Chunho Kim

In situ self-assembled homeotropic alignment layer for fast-switching liquid crystal devices

ABSTRACT We propose a novel method for homeotropic alignment of liquid crystals (LCs) utilising in situ self-assembly of a low concentration of 4-(4-heptylphenyl)benzoic acids that form hydrogen bond with the indium tin oxide (ITO) substrates. Stable homeotropic alignment in the LC device is achieved with a simple mixing process of benzoic acid derivative in LC media, and it yields electro-optical performance similar to that achieved with the conventional alignment method using polyimides. It is experimentally confirmed that an ultrathin self-assembled molecular layer of 4-(4-heptylphenyl)benzoic acid formed by hydrogen bonding on ITO substrate makes it possible to attain a reliable homeotropic alignment of LCs. Furthermore, this simple approach provides a cost-effective and stable LC alignment layer with fast response time and thermal stability. GRAPHICAL ABSTRACT

Vertical alignment of liquid crystal using an in situ self-assembled molecular layer on hydrophilic ITO electrodes

ABSTRACT Vertical alignment of liquid crystals (LCs) was implemented by forming an in situ self-assembled molecular layer on hydrophilic indium tin oxide (ITO) electrodes. Improvement in hydrophilicity of the ITO electrode according to ultraviolet (UV) irradiation time was confirmed. The self-assembled molecular layer offered stable vertical alignment of nematic LCs via hydrogen bonding between 4´-heptylbiphenyl-4-carboxylic acid and the hydroxyl groups on the ITO surface. Further, the electro-optical property of the LC cell irradiated by UV light was investigated to confirm the improved performance of the LC device.

Fabrication of photoluminescent liquid crystal device using an in situ self-assembled molecular layer of a pyrene derivative

An in situ self-assembled molecular layer of 1-pyrenesulfonic acid sodium salt as an alignment agent was formed on indium tin oxide substrates for vertically aligning liquid crystals (LCs). The thus-aligned LCs exhibited uniform vertical alignment under crossed polarisers. The electro-optical characteristic of the LC cell fabricated using this method exhibited better performance than those of conventional LC cells with a polyimide alignment layer. Because the proposed alignment method is a simple one and involves low concentrations of the alignment agent (0.05 wt%), it is highly cost-effective. Further, the pyrene derivative, when mixed with LCs, exhibited photoluminescence (PL) under ultraviolet light. Given that the proposed method resulted in highly vertically aligned LCs and the alignment agent exhibited PL, the method should find wide use in the fabrication of colour-filter-free LC displays.

Fabrication of a new homeotropic alignment layer for nematic liquid crystals using an in situ self-assembly of alkylated benzoic acid derivative

ABSTRACT We have presented a novel vertical alignment method simultaneously enhancing the alignment ability and electro-optical performances of nematic liquid crystals (LCs). The method relies on the self-assembly of 4-(4-heptylphenyl)benzoic acid and 2-carboxyethyl acrylate through hydrogen bonding with conventional polyimide and the generation of pre-tilting in the LC molecules using ultraviolet irradiation under applied voltage. The electro-optical measurements of a LC device fabricated by using this self-assembled polyimide layer exhibited strong homeotropic alignment and fast response time. The proposed alignment method may be valuable in providing new functional alignment materials for high-performance LC devices.

Simple vertical alignment of liquid crystals using photo-polymerization of alkyl acrylate monomer

ABSTRACT A new method for vertical alignment of liquid crystals (LCs) via in situ photo-polymerization of alkyl acrylate monomer on indium tin oxide, without the use of conventional polyimide layer is presented. The UV irradiation of the LC mixture doped with acrylate monomer containing long alkyl group in the confined LC cell provides uniform and stable homeotropic alignment of the LC molecules. The electro-optical characteristic of the fabricated LC cell using this method exhibits similar performance to that of the LC device prepared by using the conventional polyimide alignment layer.

Fabrication of a fast-switching liquid crystal device using reactive self-assembled polyimide alignment layer

ABSTRACT We proposed a new vertical alignment method for simultaneously improving the alignment force and electro-optical properties. The key point of the new method is the self-assembly of the reactive monomer via hydrogen bonding with the polyimide alignment layer and the formation of pre-tilt using the reactive monomer on an alignment layer. Through the self-assembly of the reactive monomer and the generation of the pre-tilt, it is possible to obtain a higher alignment force and a fast response time. As a result, through a simple additional step, we can fabricate a fast-switching liquid crystal device using a reactive self-assembled alignment layer.

Vertical alignment of liquid crystals using novel self-assembled molecular layer of alkylated benzoic acid derivatives

ABSTRACT The one-step vertical alignment of liquid crystals (LCs) was achieved by forming a novel self-assembled molecular layer of alkylated benzoic acid derivatives on an indium tin oxide substrate through hydrogen bonding. This method utilizes the simple doping of the alignment material into the LCs. The assembled molecular layer promoted the stable homeotropic LC alignment. The electro-optical characteristics of LC devices exhibited stable vertical alignments and fast response times, depending on the number of rigid aromatic rings in the alignment material. Our simplified alignment method allows the fabrication of LC devices without a separate alignment step for fast LC switching.

Vertical alignment of liquid crystals using an in situ self-assembled layer of an amphiphilic block copolymer

We have employed an in situ self-assembled layer of an amphiphilic block copolymer to vertically align liquid crystals (LCs). The amphiphilic block copolymer used consisted of hydrophobic polyethylene (PE) and hydrophilic polyethylene glycol (PEG). The degrees of vertical alignment obtained for various PEG contents and polymer concentrations were compared through polarized optical microscopy, electro-optical measurements, and contact angle measurements. The LC cell prepared using a PEG content of 50% and concentration of 0.05 wt% exhibited the most uniform vertical alignment. Moreover, the electro-optical characteristics of this LC cell were similar to those of a conventional LC cell fabricated using a polyimide alignment layer. This was because the anchoring force for a PEG content of 50% was sufficiently high, allowing for uniform alignment. Since the proposed method involves only the simple doping of a low-concentration (0.05 wt%) amphiphilic block copolymer, it is also cost effective.

Fabrication of coloured liquid crystal device using photoluminescent biomolecular chlorophyll

ABSTRACT We fabricated a coloured liquid crystal (LC) device using photoluminescent biomolecular chlorophyll. The chlorophyll molecule, which is inexpensive because of its abundance in nature, was doped in the LCs for manufacturing the coloured LC device. We confirmed the green colour property of the LC doped with chlorophyll dye using UV-vis spectroscopy. Although the LC cell filled with 0.5 wt% chlorophyll showed good vertical alignment and fast response time, the doping of LC medium with 1.5 wt% chlorophyll resulted in imperfect vertical alignment and slowed response time due to the aggregation of chlorophyll molecules at high concentration. Furthermore, the photoluminescence (PL) characteristics of the LC doped with chlorophyll were investigated using PL spectroscopy. The coloured LC cell doped with chlorophyll dye showed emission in the red wave number region under UV light. As the LC doped with chlorophyll exhibited good colour performance, conventional colour filter layers could be avoided with the employment of the proposed coloured LC device. By controlling the light source, it is possible to apply the advanced LC device for colour switching. Moreover, a full-colour-switching LC device can be realised using various biomolecular dyes that can emit other colours. Graphical Abstract

Improvement of adhesive strength and moisture barrier property of UV/heat dual-curable sealant for liquid crystal display by inclusion of inorganic plate-type filler

ABSTRACT We proposed new type of ultraviolet/heat dual-curable sealants for liquid crystal display (LCD) with low water permeability and high adhesive strength. The water permeability and adhesive strength performance of the proposed sealants containing plate-like inorganic fillers such as talc and mica were compared to those of the conventional sealant containing spherical silica filler. The surface of plate-like inorganic fillers was confirmed by field emission scanning electron microscopy and the enhanced sealant performances could be explained by the surface morphology and filler shape. As a result, the proposed sealants offered optimized water permeability and adhesive strength for LCD application.