Gyu Jin Choi

Polarized Light‐Emitting Diodes Based on Patterned MoS2 Nanosheet Hole Transport Layer

Here, this study successfully fabricates few-layer MoS2 nanosheets from (NH4 )2 MoS4 and applies them as the hole transport layer as well as the template for highly polarized organic light-emitting diodes (OLEDs). The obtained material consists of polycrystalline MoS2 nanosheets with thicknesses of 2 nm. The MoS2 nanosheets are patterned by rubbing/ion-beam treatment. The Raman spectra shows that {poly(9,9-dioctylfluorene-alt-benzothiadiazole), poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]} (F8BT) on patterned MoS2 exhibits distinctive polarization behavior. It is discovered that patterned MoS2 not only improves the device efficiency but also changes the polarization behavior of the devices owing to the alignment of F8BT. This work demonstrates a highly efficient polarized OLED with a polarization ratio of 62.5:1 in the emission spectrum (166.7:1 at the peak intensity of 540 nm), which meets the manufacturing requirement. In addition, the use of patterned MoS2 nanosheets not only tunes the polarization of the OLEDs but also dramatically improves the device performance as compared with that of devices using untreated MoS2 .

Infrared shutter using cholesteric liquid crystal.

In this paper, we propose an infrared light shutter device using cholesteric liquid crystals. The pitch of the device corresponds to the wavelengths of the infrared region with a strong thermal effect. This device is intended for use as a smart window to maintain an optimal indoor temperature by controlling the infrared radiation coming from the sun. The proposed cholesteric device switches between the planar state and the isotropic state by controlling the temperature using an electrically heated transparent electrode made of indium tin oxide. A window with a planar state that reflects infrared radiation would be used mainly in the summer, while the isotropic state that transmits infrared would be applied in the winter. The proposed device produced a variety of gray levels of transmittance based on the temperature, and thus it can provide the proper temperature for each user. The easy fabrication process gives it appeal as a functional device in the smart window market, and it compares favorably with previous light shutter devices. The infrared shutter is expected to be useful for next-generation window applications.

Optical design for single-mode and single-cell gap transflective liquid crystal displays

Generally, for transflective liquid crystal displays with different modes and different cell gaps between the refractive and transmissive parts, precise process control to pattern the electrode and match the cell gaps may reduce the yield and thus, require high cost. This paper proposes a simple transflective liquid crystal display with a single-mode and single-cell thickness without a patterned electrode to achieve better productivity. The proposed transflective liquid crystal display consists of three half-wave retardation films, two quarter-wave retardation films, and an LC layer, whose optical performance was confirmed by both simulation and experiment. The optimal optical configuration to obtain an excellent dark state in the visible range was determined by the Mueller matrices calculus, which was applied to each optical component. The calculated and experimental results showed that the proposed transflective LC structure has excellent electro-optical properties and is expected to have many liquid crystal display applications.

Optical Design for Reflective Liquid Crystal Displays

An optical design for a reflective LCD with a thicker cell gap that can have relatively higher productivity was examined theoretically. The proposed LCD was characterized by the optical axes of a half-wave retardation film, quarter-wave retardation film, and LC layer, which are 15°, −15° and 75° with the transmission axis of the polarizer, respectively. The LCs at the dark state lie down on the surface to produce half-wave retardation, whereas they rise up to produce quarter-wave retardation under an electric field in the white state. The proposed optical structure for the reflective LCD shows good dispersion characteristics at both the dark and white states.

Polarization Characteristics of an Organic Light-Emitting Diode with an Ion-Beam-Treated MoS

In this study, we present the polarization property of an organic light-emitting diode characterized by a low-energy and high-linearity ion-beam-treated MoS2 hole-transport layer deposited by using radio-frequency sputtering in order to obtain highly-polarized light. Poly (9,9-dioctylfluorene-altbenzothiadiazole), which exhibits a liquid crystal phase at a temperature higher than 180 ◦C, was used as the light-emitting material. The polarization characteristics of the organic light-emitting device fabricated under various ion-beam irradiation conditions were examined through photoluminescence experiments. The results showed that an optimal polarization ratio of 11:1 was obtained for a ion-beam-irradiated angle of 30 degrees, an irradiation time of 30 seconds, a current density of 100 μA/cm, and an ion-beam energy of 300 eV. When the optimized organic light-emitting diode was used as a backlight for a twisted nematic liquid-crystal display, its transmittance was improved by about 25% or more.

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TiO2 has displayed potential for improved catalytic activity using a newly designed morphological system. Herein, a new catalytic system of SiO2/ITO/TiO2 nanocone array structures with unbalanced RF sputtering deposition was developed and demonstrated for photoelectrocatalytic activity. The array structures were fabricated using a soft nanolithography technique with a SiO2 sphere-patterned polydimethylsiloxane stamp. TiO2 films were coated on the ITO-coated SiO2 nanocones using unbalanced RF magnetron sputtering with various external magnetic fields applied by a solenoid. The nanostructures were examined using scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction crystallography, and X-ray photoelectron spectroscopy. The photocurrent response in water splitting was found to be increased by 400% for TiO2 films on nano-patterned arrays, compared with that on a flat surface. An additional increase in photocatalytic activity was achieved via TiO2 film deposition with an external magnetic field of 6 mT. This dramatic enhancement was attributed to the surface active sites created by the additional applied magnetic field and the increased surface area of the nanocone arrays.

Hole Transport Layer

Visual evaluation of the surface anchoring energies in a nematic liquid crystal (LC) cell is characterized by the direction of the convection roll pattern that appears in the low-frequency conduction regime. The convection roll pattern in a twisted nematic LC (TNLC) cell is oriented perpendicular to the midplane LC director dominating the direction of convection flow, and its direction is determined by the relative surface anchoring energy between two surface boundaries. Thus the direction of the roll pattern generated at the TNLC cell with asymmetric LC alignment layers can provide information on the surface anchoring energies at the two boundaries. We demonstrate a method for determining the two anchoring energies through a measured midplane LC director applied to the Ericksen-Leslie equation.

Photoelectrocatalytic effect of unbalanced RF magnetron sputtered TiO2 thin film on ITO-coated patterned SiO2 nanocone arrays

The twisted nematic liquid crystal cell was developed by using a CYTOP-transferred graphene sheet as an electrode and an alignment layer. A graphene layer was synthesized by chemical vapor deposition and transferred onto a plastic substrate using a fluoropolymer known as CYTOP. As the ion-beam treatment time increased, the sheet resistance increased from 500 to 1100 Ω/sq., while the water contact angle decreased from 110.5° to 69.7°. The increased intensities of the D and G′ bands and the appearance of D + D″ and D + G′ bands in the Raman spectra indicated the formation of defects because of the ion-beam treatment. An ion-beam exposure time of 15 s was found to be the most effective for the production of CYTOP-transferred graphene and for achieving high contrast in operating cells. The ion beam detached F from the CYTOP-transferred graphene layer, and the resulting exposure of the C=C bond on the graphene surface affected the alignment of liquid crystal molecules. Based on these results, the technique described here has applications in novel, high-performance liquid crystal displays that do not require indium-tin-oxide electrodes and polyimide alignment layers. Sheets synthesized by chemical vapor deposition were transferred and simultaneously doped using fluoropolymer supporting layers.

Visual evaluation of surface anchoring strength by electrohydrodynamic convection of a nematic liquid crystal

We present a normally black patterned electrically controlled birefringence [NB-PECB] mode with patterned electrode structure to improve the low contrast ratio of the general electrically controlled birefringence [ECB] mode and the gray inversion of twist nematic mode [TN]. The electrode structure used in the patterned vertical alignment [PVA] electro-optic mode is adopted in our proposed NB-PECB mode to produce two domains that are required for improving viewing angle uniformity. A λ/2 plate is also used to generate normally black state. Simulated results show the proposed mode can improve the gray inversion and the contrast ratio, compared with the two domain TN mode and the two domain ECB mode.

Ion-beam-irradiated CYTOP-transferred graphene for liquid crystal cells

Korea Conformity Laboratories, 199, Gasandigital 1 ro, Geumcheon-gu, Seoul 153-803, Korea(Received October 16, 2014 ; revised October 28, 2014 ; accepted October 28, 2014)Abstract In this paper, in order to achieve slim and light liquid crystal display, we examine the optical conditionsthat can obtain uniform light with higher optical efficiency over whole light guide plate (LGP) through sim-ulation. Furthermore, to overcome the issues of hot spot in front of red, green, and blue light emitting diodes(RGB LEDs) source and non-uniform color mixing, we propose four shaped color mixing bars tied up withthe LGP and check the optical characteristics of the LGP with them by simulation. Consequently, we couldknow the optical conditions of improving optical efficiency and optical uniformity in the LGP through theoptical design. Also we confirmed that the issues of the hot spot and non-uniform color mixing in edgetype LED could be solved by using the ∧-shaped window color mixing bar.