Showing papers by "Seung Hee Lee published in 2012"

Devices and materials for high-performance mobile liquid crystal displays

Abstract: Mobile liquid crystal displays (LCDs), often playing a role in hand-held computers, have become indispensable electronic tools to human beings in modern society. Innovative technological developments in the devices and materials have paved a successful road toward the development of mobile LCDs. Herein, after reviewing the trends and performance requirements of mobile LCDs, organic–inorganic materials as the key components of high performance mobile LCDs are addressed. Additionally, the developing trends in mobile LCDs are discussed with respect to these materials.

Achieving high light efficiency and fast response time in fringe field switching mode using a liquid crystal with negative dielectric anisotropy

Abstract: The fringe-field switching (FFS) mode has been widely applied in high-resolution portable liquid crystal displays because of its excellent performances in transmittance, viewing angle, and operating voltage. Up to now, the FFS mode utilised a liquid crystal with positive dielectric anisotropy owing to fast response time and low operating voltage. This paper proposes a merit of using a liquid crystal with negative dielectric anisotropy. The optimised cell structures with thin cell gap and fine patterned electrode structures make the FFS mode with negative liquid crystals exhibit higher transmittance than that of the positive liquid crystals and a fast response time.

Advanced bistable cholesteric light shutter with dual frequency nematic liquid crystal

Abstract: In this paper, a direct switching between a transparent (or reflecting) planar (P) state to an opaque (or transparent) focal conic (FC) state and vice-versa of a polymer free bistable cholesteric light shutter without any homogeneous polyimide (PI) layer, is demonstrated based on the sign inversion of dielectric anisotropy of dual frequency liquid crystal (DFLC). The direct switching was achieved by applying square wave field at low (1 kHz) and high (50 kHz) frequency. As a result, the DFLC light shutter sustains bistable bright and dark states in electric field off state and exhibits excellent electro-optic performance. The direct switching from the FC to P states not only supports more uniform P state but also significantly reduces switching voltage by eliminating the high field homeotropic (H) state required for the switching in the conventional polymer stabilized cholesteric texture (PSCT) light shutter. The driving voltage applied to make a transition from the P to FC one is relatively low (3Vp-p/µm). Further, switching time from FC to P state was reduced drastically with homeotropic PI layer. Results show that dual frequency cholesteric liquid crystal (DFCLC) light shutter holds a great promise for use in energy efficient display devices and switchable windows.

Tailoring Monodomain in Blue Phase Liquid Crystal by Surface Pinning Effect

Abstract: Surface pinning effect of the optically isotropic, blue phase liquid crystal (BPLC) has been studied. Polycrystalline, platelet, multi-domain topological defects without surface treatment have been transformed to uniform monodomain in rubbed surfaces of the cell. The operating voltage was reduced by 27%, owing to a dramatic increase in the Kerr constant. Hysteresis was reduced by 63%, whereas the transmission efficiency gets doubled. The surface anchoring effect on the blue phase plays a significant role in the elastic free energy, coherence length and topological defects. BPLC displays with a low operating voltage, and reduced hysteresis can be realized with surface treatment.

Self-constructed stable liquid crystal alignment in a monomer-liquid crystal mixture system

Abstract: Here, we demonstrate excellent liquid crystal (LC) vertical alignment without using an alignment layer printing process by introducing octadecyltrichlorosilane (OTS) into the LC mixture. Further, we investigated the alignment mechanism by analysing the surfaces of the substrates. The optimum concentration of OTS was found to be about 0.03 wt%, which is 1/100 of that in the previously reported polyhedral oligomeric silsesquioxane (POSS)–LC system. Moreover, the OTS–LC system exhibited a more stable LC alignment compared with the POSS–LC system. These differences may arise from the different strengths of surface–dopant interactions; that is, the covalent bond in the OTS–LC system and the van der Waals interactions in the POSS–LC system. We also demonstrated that the method can be used in a capillary tube, which may serve as a new method facilitating the application of LCs with curved surfaces.

The biaxial lamello-columnar liquid crystalline structure of a tetrathiafulvalene sanidic molecule

Abstract: A macroscopically oriented lamello-columnar mesophase was obtained from a newly designed and synthesized symmetric tetrathiafulvalene-based molecule (symTTF11) containing four flexible alkyl chains chemically attached to the periphery of TTF mesogens (L. Wang, et al., J. Mater. Chem., 2011, 21, 60), and its phase behaviors and physical properties were investigated by the combined experimental techniques. Based on the experimental results and structure analyses, it was revealed that there are two ordered columnar phases below the isotropic phase: a columnar crystalline phase (ColK) and a columnar smectic liquid crystal phase (ColL). From two-dimensional wide angle X-ray diffraction patterns of the macroscopically oriented samples, phase structures of ColK and ColL were identified and their biaxial molecular packing structures were also proposed. Since the assembled symTTF11 columns were well organized in the ordered layer structures over a macroscopic domain, symTTF11 exhibited a good charge carrier mobility in the ColL phase.

Optimal design of quarter-wave plate with wideband and wide viewing angle for three-dimensional liquid crystal display

Abstract: In this paper, we propose an optical structure for a circular polarizer-containing film patterned retarder (FPR), which has wideband and wide-view properties in a stereoscopic three-dimensional (3D) display. The FPR consists of a patterned λ/4 A-plate, a biaxial λ/2 plate, and a positive C-plate. We calculate the phase retardation of each film in the entire visible wavelength with the Stokes vector and the Muller matrix method. We demonstrate the excellent 3D characteristics of the proposed polarizer in the oblique direction by comparing the calculated light leakage of the proposed optical configuration with that of the conventional configuration. We calculate that the crosstalk of the proposed configuration is reduced to 0.39% for the left image and 0.29% for the right image in the horizontal oblique direction and to 0.37% for the left image and 0.29% for the right image in the vertical oblique direction (polar angle = 70°). These results indicate that the proposed configuration improves crosstalk by approximately 90% compared to the conventional mode.

Multi-domain vertical alignment liquid crystal displays with ink-jet printed protrusions

Abstract: For the fabrication of protrusions in multi-domain vertical alignment liquid crystal displays (MVA-LCDs), a series of complicated and expensive photolithographic processes has to be gone through for the wide viewing angles. In this research, we proposed and demonstrated a new manufacturing approach of the protrusion utilising ink-jet printing technology. Parallel lines were ink-jet printed on the substrates and solidified by a subsequent baking process, and then used as the protrusions of MVA-LCDs. The geometric dimensions of the protrusion patterns were optimised by the comparison of experimental observations and computer simulation results. The MVA-LCD cell was fabricated by arranging the protrusion lines forming right angles between the top and the bottom lines, which resulted in cross-line protrusions. Due to the square-shaped geometric arrangement of the protrusions, the vertically aligned LCs formed the symmetric four-domains in the MVA-LCD cell. With the help of a surface polymer-stabilisation tech...

Liquid crystal micro-lenticular array assembled by a fringing field

Abstract: Based on dielectrophoretic effect, the phase separation morphology of liquid crystal (LC) in a liquid monomer can be manipulated by a fringing field. Applying the fringing field generated from interdigitated electrode upon the LC/monomer mixture, the randomly dispersed LC droplets can be assembled to grating-like stripes. When the field is removed, the LC stripes break into tiny droplets again. This process is reversible and stable. Our results show that the surface profile of each LC stripe exhibits a lenticular shape. The response time of the LC morphology converting from droplets to stripes and from stripes to droplets is ~0.96 s. Potential applications of this controllable morphology can be found in optical communications, beam steering, imaging, and displays.

Effect of the grain size on hysteresis of liquid-crystalline Blue Phase I

Abstract: — Three different blue-phase I liquid-crystal samples with different grain-size distribution were studied. Polycrystalline platelet grains reflects light in the visible region and have been observed under a polarizing optical microscope. Green and blue grain areas have been measured and then the statistical histogram by the Gaussian distribution function was fitted and the mean grain size was calculated. The effect of hysteresis on grain size has been compared for the three samples. Hysteresis depends explicitly on the grain-size distribution. Large grain size has revealed less hysteresis, while small grain size depicted pronounced hysteresis.

P-94: Nano-particle Induced VA-LCD

Abstract: A nano-material called POSS (Phenethyl-Poly Oligomeric Silsesquioxane) provides vertical alignment of LC on substrate without using conventional polymer type of vertical alignment layer. However, it still has many problems like aggregation because of high concentration doping. We synthesise new type of POSS which shows good dispersion with LC and found out that even very low 0.1 wt % into LC, uniform vertical alignment is achieved through diffusuion process from bulk LC to surface of the substrates.

Effect of cadmium sulfide nanorod content on Freedericksz threshold voltage, splay and bend elastic constants in liquid-crystal nanocomposites

Abstract: This report describes how doping liquid crystals (LC) with rod-like hexagonal semiconductor nanoprisms alters the dielectric and elastic properties of the composites as compared with a pristine nematic liquid crystal (NLC). Cadmium sulfide nanorods were synthesized via the solvothermal process and blended with a NLC. Nanorods were highly miscible with NLC and produced a topological defect-free texture up to a certain limit. A good dark state was achieved during the homeotropic configuration of the cell within that limit. Appreciable changes in splay and bend elastic constants of the LCs were observed after blending with nanorods. Long-range order was established in the hybrid system, and consequently the anisotropy was increased. The threshold voltage decreased dramatically by ?31%. Dielectric study revealed a high-frequency mode, which might be due to anchoring of the LC with nanorods.

The Qualitative Study on the Emotional Labor of Fashion Sales Personnel -Focused on Fast Fashion Sales Personnel-

Abstract: This study investigates the emotional labor of fast fashion sales personnel using the qualitative research method. The research was performed through an in-depth interview with 10 sales personnel (managers, coordinators, head cashiers, and sales associates) who experienced fast fashion brands from July to October in 2011. The results of this study provide that fast fashion brands do have specific feeling rules and display behavior. Even though the sales personnel spend an extremely short time (about 1 minute) to serve customers, most of them experience exhaustion, fatigue, burnout and self-alienation because of emotional dissonance. Sales personnel do deep-acting and surface acting simultaneously and interviewees who have longer and more sales experience express less emotional dissonance. To reduce the negative results of emotional labor, the company plays an important role through the organizational culture; in addition, deep-acting could be recommended for a beginner until they are comfortable with sales. The results of this study provide the elements of emotional labor in a fast fashion and practical suggestions for store operation and sales personnel training.

Emergence of a novel optically isotropic transient state with low frequency in a blue phase liquid crystal mixture

Abstract: The electro-optical hysteresis characteristics in a blue phase liquid crystal system showing multi-domain platelet type body-centred cubic (BP-I) structure at different levels of applied electric field have been investigated. The BP-I state possesses a large degree of hysteresis due to its compact body-centred cubic structure. The hysteresis behaviour is discussed in the context of simulation and theoretical studies reported earlier. When the cell is driven with a low frequency square wave, a novel optically isotropic state with high value of the Kerr constant has been observed. Hysteresis and response time both show relatively lower values in this state with respect to the multi-domain BP-I state.

Study on the light leakage mechanism of a blue phase liquid crystal cell with oblique interfaces

Abstract: The mechanism of light leakage in the dark state of a blue phase liquid crystal display cell which has protruded electrodes was investigated. We have performed a hybrid numerical simulation by combining the geometrical optics with the extended Jones matrix method. The light leakage in the cell was caused by changes in the polarization state which has been explained by the asymmetric amplitude change of transverse electric and transverse magnetic fields at the oblique interface and the change in an effective angle between crossed polarizers by the light path refraction. Based on our analysis, light leakage can be suppressed by the matching of the refractive indices of adjacent materials to the interface of the protruded electrodes whose surfaces are not parallel to the substrate.

Analysis of optical bounce associated with two-step molecular reorientation in the fringe-field switching mode

Abstract: The dynamic electro-optic response of liquid crystals (LCs) having positive dielectric anisotropy has been investigated in the fringe-field switching device. Simulation and experimental results show a huge optical bounce near the centre of the pixel electrodes of the device during decaying time of the LC director from an operating voltage. The causality of the said behaviour has been detected as two-step molecular relaxation, first tilt down and then rotating of the LC, which is associated with splay/bend and then twist relaxation, respectively. Reduction of dielectric anisotropy of the used LC material in the device has been suggested as a remedy of reducing optical bounce. The reduction of rotational viscosity also produces optical bounce of lesser duration; however, the intensity of the bounce remains invariant.

A wide-view twisted nematic liquid crystal cell with improved image quality obtained by decreasing γ-curve distortion

Abstract: Wide-view (WV) film, which was introduced by Fuji Co., has been commercialized to block light leakage of twisted nematic (TN)-liquid crystal (LC) cells in a dark state when the viewing angles are in the off-axis direction. In this investigation, we designed an optical compensation structure of a TN-LC cell with an improved γ-curve, as well as a viewing angle property in the dark state. Basically, we used a pair of optical films consisting of a positive and a negative A-plate in order to avoid affecting the viewing angle property in the dark state. Then, we optimized the positive and negative A-plate to improve the viewing angle property in the gray levels by calculating polarization variations as a function of the optical axis and the retardation of the pair of the A-plates. From this calculation, we were able to show that the proposed normally white (NW) WV-TN cell exhibits wide viewing angle performance, including good γ-curve stability.

Film compensation to improve the dark state in the oblique directions of the horizontal-switching mode

Abstract: In order to compensate for light leakage in the oblique viewing directions of the dark state of the horizontal-switching mode such as the in-plane switching and fringe field switching modes. The viewing angle performance of the horizontal-switching mode with an compensation film has been evaluated on a case by case basis according to the retardation values of the films. Consequently, we found that a much better dark state can be achieved by using an optimised compensation film.

P-90: Surface Pinning Effect on Blue-Phase Liquid Crystal

Abstract: This paper reports surface treatment effect on liquid crystalline blue phase morphology and electro-optical performances at various temperatures. We have succeeded to achieve homogeneous monodomain blue phase morphology by surface treatment. Operating voltage and hysteresis of the proposed device is greatly reduced compared to the non surface treated cell, owing to dramatic increase in Kerr constant. Uniform mono domain morphology has better electric field withstand capability so that its original texture restored after electric field addressing.

The Static Electricity Resistant Liquid Crystal Display Driven by Fringe Electric Field

Abstract: In this paper, we propose the static electricity resistant liquid crystal display (LCD) driven by fringe electric field. In the device, the conductive Cr metal playing role as black matrix (BM) in the top glass substrate is connected to the common electrode on the bottom substrate by the Ag transfer dotting. In addition, the common electrode also contacts to border chassis, allowing it to serve as the path for discharging the static charge generated on the top glass surface when peeling off the protective layer of the polarizer. Therefore, the electrostatic charges near BM or in the active region move fast enough to the CrBM for neutralization, while keeping the display in the neutral state electrically. The proposed LCD utilizes a liquid crystal mixture with negative dielectric anisotropy, allowing the device to be in much stable state against noise field generated by CrBM or external electrostatic discharge (ESD). We expect that this proposed structure is suitable to electrostatic resistant fringe-field switching (FFS)-LCD with a high image quality.

A transflective liquid crystal display using polymer stabilised vertical alignment mode

Abstract: A transflective liquid crystal display using polymer-stabilised vertical alignment mode with fishbone-shaped electrode structure is proposed. Using the polymer stabilisation technique and patterned direction of pixel electrode structure, initial surface tilt angle and azimuthal angle can be well defined such that the liquid crystal is almost vertically aligned in the transmissive part while it has a certain pretilt angle that satisfies effective retardation of λ/4 with azimuthal direction of 45o with respect to the analyser in the reflective part. With optimisation of cell structures and surface tilt angles in the transmissive and reflective parts, a high performance transflective display showing excellent readability in any environment light conditions and single-gamma characteristic without using any retardation film or in-cell retarder can be achieved.