Liquid Crystal Alignment Characteristics Research Articles

Electro-Optical Characteristics of Solution-Derived Zinc Oxide Film According to Number of Rubbing Iterations for Liquid Crystal Alignment

Zinc oxide (ZnO) films reportedly exhibit a rubbing effect for liquid crystal (LC) alignment. In this study, we investigated the LC alignment characteristics of solution-derived ZnO films according to the number of rubbing iterations. Uniform and homogeneous LC alignments were achieved on the rubbed ZnO films, regardless of the number of rubbing iterations. As the number of rubbing iterations increased, the surface energies of the rubbed ZnO films were similar to 42.20 mJ/m2, but the polar energy increased from 4.288 mJ/m2 to 6.470 mJ/m2. Additionally, the electro-optical characteristics of the twisted-nematic (TN) LC cells fabricated by rubbed ZnO films showed the best performance when the number of rubbing iterations was at five. By confirming that the ZnO film with improved physical, chemical, and electro-optical characteristics compared to the polyimide film achieved a perfect LC alignment through the conventional rubbing process, it indicates that the ZnO film can be an excellent substitute for the LC alignment film. In addition, it is expected that it can be applied to an LC-based virtual reality (VR)/augmented reality (AR) display system that requires a very fast response time through the excellent EO characteristics of the TN LC cell of the solution-derived ZnO film through the rubbing process.

Self-aligned liquid crystals and enhanced electro-optical properties on solution-processed aluminum gallium tin zinc oxide surfaces

An aluminum gallium tin zinc oxide (AlGaSnZnO) film was fabricated by brush coating to create a directional micro/nano structure. Atomic force microscopy was used to observe the physical change in the film surface after brush coating, and the difference in the surface modification according to the curing temperature was analyzed through line profile and surface roughness studies. Changes in the chemical composition of the surface were analyzed through X-ray photoelectron spectroscopy. The liquid crystal (LC) alignment characteristics were analyzed through polarized optical microscopy, pretilt angle, and thermal stability tests. It was confirmed that the orientation direction of LC molecules changed from vertical to horizontal due to the difference in the surface modification at a curing temperature of 180 °C or higher, and stable LC orientation and excellent electro-optical properties were achieved. Brush-coated AlGaSnZnO film is an effective alignment film that controls the LC alignment characteristics with a simple process and can be applied in various fields.

Superior Performance in Liquid Crystal Alignment of Polystyrene-Block-Poly(ethylene-ran-butylene)-Block-Polystyrene-Graft-Maleic Anhydride Film Irradiated with Ion Beam

We studied the change of the surface and liquid crystal (LC) alignment characteristics of Polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene-graft-maleic anhydride (SEBS-g-MA) film according to ion beam (IB) irradiation. Atomic force microscopy (AFM) analysis revealed that IB irradiation reduces peaks and creates a wrinkled structure. X-ray photoelectron spectroscopy (XPS) analysis revealed that IB irradiation disintegrates the C–C bonds and makes C–O bonds, creating a dipole-dipole interaction, which improves the LC alignment characteristics. Through polarized optical microscopy (POM) analysis, it was found that the best LC alignment characteristics can be obtained after 120 s of IB irradiation. It was found that the anisotropy caused by the ion beam contributed more to the liquid crystal alignment in homogeneous direction than the one caused by the wrinkle structure.

The effects of surface morphological mechanics on liquid crystal alignment characteristics using photo-activators

Photo-activators, which produce carbon dioxide, are highly reactive radical generators upon UV light illumination. In this study, the generated radicals selectively reacted with the polyimide (PI) main chains and created a liquid crystal alignment layer with a high level of azimuthal anchoring energy. The thickness of the photo-irradiated PI alignment layer was reduced dramatically by photo-induced radical crosslinking, which induced surface wrinkling and roughness. Moreover, the carbon dioxide and methane gases that were generated during photo-irradiation produced many micro-pores, which also attributed to holding the LC molecules tightly on the PI surfaces. The level of azimuthal anchoring energy that was obtained by photo-alignment was better than what was obtained by the rubbing method with the same PI, specifically, the maximum value of the photo-alignment with the photo-activator was 6.92 x 10−5 J/m2 than 1.11 x 10−5 J/m2 of the rubbing. We proposed a mechanism based on a high anchoring energy, a rough surface, a hydrophilic surface, and rapid photo-reactions.

Ion-beam irradiation modified chemical and physical surface characteristics of polyethylene glycol film for liquid crystal aligning

ABSTRACTThe characteristics of ion beam (IB) irradiated polyethylene glycol (PEG) films were analyzed to determine their potential use as a liquid crystal (LC) alignment layer. The transmittance of the PEG films at various IB intensities was measured to verify the possibility of using it as an alignment layer. Anti-parallel cells with the PEG film were fabricated to examine the LC alignment characteristics. Perfectly dark cross-polarized microscopy images were obtained at an intensity of 700 eV, which indicates uniform LC alignment, and pre-tilt angle measurements supported this finding. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analyses were conducted to determine the effect of the IB irradiation on the PEG film on the surface modification. After exposure at an IB intensity of 700 eV where uniform LC alignment was achieved, remarkable chemical composition modification of the PEG film, including the reduction of C-O bonds that affected the uniform LC alignment, was observed via the XPS analysis. Furthermore, the AFM analysis revealed that low kurtosis value was obtained at this IB intensity. Therefore, we are convinced that the PEG films irradiated at this low IB intensity show potential as alternative alignment layer in LC applications.

Electro-optical Properties of Twisted Nematic Liquid Crystal Cell with Silver Nanowire Network Electrodes

This paper introduces liquid crystal (LC) alignment and its electro-optical properties in the LC cells with silver nanowire (AgNW) networks. The AgNW network was used as an electrode of LC cell as a substitute for an indium-tin-oxide (ITO) film. LC alignment characteristics in the LC cell using AgNW networks, which have two different sheet resistances of 60 Ω/㎡ and 80 Ω/㎡, were observed. The LC alignment characteristics including pretilt angle, LC alignment state, and thermal stability are similar irrespective of sheet resistance of AgNW network. However, twisted-nematic (TN)-LC cell normally operated when using AgNW network with sheet resistance of 80 Ω/㎡. Electrooptical properties of TN-LC cell exhibited competitive performance compared to those of TN-LC cell based on conventional ITO electrode, which allow new approaches to replace conventional ITO electrode in display technology.

Electro-Optical Characteristics of Liquid Crystal Device With Nanoscale Molybdenum Trioxide $(\hbox{MoO}_{3})$ Thin Films

The liquid crystal (LC) alignment of molybdenum trioxide (MoO3) thin layers through ion-beam (IB) irradiation leads to LC alignment characteristics that are superior to those obtained with rubbed polyimide (PI) layers. We found that LC alignment states on a thin MoO3 layer are influenced by changes in IB irradiation energy. X-ray photoelectron spectroscopy analysis showed that the binding intensities of the Mo peaks that significantly decreased at IB irradiation energies greater than 2400 eV can homogeneously align with the LC and MoO3 molecules. An examination of the electro-optical characteristics revealed that MoO3 gave rise to a fast LC response time when compared to the PI layer.

Electrooptical Properties of Single-Walled Carbon-Nanotube Mixed Liquid-Crystal Cells With Rubbed and Ion-Beam-Treated Alignment Layers

The electrooptical properties of single-walled carbon-nanotube (SWCNT) mixed liquid-crystal (LC) cells fabricated by rubbing and ion-beam (IB) process are investigated. The increase of SWCNT concentration in the LC mixture led to a decrease in the threshold voltage of twisted nematic cells by reducing the screening effect. Threshold voltages of 1.3 and 1.7 V are lowered to 0.8 and 1.2 V, respectively, with SWCNT mixtures up to 10×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> wt%. LC alignment characteristics including LC orientation state, transmittance spectra, pretilt angle, and thermal stability of LC cells are also observed.

Vertically Aligned Liquid Crystals on Tantalum Oxide Thin Films Using Ion Beam Irradiation Processing

In this study, we explored the effects of ion beam (IB) bombardment on the orientation of liquid crystal (LC) molecules on surfaces. Increasing the IB exposure time resulted in improved LC alignment characteristics. Through X-ray photoelectron spectroscopy analysis, it was also confirmed that IB irradiation caused Ta 4f and O 1s peak shifts in the positive direction. The breaking phenomenon of the O 1s bonds may cause anisotropic dipole–dipole forces to stably align the vertical LC molecules. Finally, measuring the contact angles on the IB irradiated surfaces, we concluded that increasing the IB irradiation time strengthened the anisotropic surface energy on the surfaces, and stable vertical LC alignment was achieved via the transformed surface energies.

Liquid Crystal Alignment on Fluorinated Amorphous Carbon Films Treated by H2/Ar Plasma Beam

The liquid crystal alignment of fluorinated amorphous carbon (a-C:F) films which were deposited on the indium tin oxide (ITO) glass have shown to be improved by plasma beam processing. The experimental results indicated that excellent alignment characteristics regarding the range of pretilt angles and the alignment uniformity in a-C:F alignment films were improved by using the H2/Ar plasma beam processing. In this study, measurements of a-C:F film properties and liquid crystal (LC) cell characteristics were performed to investigate the relationship between the ratio of H2/Ar gaseous mixture and the LC alignment characteristics. The use of different gases allows enhanced control of the final surface chemical and physical properties. It was observed from the atomic force microscopy (AFM) results that the ion bombardment of films has a minimal impact on the alignment properties. Moreover, the X-ray photoemission spectroscopy (XPS) results showed that the H2/Ar plasma beam processing induces bond-breaking and chain scission. The results also indicated that the C–F bonds are broken by Ar+ and H radicals, while the benzene rings have carbon dangling bonds for the formation of C–O bonds in a-C:F films. Finally, the F/C ratio of a-C:F films was observed to be controlled by the H2/Ar ratio and the pretilt angles could be obtained from measurements of LC cells shown on a-C:F films.

Liquid Crystal Alignment Characteristics on the NDLC Thin Film Deposited using PECVD and Sputter

Applying inorganic thin film to liquid crystal (LC) alignment layer has been observed for the alternative of the LC alignment method. In this study, LC alignment characteristics of the nitrogen doped diamond like carbon (NDLC) thin film which was deposited using different deposition method–sputtering and plasma enhanced chemical vapor deposition (PECVD) system was investigated. The difference of the thin film deposition method and the rate of composition gas can affect the LC alignment. The NDLC thin film that was deposited using sputter showed uniform LC alignment at the 1200 eV of the ion beam intensity while the NDLC thin film that was deposited using PECVD showed uniform LC alignment at the 1800 eV of the ion beam intensity in our previous research. The uppermost of the thermal stability of NDLC thin film was 200°C. However, NDLC thin film deposited using sputter showed stability at high temperature, compared with NDLC thin film deposited using sputter.

Synthesis and properties of photoalignable aromatic polyesters containing phenylenediacrylate units in their backbones and n‐alkyl moieties in their side groups

AbstractA series of poly[oxy(4‐n‐alkyl‐3,5‐benzoate)oxy‐1,4‐phenylenediacryloyl]s (PPDA‐CnBZ polymers) with high molecular weights was synthesized. These polymers exhibit excellent solubility in some common organic solvents and produce good quality films using conventional spin‐casting and drying processes. The polymers are thermally stable up to 357–362 °C in a nitrogen atmosphere; their glass transition temperatures are greater than 121 °C. The photoreactions and photoalignments of the polymers were investigated using ultraviolet‐visible and infrared spectroscopy, and their liquid crystal (LC) alignment properties were examined. The phenylenediacrylate (PDA) chromophores in the polyesters were found to mainly undergo photocyclization upon ultraviolet light irradiation. Irradiation of the polyester films with linearly polarized ultraviolet light (LPUVL) induces preferential orientation of the polymer main chains, while the unreacted PDA chromophores are aligned along the direction perpendicular to the electric vector of the LPUVL. All the films irradiated with LPUVL were found to align LCs in a direction perpendicular to the electric vector of the LPUVL. Moreover, these LC alignments persisted even on irradiated films annealed at temperatures up to 210 °C, which is much higher than the glass transition temperatures of the polyesters. These LC alignment characteristics are due to the anisotropic interactions of the LC molecules with the oriented polymer chains and with the unreacted PDA chromophores. LC alignments on the polyester film surfaces have homeotropic to homogeneous characteristics, depending on the length of the n‐alkyl side group, providing strong evidence that the n‐alkyl side groups of the polyesters play a critical role in determining the pretilt angles of the LCs. The LC pretilt angles were also found to be influenced by the thermal annealing history of the irradiated films. In summary, the excellent properties of the PPDA‐CnBZ polymers make them promising candidate materials for use as LC alignment layers in advanced LC display devices. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1322–1334, 2004