Jeong Min Han

Liquid Crystal Alignment Effects for Nematic Liquid Crystal on Homeotropic Polyimide Surface Using New Ion-Beam Source

We have studied the liquid crystal (LC) alignment and tilt angle generation of a nematic liquid crystal (NLC) on a homeotropic polyimide (PI) surface using a new ion beam source. The tilt angle of the NLC on the homeotropic PI surface for all incident angles is about 38° and this angle has a stabilization trend. A good LC alignment of the NLC on the homeotropic PI surface by to exposure ion beam of 45° incidence angle was observed. Also, the tilt angle of the NLC on the homeotropic PI surface by to exposure ion beam of 45° incidence angle had a tendency to decrease as ion beam energy density increased. Thus, we demonstrated that the tilt angle could be controlled from vertical to horizontal directions. Also, the alignment character of the NLC on the homeotropic PI surface with respect to ion beam energy was good at more than 1500 eV. Finally, a superior LC alignment thermal stability on the homeotropic PI surface subject to ion beam exposure was achieved.

Homogeneous alignment of liquid crystals on low-temperature solution-derived gallium oxide films via IB irradiation method

ABSTRACT In this paper, solution-derived gallium oxide (GaO) films are fabricated for the homogeneous alignment of liquid crystals (LCs) after an ion-beam (IB) irradiation process. GaO thin films are prepared under a variety of temperatures and different IB irradiation intensities, and the physicochemical performances of the fabricated GaO thin films are analysed using a UV-vis spectrometer, an atomic force microscope, and X-ray photoelectron spectroscopy. A higher transmittance of 85.40% from GaO thin film is obtained compared with that of polyimide (PI) film (83.52%), which indicates the feasibility for a GaO thin layer to substitute for a conventional PI layer as an alignment layer. LCs are found to align on the GaO thin film after pre-baking at 100°C and homogeneous and uniform low-IB intensity irradiation. We also determined the electro-optical (EO) characteristics of the twisted nematic (TN) cells fabricated with GaO thin layers and found them to be similar to those of cells fabricated with PI layers. Overall, GaO films achieved via the IB irradiation method are promising LC alignment layers due to the method’s low-temperature solution-derived process. GRAPHICAL ABSTRACT

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.

Homogeneously aligned liquid crystal molecules on unidirectional buckle pattern of polyurethane films

ABSTRACT We present the fabrication of an anisotropic structure on a polyurethane (PU) film for aligning liquid crystal (LC) molecules with ion beam (IB) irradiation at an incident angle. The obtained anisotropic structure assembles the LC cells in a layer that aligns LC molecules uniformly. Polarised optical microscopy images of an LC cell fabricated with IB-irradiated PU were captured to confirm the LC alignment state and compared with those fabricated with non-treated PU. To analyse the effects of the IB irradiation, X-ray photoelectron spectroscopy and field-emission scanning electron microscopy were used to investigate the chemical and morphological modifications, respectively. IB irradiation modifies the chemical structure of PU, which indicates that a new skin layer is formed on the PU films. This skin layer generates an in-plane compressive strain, thereby creating buckles on the PU films. Simultaneously, the physical collision of the reactive Ar+ ions during IB irradiation induces a directional strain on the surface, thereby forming a unidirectional structure of buckles along the direction of IB irradiation. The PU film annealed at 200°C showed the high average transmittance of 88.9%, which is appropriate as an alignment layer. Graphical Abstract

Liquid crystal aligning capabilities on surface-reformed indium-doped zinc oxide films via ion-beam exposure

ABSTRACT We investigated the characteristics of a solution-processed indium-doped zinc oxide (In:ZnO) film formed via ion-beam (IB) irradiation as a liquid crystal (LC) alignment layer. The In:ZnO film was deposited using solution processing and cured at various temperatures. Uniform LC alignment was observed at all curing temperatures in cross-polarised optical microscopy images. A regular pre-tilt angle supported these results and showed homogeneous LC alignment. Several surface analyses were conducted to evaluate the effect of IB irradiation on the In:ZnO film surface. X-ray diffraction analysis showed an amorphous structure both before and after IB irradiation, and physical surface reformation was observed using atomic force microscopy. Root mean square surface roughness was reduced and a smooth surface was achieved after IB irradiation. X-ray photoelectron spectroscopy was used to detect chemical surface reformation. It was found that the IB irradiation broke the metal-oxide bonds and increased the occurrence of oxygen vacancies, which affected the van der Waals forces between the LC molecules and the In:ZnO film surface. Electrical performance was observed to identify the possibility of using the In:ZnO film in LC applications. Enhanced electro-optical performance was measured and zero residual DC voltage which was verified using a capacitance-voltage curve was achieved. GRAPHICAL ABSTRACT

Liquid crystal alignment on ion-beam irradiated bismuth-doped tin oxide films and their application to liquid crystal displays

ABSTRACT Uniform and defect-free homogeneous alignment of liquid crystal (LC) molecules on solution-derived bismuth-doped tin oxide (TBO) films has been achieved using ion-beam (IB) irradiation. We performed measurements and physicochemical analysis to verify and establish the cause of the successful LC alignment. In addition, we measured the electro-optical characteristics of twisted-nematic cells with IB-irradiated TBO films to explore the suitability of this material for liquid crystal displays (LCDs). The results indicate that this approach will allow the fabrication of high-performance enhanced LCD devices. Graphical Abstract

Inducement of homogeneous liquid crystal alignment on surface-reformed polyurethane films via manipulation of ion-beam irradiation incidence angle

ABSTRACT We investigated the induced uniform alignment of liquid crystals (LCs) on a polyurethane (PU) ion-beam (IB) irradiated at a certain incidence angle. Polarized optical microscopy (POM) and the crystal rotation method were used to evaluate the LC alignment state. On PU films IB-irradiated with incidence angles of 60° or less, the LC cells exhibit a clear and uniform alignment. At an incidence angle of 75°, light scattering was observed via POM, indicating the randomly aligned state of the LC cells. Moreover, the calculated pretilt angles for LC cells on IB-irradiated PU at incidence angles of 15°, 30°, 45°, and 60° were 0.05°, 0.05°, 0.20°, and 0.18°, respectively. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to confirm the effect of varying the IB irradiation incidence angle. The PU surface irradiated by IB was modified differently as the incidence angle changed. IB irradiation at low incidence angles broke C–C, C–O/C–N, and N–C(O)–O bonds and caused C=O bond formation by the dissociation of the N–C(O)–O bonds. The van der Waals interactions between the PU and LC matched the LC molecular interactions and the anisotropic characteristics could be maintained, thereby achieving a stable homogeneous LC alignment.

Free residual DC voltage for nematic liquid crystals on solution-derived lanthanum tin oxide film

ABSTRACT We present the liquid crystal (LC) alignment properties of solution-derived lanthanum tin oxide (LaSnO) films cured at various temperatures and exposed to ion-beam (IB) irradiation. Using a solution process, LaSnO films were deposited on the indium-tin-oxide glass substrates and IB irradiation was used as an alignment method. Homogeneous and uniform LC alignment was achieved and observed by cross-polarised optical microscopy. Pre-tilt angle results with low standard deviation supported the notion of uniform LC alignment. The LaSnO film cured at 300°C showed nearly zero capacitance–voltage hysteresis. The change of the surface morphology of the LaSnO film due to IB irradiation was observed by atomic force microscopy. The effects of IB irradiation on the LC alignment layer were further demonstrated by X-ray photoelectron spectroscopy. The strong IB irradiation broke the metal–oxide bonds present, which in turn induced an increased number of oxygen vacancies on the whole surface. Uniform LC alignment was attributed to surface reformation and van der Waals forces. Graphical Abstract

Organic Thin Film Transistors Fabricated with Soluble Pentacene Active Channel Layer and NiO x Electrodes

We report on the fabrication of soluble pentacene-based thin-film transistors (TFTs) that consist of NiO x , poly-vinyl phenol (PVP), and Ni for the source-drain (S/D) electrodes, gate dielectric, and gate electrode, respectively. The NiO x S/D electrodes of which the work function is well matched to that of soluble pentacene are deposited on a soluble pentacene channel by sputter deposited of NiO target and show a moderately low but still effective transmittance of ∼ 65% in the visible range along with a good sheet resistance of ∼ 40 Ω/□. The maximum saturation current of our soluble pentacene-based TFT is about 15 µA at a gate bias of − 30 V showing a high field effect mobility of 0.03 cm2/Vs in the dark, and the on/off current ratio of our TFT is about 104. It is concluded that jointly adopting NiO x for the S/D electrodes and PVP for gate dielectric realizes a high-quality soluble pentacene-based TFT.

Application of Zinc Oxide-Silver Electrode for Bottom Emission Organic Light Emitting Diode

The process for depositing indume-tin oxide (ITO) electrode on polymer substrate is not readily applicable at high temperature and shows poor electrical properties than the conventional ITO deposition process on glass substrate. Especially, the deformation of polymer substrate and residual stress yield the crack which degrade the performance of organic light emitting diode (OLED) unit. In this study, we developed low resistance ITO electrode, AZO(ITO-ZnO-Ag-ZnO-ITO) to apply to the fabrication of the bottom emission OLED unit. The performance of the AZO exhibited low surface resistance as 8 ohm and the ITO electrodes were additionally deposited to guarantee the same work function as conventional ITO. The emissive efficiency was 92% at 550 nm which almost corresponds to that of the conventional ITO.