Hong Gyu Park

Thermal and electro-optical properties of cerium-oxide-doped liquid-crystal devices

ABSTRACT Doping with cerium (IV) oxide (CeO2) nanoparticles can significantly enhance the thermal stability and electro-optical (EO) properties of nematic liquid crystal (NLC) systems. Thermal stability was improved without aggregation and reduction of transmittance by adding CeO2 nanoparticles in a liquid crystal medium. In particular, the EO properties greatly improved in CeO2-dispersed NLC cells. The threshold voltage reduced from 3.027 to 2.279 V and the response time decreased from 13.097 to 9.970 ms with increased CeO2 nanoparticles in the NLC cells. The improved properties of liquid crystals doped with CeO2 nanoparticles depend on the anchoring energy and the electric field of the CeO2-dispersed liquid crystal displays. Graphical Abstract

Tailoring the Orientation and Periodicity of Wrinkles Using Ion-Beam Bombardment.

The present study demonstrates that surface reformation in polydimethylsiloxane can be controlled using ion-beam (IB) irradiation. This can be done by simply varying the IB incidence angle and requires no change in the energy source. By controlling the incidence angle of IB irradiation, we were able to continuously control the pattern of the wrinkle structure, that is, a randomly formed pattern or an anisotropic one. Moreover, the directional characteristics of the wrinkle pattern control the alignment of liquid crystal molecules. This control is a function of the incidence angle of the IB. These simple methods can provide considerable flexibility in the fabrication of wrinkle structures.

Hysteresis-free, energy efficient twisted nematic liquid crystal systems based on IB-irradiated gallium-doped tin oxide films

We successfully demonstrated the high performance of liquid crystal (LC) devices without capacitance hysteresis using ion-beam (IB)-irradiated GaSnO in solution. This method is widely used for obtaining inorganic films, although it yields poor crystallinity, owing to its cost effectiveness and high flexibility in combining materials. Atomic force microscopy and X-ray photoelectron spectroscopy analyses determined that IB irradiation on the films had an effect, which enabled the elucidation of the mechanism for LC alignment. Strong IB irradiation on GaSnO induced the formation of spherical particles on the films and rearrangement of the chemical bonds (linkage of Ga–Sn–O and gradually oxidized films), which could preserve the anisotropic characteristics to unidirectionally align LC molecules on the surface. Because rough surfaces effectively increase the electric field, the strongly restructured GaSnO surface led to a decrease in threshold voltages. Moreover, strong IB irradiation effectively released volume charges when the states of LC molecules were switched “on” and “off”, which is attributed to a reduction in oxygen bonding derived from hydroxyl groups responsible for capturing neutral electrons. Therefore, an IB-irradiated GaSnO film is appropriate as an alternative alignment layer for advanced, energy efficient, hysteresis-free LC display devices.Graphical AbstractWe successfully demonstrated the high performance of liquid crystal (LC) devices without capacitance hysteresis using ion-beam (IB)-irradiated GaSnO in solution. This method is widely used for obtaining inorganic films, although it yields poor crystallinity, owing to its cost effectiveness and high flexibility in combining materials. (a) Schematic of the IB irradiation of the solution-derived GaSnO film. (b) Alignment state of the LCs as a function of IB intensity. (c) Transmittance graph using the crystal rotation method to calculate the pretilt angle. (d) Transmittance as a function of voltage of the twisted nematic (TN) cells fabricated from IB-irradiated GaSnO at different intensities compared to the rubbed PI method (e) Capacitance of the TN cells fabricated from IB-irradiated GaSnO at 2200xa0eV, which exhibited hysteresis-free characteristic of TN cells. Because rough surfaces effectively increase the electric field, the strongly restructured GaSnO surface led to a decrease in threshold voltages. Moreover, strong IB irradiation effectively released volume charges when the states of LC molecules were switched “on” and “off”, which is attributed to a reduction in oxygen bonding derived from hydroxyl groups responsible for capturing neutral electrons. Therefore, an IB-irradiated GaSnO film is appropriate as an alternative alignment layer for advanced, energy efficient, hysteresis-free LC display devices.

Superior electro-optical performance in vertically aligned liquid crystal devices based on aluminum oxide films

ABSTRACT We achieved vertically aligned (VA) liquid crystals (LCs) on aluminum oxide (Al2O3) films deposited via e-beam evaporation using a rubbing treatment. Uniform and vertical LC alignment was achieved and high thermal stability was obtained using these substrates. By analyzing measurements from optical retardations, we confirm that the LC orientation is adjustable using rubbing treatment. The superior electro-optical characteristics of the VA cells based on Al2O3 films are measured and compared with those based on polyimide layers, indicating that this approach will allow the fabrication of high-performance, advanced LC displays using a conventional rubbing process.

Homeotropic alignment behavior of liquid crystal molecules on self-assembled monolayers with fluorinated alkyl chain

This study demonstrates the orientation characteristics of liquid crystals (LCs) on fluorinated self-assembled monolayers (FSAMs) deposited using the gas phase method. The authors confirmed that the FSAMs were properly deposited on indium-doped tin oxide glass substrates using x-ray photoelectron spectroscopy and atomic force microscopy. Compared with conventional polyimide (PI) layers, the authors confirmed that the FSAM is an alternative to the conventional PI layer. Regardless of the positive and negative LC characteristics, the LC molecules were vertically aligned on the FSAMs, with hydrophobic properties. In addition, an LC cell using an FSAM showed faster response times than that with a conventional PI layer when the electro-optical characteristics were examined. Therefore, these results indicate that FSAMs are suitable for LC applications as homeotropic LC alignment layers.This study demonstrates the orientation characteristics of liquid crystals (LCs) on fluorinated self-assembled monolayers (FSAMs) deposited using the gas phase method. The authors confirmed that the FSAMs were properly deposited on indium-doped tin oxide glass substrates using x-ray photoelectron spectroscopy and atomic force microscopy. Compared with conventional polyimide (PI) layers, the authors confirmed that the FSAM is an alternative to the conventional PI layer. Regardless of the positive and negative LC characteristics, the LC molecules were vertically aligned on the FSAMs, with hydrophobic properties. In addition, an LC cell using an FSAM showed faster response times than that with a conventional PI layer when the electro-optical characteristics were examined. Therefore, these results indicate that FSAMs are suitable for LC applications as homeotropic LC alignment layers.

Liquid crystal alignment on ion-beam irradiated homogeneous hafnium strontium oxide films deposited by sol-gel process

We investigated homogeneous liquid crystal alignment on hafnium strontium oxide films deposited by a solution process, where the liquid crystal alignment was induced by ion-beam irradiation. Uniform liquid crystal alignment was achieved on the HfSrO films grown at 300u2009°C. We confirmed the effect of surface morphology and chemical composition of HfSrO films using field-emission scanning electron microscopy and X-ray photoelectron spectroscopy. In addition, we observed the electro-optical characteristics of the twisted-nematic liquid crystal cells based on hafnium strontium oxide films to verify the possibility of their application in liquid crystal displays.Graphical Abstract

Stable and fast switching of liquid crystals on solution-derived compound oxide films irradiated by ion beam

We fabricated stable, homogeneous, fast-switching liquid crystal cells on solution-processed zinc tin oxide films treated by ion-beam irradiation >1200u2009eV. The ion-beam-irradiated zinc tin oxide films were amorphous and demonstrated good transparency. X-ray photoelectron spectroscopy analysis was used to determine the liquid crystal alignment mechanism with respect to the ion-beam-irradiated zinc tin oxide film. The total response time of electrically controlled birefringence-liquid crystal cells on 2200-eV ion-beam-irradiated zinc tin oxide films was 5.04u2009ms; the threshold voltage was reduced by 15% compared with rubbed polyimide. Thus, ion-beam-irradiated zinc tin oxide films showed great potential as an alignment layer for high-performance electrically controlled birefringence-liquid crystal cells in industrial applications.Graphical abstract

Unidirectional alignment of liquid crystals on solution-derived hafnium tin oxide films via ion-beam irradiation

We demonstrated the performance of solution-derived hafnium doped tin oxide films annealed at various temperatures via ion-beam irradiation. The Hf:SnO2 films were employed as liquid crystal alignment layers, which were deposited from solution to replace sputtering methods. Homogeneous and uniform liquid crystal alignment was achieved at all annealing temperatures, yielding a high-performance liquid crystal system. Field-emission scanning electron microscopy revealed the surface characteristics of the hafnium doped tin oxide films at various annealing temperatures, and various surface phenomena were observed. High-performance electro-optical properties were observed for the twisted nematic LC cells of hafnium doped tin oxide films annealed at various temperatures. The lowest threshold voltage (2.042u2009V) and fastest response time (8.494u2009ms) were achieved at an annealing temperature of 200u2009°C. Our results revealed the potential of solution-derived hafnium doped tin oxide films as liquid crystal alignment layers.Graphical Abstract