Gi Seok Heo

Electro-optical switching of liquid crystals sandwiched between ion-beam-spurted graphene quantum dots-doped PEDOT:PSS composite layers

UNLABELLED Graphene quantum dots (GQDs)-doped PEDOT PSS composite layers were utilized to align liquid crystals (LCs) via an ion-beam (IB)-spurting pre-treatment process. LCs were homogeneously aligned between sandwiched GQDs/ PEDOT PSS composite thin layers, and the alignment of LCs was found to be affected by both the quantity of doped GQDs and IB-spurting intensity. Competitive electro-optical switching properties and non-residual DC performance of the cell equipped with GQDs/ PEDOT PSS composite alignment layers were obtained because of the enhanced field effect and charge transport induced by doped GQDs. Notably, using IB-spurted GQDs/ PEDOT PSS layers as alignment layers for next generation high-performance liquid crystal display (LCD) is promising.

Origin of Mechanoluminescence from Cu-Doped ZnS Particles Embedded in an Elastomer Film and Its Application in Flexible Electro-mechanoluminescent Lighting Devices

Mechanically driven light emission from particles embedded in elastomer films has recently attracted interest as a strong candidate for next-generation light sources on display devices because it is nondestructive, reproducible, real-time, environmentally friendly, and reliable. The origin of mechanoluminescence (ML) obtained from particles embedded in elastomer films have been proposed as the trapping of drifting charge carriers in the presence of a piezoelectric field. However, in this study, we propose a new origin of ML through the study of the microstructure of a Cu-doped ZnS particles embedded in an elastomer composite film with high brightness using transmission electron microscopy (TEM) to clearly demonstrate the origin of ML with respect to the microstructure of ML composite films. The TEM characterization of the ML composite film demonstrated that the Cu-doped ZnS particles were fully encapsulated by a 500 nm thick Al layer, which acts as an electron source for ML emission. Furthermore, we fabricated a flexible electro-mechanoluminescence (EML) device using a Cu-doped ZnS particles embedded in a flexible elastomer composite film. Our research results on a new emission mechanism for ML and its application in flexible light generating elastomer films represent an important step toward environmentally benign and ecofriendly flexible electro-mechanoluminescent lighting devices.

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

Homogeneous liquid crystal alignment characteristics on solution-derived HfYGaO films treated with IB irradiation

Solution-derived HfYGaO films have been treated by ion beam (IB) irradiation and used as liquid crystal (LC) alignment layers. Solution processing was adopted due to its simplicity, high throughput, and facile composition modification. Homogeneous and uniform LC alignment was achieved on the IB-irradiated HfYGaO films, and when these films were adopted in twisted nematic (TN) cells, electro-optical performance comparable to that of TN cells with conventional polyimide layers was achieved, with almost no capacitance-voltage hysteresis. Moreover, LC cells based on IB-irradiated HfYGaO films had a high thermal budget. The proposed IB-irradiated solution-derived HfYGaO films have considerable potential for use in advanced LC applications.

Solution-Derived Zn-Doped GaO Films as Alignment Layers for Twisted-Nematic Liquid Crystal Displays Using Ion-Beam Bombardment

This letter demonstrates liquid crystal (LC) alignment on solution-derived Zn-doped GaO (ZnGaO) films using an ion beam (IB) method. Homogeneous and uniform LC alignment was achieved on IB-irradiated ZnGaO films. X-ray photoelectron spectroscopy was used to analyze the chemical bonding states of the IB-irradiated ZnGaO surfaces to verify the compositional behavior for LC alignment. In addition, ZnGaO films with twisted nematic cells exhibited electro-optical characteristics comparable with those of a conventional polyimide layer for potential LC display applications.

Effect of Sputtering Powers on Mg and Ga Co-Doped ZnO Thin Films with Transparent Conducting Characteristics

ZnO thin films co-doped with Mg and Ga (MxGyZzO, x + y + z = 1, x = 0.05, y = 0.02 and z = 0.93) were prepared on glass substrates by RF magnetron sputtering with different sputtering powers ranging from 100 W to 200 W at a substrate temperature of 350 oC. The effects of the sputtering power on the structural, morphological, electrical, and optical properties of MGZO thin films were investigated. The X-ray diffraction patterns showed that all the MGZO thin films were grown as a hexagonal wurtzite phase with the preferred orientation on the c-axis without secondary phases such as MgO, Ga2O3, or ZnGa2O4. The intensity of the diffraction peak from the (0002) plane of the MGZO thin films was enhanced as the sputtering power increased. The (0002) peak positions of the MGZO thin films was shifted toward, a high diffraction angle as the sputtering power increased. Cross-sectional field emission scanning electron microscopy images of the MGZO thin films showed that all of these films had a columnar structure and their thickness increased with an increase in the sputtering power. MGZO thin film deposited at the sputtering power of 200 W showed the best electrical characteristics in terms of the carrier concentration (4.71 × 1020 cm−3), charge carrier mobility (10.2 cm2 V−1 s−1) and a minimum resistivity (1.3 × 10−3Ωcm). A UVvisible spectroscopy assessment showed that the MGZO thin films had high transmittance of more than 80 % in the visible region and that the absorption edges of MGZO thin films were very sharp and shifted toward the higher wavelength side, from 270 nm to 340 nm, with an increase in the sputtering power. The band-gap energy of MGZO thin films was widened from 3.74 eV to 3.92 eV with the change in the sputtering power.

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

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 >1200 eV. 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 5.04 ms; 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