Sungho Choi

Phase studies of SrOAl2O3 by emission signatures of Eu2+ and Eu3+

Phase transformation sequences of SrOAl2O3 have been investigated at 1000 and 1250 °C through XRD technique along with emission signatures of Eu2+ and Eu3+. Two phases, i.e., [Sr3Al2O6] and [SrAl2O4], have been observed at 1000 °C as stable phases. These phases were found to be transformed into [SrAl2O4] at 1250 °C.

Tunable Color Emission in a Zn1−xCdxGa2O4 Phosphor and Solid Solubility of CdGa2O4 in ZnGa2O4

Abstract Tunable color emission was achieved through forming solid solution between ZnGa 2 O 4 and CdGa 2 O 4 , i.e., Zn 1−x Cd x Ga 2 O 4 . The solid solubility limit of CdGa 2 O 4 in ZnGa 2 O 4 was found to be 0.6 mole fraction. Thus, solid solution can exist in 0 ≤ x ≤ 0.6 compositions in the Zn 1−x Cd x Ga 2 O 4 . Emission color was tunable between 352 and 520 nm in Zn 1−x Cd x Ga 2 O 4 (0 ≤ x ≤ 0.6). Our approach has demonstrated an innovative way of tuning color through the formation of solid solution between CdGa 2 O 4 and ZnGa 2 O 4

Polyethylenimine-Mediated Electrostatic Assembly of MnO2 Nanorods on Graphene Oxides for Use as Anodes in Lithium-Ion Batteries.

In recent years, the development of electrochemically active materials with excellent lithium storage capacity has attracted tremendous attention for application in high-performance lithium-ion batteries. MnO2-based composite materials have been recognized as one of promising candidates owing to their high theoretical capacity and cost-effectiveness. In this study, a previously unrecognized chemical method is proposed to induce intra-stacked assembly from MnO2 nanorods and graphene oxide (GO), which is incorporated as an electrically conductive medium and a structural template, through polyethylenimine (PEI)-derived electrostatic modulation between both constituent materials. It is revealed that PEI, a cationic polyelectrolyte, is capable of effectively forming hierarchical, two-dimensional MnO2-RGO composites, enabling highly reversible capacities of 880, 770, 630, and 460 mA·h/g at current densities of 0.1, 1, 3, and 5 A/g, respectively. The role of PEI in electrostatically assembled composite materials is clarified through electrochemical impedance spectroscopy-based comparative analysis.

Superior Lithium Storage Performance using Sequentially Stacked MnO2/Reduced Graphene Oxide Composite Electrodes

Hybrid nanostructures based on graphene and metal oxides hold great potential for use in high-performance electrode materials for next-generation lithium-ion batteries. Herein, a new strategy to fabricate sequentially stacked α-MnO2 /reduced graphene oxide composites driven by surface-charge-induced mutual electrostatic interactions is proposed. The resultant composite anode exhibits an excellent reversible charge/discharge capacity as high as 1100 mA h g(-1) without any traceable capacity fading, even after 100 cycles, which leads to a high rate capability electrode performance for lithium ion batteries. Thus, the proposed synthetic procedures guarantee a synergistic effect of multidimensional nanoscale media between one (metal oxide nanowire) and two dimensions (graphene sheet) for superior energy-storage electrodes.

Electro optical performance characteristic of in-plane switching cell treated on nitrogen-doped diamond-like carbon thin film surfaces by ion beam alignment

The nematic liquid crystal (NLC) alignment capability achieved by ion beam alignment on a nitrogen-doped diamond-like carbon (NDLC) thin film was investigated. To characterize the NDLC thin film, we analyzed the obtained atomic force microscopy (AFM) images and pretilt angle. Moreover, we investigated the electro optical (EO) performance characteristics of ion-beam-aligned in-plane switching (IPS) cell with the NDLC thin film. The EO performance characteristics of the ion-beam-aligned IPS cell were observed by ion beam exposure on the NDLC thin film and compared with those of a rubbing aligned polyimide (PI) thin film. Finally, similarly prominent capacitance–voltage (C–V) characteristics were achieved in the ion-beam-aligned NDLC thin film compared with the rubbing-aligned PI thin film.

3D-stacked carbon composites employing networked electrical intra-pathways for direct-printable, extremely stretchable conductors.

The newly designed materials for stretchable conductors meeting the demands for both electrical and mechanical stability upon morphological elongation have recently been of paramount interest in the applications of stretchable, wearable electronics. To date, carbon nanotube-elastomeric polymer mixtures have been mainly developed; however, the method of preparing such CNT-polymer mixtures as stretchable conductors has been limited to an ionic liquid-mediated approach. In this study, we suggest a simple wet-chemical method for producing newly designed, three-dimensionally stacked carbon composite materials that facilitate the stable morphological elongation up to a strain of 300% with normalized conductivity variation of only 0.34 under a strain of 300%. Through a comparative study with other control samples, it is demonstrated that the intraconnected electrical pathways in hierarchically structured composite materials enable the generation of highly stretchable conductors. Their direct patternability is also evaluated by printing on demand using a programmable disperser without the use of prepatterned masks.

In situ synthesis of amorphous RuO2/AZO as a carbon-free cathode material for Li–O2 batteries

The composite of amorphous RuO2 as an electrocatalyst and aluminum-doped ZnO (AZO) as a cathode material was synthesised and developed into a carbon-free cathode material for Li–O2 batteries for the first time. The amorphous RuO2/AZO carbon-free cathode exhibits a noticeably reduced overpotential as well as an enhanced specific capacity.

Electro-Optical Characteristics of Vertical Alignment Cell by Ion-Beam Exposure on the SiC Thin Film Layer

We studied the electro-optical (EO) characteristics and nematic liquid crystal (NLC) aligning capabilities with the new alignment material of the Silicon Carbide (SiC) thin film using two kinds of ion beam (IB) gun. The SiC thin film exhibits good chemical and thermal stability. A vertical alignment of nematic liquid crystal by atomic beam exposure on the SiC thin film surface was achieved via all IB exposure system. The generated NLC tilt angle is about 87° using Kaufman type ion gun. The about 88° of stable tilt angle was achieved with incident angle of 10° using duoPIGatron type ion gun. An excellent voltage-transmittance (V-T) and response time curve of the IB-aligned VA-LCD using duoPIGatron type ion gun was observed. The V-T hysteresis characteristics of IB-aligned VA-LCD are almost the same as that of the rubbing-aligned VA-LCD. Consequently, the vertical alignment effect of liquid crystal and the good EO characteristics by the atomic beam alignment method on the SiC thin film layer can be achieved.

Solution Processed High-k Lanthanide Oxides for Low Voltage Driven Transparent Oxide Semiconductor Thin Film Transistors

Thin film transistor (TFT) gate dielectrics must be smooth, dense free from pinholes and charge-trapping defects and exhibit low-leakage current under applied bias. Certainly the gate dielectric having the high dielectric constant (high-k) need for the low operating voltage and high field effect achievement at the recent semiconductor TFTs. Given recent advances in vacuum-deposited oxide semiconductors, stable and complementary solution-processed all-oxide devices offer a new approach, provided suitable deposition chemistries are developed. An operational solution-processed oxide gate dielectric represents a vital part of this vision, and the stringent requirements on film quality and electrical characteristics make it a challenging target and key development milestone. Recently the lanthanide oxides have attracted much attention to replace SiO2 as the potential candidate material for high-k gate dielectrics. They have many advantages such as high capacitance with low leakage current and very good thermodynamic stability with respect to various substrates. In this work, we have investigated the microstructure and dielectric properties such as dielectric constant, leakage current and breakdown voltage characteristics of a solution-processed lanthanide oxides; Y2O3 and Gd2O3 thin films. Moreover the switching property of a bottomgate structured TFTs combined with the Zn-Sn-O (ZTO) channel layer was also examined to confirm the expected performance of high-k gate insulators. Lanthanide oxide thin-film was formed using the solgel method with the precursor solution containing Gd(NO3)3·6H2O, (or Y(NO3)3·6H2O) 2-methoxyethanol, and polyvinylpyrrolidone (PVP). Thin films were prepared by depositing precursor solutions on ITO/glass substrate using spin casting method followed by annealing at 400 in air. The resultant films are semicrystalline phase with homogeneous, smooth and highly transparent under visible wavelength. The relative dielectric constant is 12 to 10 kHz where leakage current density equals 1x10 A/cm at 1.0 MV. The crosssectional TEM image of the solution processed Gd2O3 film is presented in the Fig. 1. The film has smooth and dense cross-sectional image. It can be seen that the overall film thickness is about 100 nm within our synthetic conditions. The electron diffraction patterns is represented on inset image. The rings correspond to the lattice spacings of the cubic phase Gd2O3 (JCPDS #12-0797). We fabricated ZTO TFTs, also grown by spin casting on gate dielectric of Y2O3 and Gd2O3. The corresponding output and transfer curves are presented in Fig. 2. Devices on Gd2O3 had a mobility of 1.09 cm/Vs with a threshold voltage of 6.8 V and subthreshold slope of 0.85 V/decade, while those of using Y2O3 layer were 5.66 cm/Vs, 2.1 V and 1.7 V/decade. (Note that the device property of ZTO/SiO2(100nm) TFT were 0.29 cm/Vs, 32.9 V and 4.4 V/decade) The overall trends show that device with high capacitance oxide, especially on Gd2O3, were properly working within low power consumption. Both the low voltage-driven switching property and the gradual increasing of semiconductor channel mobility with high-k dielectrics were presumably because of an improved interface. Device performance based on these high-k materials can lead to marked improvements in solution-based amorphous oxide TFTs for large area device.

Understanding the Critical Role of the Ag Nanophase in Boosting the Initial Reversibility of Transition Metal Oxide Anodes for Lithium-Ion Batteries

The initial reversible capacity, a critical impediment in transition metal oxide-based anodes, is augmented in conversion-reaction-involved CoO anodes for lithium-ion batteries, by incorporating a chemically synthesized Ag nanophase. With an increase in the added amount of Ag nanophase from 5 to 15 wt %, the initial capacity loss decreases linearly up to 31.7%. The Ag nanophase maintains its pristine metallic nature without undergoing phase transformations, even during repeated vigorous electrochemical reactions of the active CoO phase. Complementary ex situ chemical/physical analyses suggest that the Ag nanophase promotes the catalytic generation of reversible gel-like/polymeric films wherein lithium ions are stored capacitively in the low-voltage region below 0.7 V during discharging. These scientific findings would provide a heretofore unrecognized pathway to resolving a major issue associated with the critical irreversibility in conversion-type transition metal oxide anodes.