron spectroscopy (XPS) confirmed an active thermal oxidation effect at a high curing temperature, contributing to the formation of a unidirectional structure. This unidirectional IGO structure functioned as a guide for LCs and induced a uniform alignment state, verified by polarized optical microscopy (POM) and a pre‐tilt angle analysis. The IGO film also revealed improved thermal endurance to the LC alignment state and optical transmittance compared with the conventional rubbed polyimide (PI) layer. Given this performance, the one‐dimensional pattern‐imprinted IGO film can be a promising LC alignment layer for an advanced LC system.

Physicochemically reformed tin oxide film fabricated for self-alignment of liquid crystals

This study compressed the conventional two-step process of brush coating and orientation for liquid crystal (LC) alignment layer manufacturing into a one-step process. Surface modification and LC alignment were performed by brush coating with a tin oxide solution prepared via the solgel process at different curing temperatures. The effect of the curing temperature on atomic bonds was analyzed through x-ray photoelectron spectroscopy, the physical morphology of the film surface was characterized by scanning electron microscopy and atomic force microscopy, and the LC alignment of the brush-coated tin oxide film was verified via polarized optical microscopy and pretilt angle measurements. The shear stress generated during the brush coating process formed a partial groove structure; the resulting anisotropy aligned the LC molecules homogeneously. The brush-coated film showed excellent LC orientation properties, confirming the potential of the proposed and efficient fabrication process as a new orientation technology.

Anisotropic Surface Formation Based on Brush-Coated Nickel-Doped Yttrium Oxide Film for Enhanced Electro-Optical Characteristics in Liquid Crystal Systems

This paper introduces anisotropic nickel yttrium oxide (NYO) film formed by the brush coating technique. X-ray photoelectron spectroscopy confirmed well-formed NYO film after the curing process, and the morphology of the surface was investigated using atomic force microscopy. The shear stress driven from brush hair movements caused the nano/micro-grooved anisotropic surface structure of NYO. This anisotropic surface induced uniform liquid crystal (LC) alignment on the surface, which was confirmed by pre-tilt angle analysis and polarized optical microscopy. The contact angle measurements revealed an increase in hydrophilicity at higher temperature curing, which contributed to homogenous LC alignment. The NYO film achieved good optical transmittance and thermal stability as an LC alignment layer. In addition, the film demonstrated good electro-optical properties, stable switching, and significantly enhanced operating voltage performance in a twisted-nematic LC system. Therefore, we expect that this brush coating method can be applied to various inorganic materials to achieve an advanced LC alignment layer.

Monomolecular vertical alignment layer with room temperature processibility for flexible liquid crystal displays

In this article, we disclose a vertical alignment material using alkylphosphonic acid for liquid crystals that only requires simple room temperature treatments to secure good vertical alignment for liquid crystals. Alkylphosphonic acid forms a monomolecular layer on the substrate providing strong vertical alignment. The proposed alignment layer shows decent electro-optical properties, such as high voltage holding ratio, low residual direct current voltage, low image sticking parameter, etc. Later, we combine the alkylphosphonic acid with photo-rewritable azo-dye to fabricate a hybrid alignment of nematic cells and study its stability and anchoring energy. The material shows great compatibility with flexible substrates. Thus, the proposed alignment layer of alkylphosphonic acid can potentially find application as the alignment layer for flexible liquid crystal displays.

Temperature-controlled liquid crystals alignment on solution-derived bismuth lanthanum tin oxide films prepared through brush process

ABSTRACT In this study, a bismuth – lanthanum–tin oxide film was fabricated via brush coating to obtain a process-efficient liquid crystal (LC) alignment layer. The brush-coating solution was prepared through a sol – gel process, applied onto an indium tin oxide glass, and cured at various temperatures. The shearing stress caused by the brush coating process formed a micro-/nanogroove structure, as confirmed via atomic force microscopy and line profile analyses. The chemical composition of the surface was analysed through X-ray photoelectron spectroscopy, and the optical transmittance of the film was measured. As regards the LC alignment, polarised optical microscopy observations, thermal stability tests, and pretilt angle measurements were performed, confirming a stable alignment; the grooves generated during the shearing and hardening phases were most clearly formed after curing at 180°C, which led to stable LC orientation.

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.

Tunable Liquid Crystal Alignment and Driving Mode on Lanthanum Aluminum Zirconium Zinc‐Oxide Thin Film Achieved by Convenient Brush‐Coating Method

AbstractTunable liquid crystal (LC) alignment with twisted nematic (TN) and vertical alignment (VA) driving modes is achieved by a convenient brush‐coating method. The brush‐coating process combines film deposition and alignment treatment processes, and lanthanum aluminum zirconium zinc oxide (LaAlZrZnO) is used for the LC alignment layer. X‐ray photoelectron spectroscopy results confirm LaAlZrZnO film formation with fine optical transparency. Atomic force microscopy and line profile data indicate nano/micro surface structure with dense or directional morphologies induced by shear stresses according to film curing temperature. Low‐temperature‐cured films exhibit hydrophobic properties with low surface energy, whereas high‐temperature‐cured films show hydrophilicity with high surface energy. The geometric constraints and chemical affinities of the films induce homeotropic and homogeneous LC alignments on the surface according to the film curing temperature, respectively. The LC alignment state is verified by polarized optical microscopy and pretilt angle analysis. The brush‐coated LaAlZrZnO film demonstrates high thermal budget for advanced LC systems. The VA‐mode and TN‐mode LC cells based on the LaAlZrZnO film enable high electro‐optical performances and low power consumption. Therefore, the brush‐coating method is expected to be adoptable for next‐generation LC device applications.

Self aligned liquid crystals on sol-gel-processed zirconium oxide surface via nanoimprinting lithography

In this paper, a nanopattern was effectively formed on a zirconium oxide (ZrO) film, and a uniform liquid crystal (LC) alignment layer was developed through it. One-dimensional (1D) nanopatterns from polydimethylsiloxane (PDMS) molds were transferred to ZrO films via nanoimprinting lithography (NIL). The pattern height of the PDMS mold was 30 nm, and the ZrO film was cured at 100 °C and 200 °C. The transfer of nanopatterns was analyzed by observation of surface modification through atomic force microscopy. The ZrO solution injected between the molds in a sol state by capillary force did not sufficiently solidify at 100 °C, but at 200 °C, sufficient pattern height and period were secured. Using X-ray photoelectron spectroscopy, it was confirmed that ZrO used as an alignment layer was well deposited on the substrate surface. An increase in the contact angle with pattern formation was also confirmed, indicating a decrease in surface energy and hydrophobicity of the surface. A polarized light microscope and pretilt angle were analyzed using a liquid crystal cell manufactured with an antiparallel structure, and the excellent alignment characteristics of the 200 °C cell were confirmed. The vertical alignment mode was confirmed in the voltage-transmittance curve, and a good threshold voltage was observed. Through these analysis methods, the nanopattern was successfully transferred to the ZrO film through NIL, and excellent performance as a liquid crystal alignment layer was confirmed.

Photodesign and fabrication of surface relief gratings on films of polyimide-based supramolecular systems obtained using host-guest strategy

New supramolecular structures were attained by an original approach based on mixing the concepts of polyamidic acid (PAA) host and an azochromophore guest with hydrogen bonding intermolecular interactions. The materials have been obtained and characterized for use as liquid crystal (LC) alignment layers. The azochromophore was introduced in the PAA solution in different ratio followed by casting on glass plate and thermal treatment to realize a film with imide structure, confirmed by Fourier-transform infrared spectroscopy (FTIR). Thermal analyses indicated that the dye loading in the polymer host slightly reduces the temperature of thermal decomposition from 508 to 466 °C, while glass transition temperature ranges between 244 and 213 °C. UV–Vis data proved that the samples exposure at the wavelengths of 355 nm is beneficial for surface relief gratings (SRGs) formation. Atomic force microscopy (AFM) evidenced the surface structuring of the samples and the changes in the spatial and functional contact area characteristics as a function of azo-dye amount. The attained grating periodicity was about 1 μm, while the depth ranged between 35 and 100 nm depending of the system composition. Surface anisotropy, bearing properties and fluid retention are enhanced upon chromophore incorporation. LCs anchoring energy increases with azochromophore percent up to 22.9 ∙ 10 −16 μJ/m 2 , which is desirable for LC display applications. • Supramolecular structures, based on a polymer matrix and an azo-dye, are achieved. • Transparent flexible polyimide films with heat resistance up to 466 °C are attained. • As the azo-dye loading increases LASER-made SRGs depth increases from 37 to 81 nm. • Anisotropy, bearing and fluid retention are enhanced upon azo-dye incorporation. • LC anchoring energy increases with azochromophore percent up to 22.9 ∙ 10 −16 μJ/m 2 .

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.

Photo‐isomerization, photodimerization, and photodegradation polyimides for a liquid crystal alignment layer

AbstractWe review the important features of LC orientation for the alignment layer, namely, production method, alignment mechanisms, and required properties. Photoalignment, which could eliminate the disadvantages of the rubbing method is explored. Applicable materials and techniques for photoalignment are also examined. Many materials for photoalignment layers have been proposed, but polyimide‐based materials are commonly used because of their transparency and insulation properties, which are needed for optical electronic devices. Furthermore, photoisomerization, photodimerization, and photodecomposition as photoalignment methods are reviewed, and specific photoreactive groups and alignment mechanisms are used to induce LC alignment. In summary, polyimides cause photodecomposition and have the following disadvantages; (1) requirement of relatively short‐wavelength ultraviolet (UV) irradiation, (2) difficulty in reorienting polymer chains owing to their rigid polymer matrix, and (3) low‐photo reactivities. The solutions for each disadvantage are already proposed, but the required properties should be determined during material design. Polyimide‐based materials introduced with the azobenzene and cinnamate groups are proposed and found to have the ability of fulfilling the required properties. The mechanism of polyimide photodecomposition is not fully understood. It likely has the advantage in terms of LC alignment stability because of the absence of E/Z isomerization. Therefore, the alignment mechanism and availabilities should be further explored.

Vertical Alignment of Liquid Crystal on Sustainable 2,4-Di-tert-butylphenoxymethyl-Substituted Polystyrene Films.

We synthesized sustainable 2,4-di-tert-butylphenoxymethyl-substituted polystyrenes (PDtBP#, # = 88, 68, 35, and 19, where # is molar percent contents of 2,4-di-tert-butylphenoxymethyl moiety), using post-polymerization modification reactions in order to study their liquid crystal (LC) alignment behaviors. In general, LC cells fabricated using polymer film with higher molar content of 2,4-di-tert-butylphenoxymethyl side groups showed vertical LC alignment behavior. LC alignment behavior in LC cell was related to the surface energy of the polymer alignment layer. For example, when the total surface energy value of the polymer layer was smaller than about 29.4 mJ/m2, vertical alignment behaviors were observed, generated by the nonpolar 2,4-di-tert-butylphenoxymethyl moiety with long and bulky carbon groups. Orientation stability was observed at 200 °C in the LC cells fabricated using PDtBP88 as the LC alignment layer. Therefore, as a natural compound modified polymer, PDtBP# can be used as a candidate LC alignment layer for environmentally friendly 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.

Physicochemically modified high-resolution tin oxide thin film using soft imprinting.

We carried out nanoimprinting lithography on solution-processed tin oxide (SnO) film for use as a liquid crystal (LC) alignment layer, for which we used a parallel configuration. To transfer the nanostructures onto the SnO film, we conducted an experiment according to curing, from which fine nanostructures on the SnO film were obtained at a curing temperature of 200 °C. These acted as a guide for the arrangement of the LC molecules and induced geometric restriction which minimized elastic distortion energies, and so the LC molecules could be aligned in the direction of the nanostructures. The LC alignment state was investigated using polarized optical microscopy, and the pre-tilt angle was measured using a crystal rotation method. With high thermal endurance and drastically low power consumption, the nano patterned SnO was shown to be a promising candidate for LC applications. The nanopatterning process combined with nanoimprinting lithography and solution-processed inorganic materials exhibited the possibility of broadening the features of nanostructure-mounted applications, including LC devices.