Hong-Gyu Park

Vertically aligned liquid crystals on a γ-Al2O3 alignment film using ion-beam irradiation

Using ion-beam (IB) irradiation, liquid crystals (LCs) were vertically aligned (VA) on a γ-Al2O3 alignment film. Atomic-layer deposition was used to orient the LCs on high-quality γ-Al2O3 alignment films. The LC molecule orientation indicates the vertical direction of the atomic-layer-deposited γ-Al2O3 alignment films. X-ray photoelectron spectroscopy showed that IB irradiation changed the chemical structure, shifting the Al–O binding energy and altering the Al–O bonding intensity. The low-voltage transmittance characteristics of the VA LC displays on the γ-Al2O3 alignment films were also measured, showing reduced voltage and power requirements.

Enhancement of electro-optic properties in liquid crystal devices via titanium nanoparticle doping

We investigated the properties of nematic liquid crystal device (NLC) doped with titanium (Ti) nanoparticle (~100nm). The electro-optic (EO) properties of LCs changed according to Ti nanoparticle doping concentration. Ti nanoparticles in the NLC cells focused the electric field flux and strengthened the electric field. Further, Ti nanoparticles in NLC molecules may trap charged ionic impurities and suppress the screen effect, leading to a stronger electric field and the van der Waals dispersion interactions between NLC molecules and the alignment layers. We also simulated the boundary conditions of the Ti nanoparticles in the electric field using Ansoft Maxwell software. Our experimental results agreed with the phenomenon predicted by software simulation based on general physical theory. Synthetically, at a 1.0 wt. % Ti nanoparticle doping concentration, the NLC cells showed the best EO properties, such as a low threshold voltage (1.25V), fast response time (13.2ms), and low pretilt angle (3.90°).

Vertical alignment of liquid crystals on a fully oxidized HfO2 surface by ion bombardment

High-performance liquid crystals (LCs) driven at a 0.9 V threshold were demonstrated on very thin HfO2 films with vertical (homeotropic) alignment by ion bombardment. Atomic layer deposition was used to obtain LC orientation on ultrathin high-quality films of double-layer HfO2/Al2O3. X-ray photoelectron spectroscopy indicated that full oxidization of HfO2 film surfaces was induced by ion bombardment, shifting the Hf 4f spectra to lower binding energies. The increased intensities of the Hf 4f peaks after ion bombardment confirmed that nonstoichiometric HfOx was converted to the fully oxidized HfO2 surfaces.

High-performance ZnO thin-film transistor fabricated by atomic layer deposition

We report the fabrication and characteristics of a ZnO thin-film transistor (TFT) using a 50 nm thick ZnO film as an active layer on an Al2O3 gate dielectric film deposited by atomic layer deposition. Lowering the deposition temperature allowed the control of the carrier concentration of the active channel layer (ZnO film) in the TFT device. The ZnO TFT fabricated at 110 °C exhibited high-performance TFT characteristics including a saturation field-effect mobility of 11.86 cm2 V−1 s−1, an on-to-off current ratio of 3.09 × 107 and a sub-threshold gate-voltage swing of 0.72 V decade−1.

Tin dioxide inorganic nanolevel films with different liquid crystal molecular orientations for application in liquid crystal displays (LCDs)

We generated SnO2 alignment films with one of two liquid crystal (LC) molecular orientations: a stable, vertical orientation or a homogeneous orientation. We changed the molecular orientations by altering the growth temperature of radio-frequency (RF) magnetron sputtering. An ion-beam (IB) advanced system was used to achieve uniform alignment. We obtained two LC molecular orientations as demonstrated by the shift of the Sn 3d peaks and the altered-sub peaks of Sn2+, Sn4+ intensity on X-ray photoelectron spectroscopy (XPS) images of the various films. The electro-optical (EO) characteristics of the TN-LC and VA-LC displays on the high-k SnO2/HfO2 double films were measured; a reduction in threshold voltage, response time, and low power consumption were observed.

Homogeneous self-aligned liquid crystals on wrinkled-wall poly(dimethylsiloxane) via localised ion-beam irradiation

We demonstrate self-aligned liquid crystals (LCs) using a wrinkled-wall polydimethylsiloxane (PDMS) wrinkle structure, which is a key factor to obtain a stable homogeneous alignment state with positive LCs. We constructed the wrinkled walls via localised surface exposure to IB radiation, which passed through a long length localised pattern mask. The creation of the wrinkled wall helped to align the LC molecules homogeneously because the wrinkled wall acted as a guide for the arrangement of positive LC molecules. In addition, we confirmed the stability of the alignment state as the width of the wrinkled wall was changed. Although this wrinkled-wall method is a non-contact method, LC alignment is achieved via an anisotropic topographical guide, which provides the LC molecules with stable homogeneous alignment.

No-Bias-Bend Liquid-Crystal Display with an Intermediate Angle on Blended Polyimide via Ion-Beam Irradiation

We created a no-bias-bend (NBB) pi cell on a blended polyimide (PI) layer with intermediate pretilt angles using the ion-beam method. Various pretilt angles were easily obtained depending on the concentration of homeotropic PI. The plot of surface tension corresponded to the variation of the pretilt angle. We also observed a uniform alignment of the liquid crystals on the blended PI layer. The NBB pi cell had a low threshold voltage of 0.9 V and a fast response time of 5.4 ms.

Superior electro-optic properties of liquid crystal system using cobalt oxide nanoparticle dispersion

In this article, we study the electro-optical (EO) properties of the homogeneous aligned nematic liquid crystal (N-LC) doped with cobalt oxide (Co3O4) nanoparticles (NPs). The EO characteristics of Co3O4 doped N-LC are higher performance, indicating lower threshold voltage (1.33 V), faster rising time (1.479 ms), and faster falling time (9.343 ms) than pure liquid crystal (LC) cells. We have demonstrated these results by investigating the relationship between dielectric constants and LC device properties. Furthermore, we proved NPs doped N-LC cells drive low power operation without capacitance hysteresis. Our experimental results were verified by software simulation based on general physical properties.

Superior fast switching of liquid crystal devices using graphene quantum dots

We report the doping effect of graphene quantum dots (QDs) in a nematic liquid crystal (NLC) device. The use of graphene QDs leads to improved electro-optical (EO) properties, such as faster response times and a decreased threshold voltage. A 14.8% reduction in the threshold voltage (3.165 V), a 30% decrease in the falling time (4.9 ms) and a 63.6% decrease in the rising time (2.4 ms) were observed for twisted nematic (TN) LC devices (LCDs) that were doped with QDs. We confirm that the doping of the TN-LCD with graphene QDs reduces the field-screening effect, which is caused by impurity ions, whereas the other properties of conventional TN-LCDs are maintained.

Control of the wrinkle structure on surface-reformed poly(dimethylsiloxane) via ion-beam bombardment

We investigated the surface reformation of poly(dimethylsiloxane) (PDMS) elastomers by means of ion beam bombardment for fabricating wrinkle structures. Oxidation on the PDMS surface formed a silica-like outer layer that interacted with the inner PDMS layer, leading to the formation of wrinkle structures that minimized the combined bending energy of the outer layer and stretching energy of the inner layer. In addition, we controlled the amplitude and period of the wrinkle structures by adjusting the PDMS annealing temperature. As the PDMS annealing temperature was increased, the amplitude and period of the wrinkles formed by IB irradiation changed from 604.35 to 69.01 nm and from 3.07 to 0.80 μm, respectively.