Sungjun Choi

Structural transition of crystalline Y2O3 film on Si(111) with substrate temperature

Abstract Crystalline Y 2 O 3 films on Si(111) were grown by ionized cluster beam (ICB) deposition in an ultra high vacuum (UHV). The crystallinity of the films deposited at several different temperatures was studied using X-ray diffraction (XRD) and reflection of high-energy electron diffraction (RHEED), and the chemical states of the films was investigated using X-ray photoelectron spectroscopy (XPS). The transformation from monoclinic to cubic structure was observed upon the increase of the substrate temperature from 100°C to 500°C. The single crystal cubic structure was obtained at substrate temperatures over 500°C. The stoichiometry and binding state in the films were gradually changed to a cubic Y 2 O 3 structure with the increase of the substrate temperature. The transformation of the film structure from a monoclinic structure to a cubic structure was also observed by post annealing treatment in an oxygen ambient.

HYDROPHILIC GROUP FORMATION ON HYDROCARBON POLYPROPYLENE AND POLYSTYRENE BY ION-ASSISTED REACTION IN AN O2 ENVIRONMENT

Abstract Polystyrene (PS) and polypropylene (PP) films were modified by ion-assisted reaction in order to understand chemical reaction between the polymer and the O2 gas during low-energy Ar+ ion irradiation and to improve wettability of the polymers to water. The ion dose of Ar+, the ion beam energy, and the oxygen gas flow rate were changed from 5×1014 to 1×1017/cm2, from 0 to 1.2 keV and from 0 to 8 ml/min, respectively. Contact angles of water on polymers modified by Ar+ ion irradiation without blowing oxygen gas changed from 76° to around 40°, but those of water on polypropylene and polystyrene modified with blowing oxygen gas drop to 22° and 19°, respectively. X-ray photoelectron spectroscopy analysis shows that the hydrophilic groups were formed on the surface of polymers by chemical reaction between the unstable chains induced by the ion irradiation and the blown oxygen gas, and the hydrophilic groups were identified as CO bond, (CO) bond and (CO)O bond. The contact angle of water on polymers depends on the hydrophilic groups formed on the polymer surface by the ion-assisted reaction.

Catalytic behavior of metal catalysts in high-temperature RWGS reaction: In-situ FT-IR experiments and first-principles calculations

High-temperature chemical reactions are ubiquitous in (electro) chemical applications designed to meet the growing demands of environmental and energy protection. However, the fundamental understanding and optimization of such reactions are great challenges because they are hampered by the spontaneous, dynamic, and high-temperature conditions. Here, we investigated the roles of metal catalysts (Pd, Ni, Cu, and Ag) in the high-temperature reverse water-gas shift (RWGS) reaction using in-situ surface analyses and density functional theory (DFT) calculations. Catalysts were prepared by the deposition-precipitation method with urea hydrolysis and freeze-drying. Most metals show a maximum catalytic activity during the RWGS reaction (reaching the thermodynamic conversion limit) with formate groups as an intermediate adsorbed species, while Ag metal has limited activity with the carbonate species on its surface. According to DFT calculations, such carbonate groups result from the suppressed dissociation and adsorption of hydrogen on the Ag surface, which is in good agreement with the experimental RWGS results.

Effects of Cu seeding layer on Si grown by partially ionized beam in plasma enhanced chemical vapor deposition

Plasma enhanced chemical vapor deposition of the Cu films on Si substrate was investigated in which 30 A Cu-seed layer on the substrate was formed by partially ionized beam prior to deposition. In order to elucidate the difference in growth mechanism of Cu film between on Cu-seed layer and on bare Si, the initial stage of Cu-seed layer grown by partially ionized beam was studied by transmission electron microscopy and different nucleation formation processes from conventional method was shown. A high deposition rate and an improved adhesion strength were achieved when thick Cu film on the Cu seeded Si substrate was deposited by plasma enhanced chemical vapor deposition.

Collateral hydrogenation over proton-conducting Ni/BaZr0.85Y0.15O3−δ catalysts for promoting CO2 methanation

Despite the importance of CO2 methanation for eco-friendly carbon-neutral fuel recycling, the current technologies, relying on catalytic hydrogenation over metal-based catalysts, face technological and economical limitations. Herein, we employ the steam hydrogenation capability of proton conductors to achieve collateral CO2 methanation over the Ni/BaZr0.85Y0.15O3−δ catalyst, which is shown to outperform its conventional Ni/Al2O3 counterpart in terms of CH4 yield (8% higher) and long-term stability (3% higher for 150 h) at 400 °C while exhibiting a CH4 selectivity above 98%. Moreover, infrared and X-ray photoelectron spectroscopy analyses reveal the appearance of distinct mobile proton-related OH bands during the methanation reaction.

Thermally Induced S-Sublattice Transition of Li3PS4 for Fast Lithium-Ion Conduction

The ion-transport phenomenon, determined by the interaction of strain and electrostatic energy, is one of the most important examples that confirms the effects of the polymorphism and atomic morphology. We investigated the correlation between the structural morphology and Li-ion conduction characteristics in α-Li3PS4, a high-temperature phase of the Li3PS4, using ab initio molecular dynamics (AIMD) calculations. We successfully reproduced the thermal disorder and partial occupancy observed at high temperatures by AIMD and confirmed the Li-ion sites and its migration pathways. The activation energy and Li-ion conductivity of α-Li3PS4 at room temperature were predicted to be about 0.18 eV and 80 mS cm-1, respectively, indicating that α-Li3PS4 is one of the fastest Li-ion conductors known so far. The fast Li-ion conduction in α-Li3PS4 is mainly caused by the BCC S-sublattice and tetrahedron-tetrahedron pathway with fully occupied Li-ion sites. Therefore, α-Li3PS4 having a BCC S-sublattice offers a promising structural morphology for effective Li-ion conduction.

Configuring PSx tetrahedral clusters in Li-excess Li7P3S11 solid electrolyte

We demonstrate that the Li-ion conductivity can be improved by adding a certain amount of Li (x = 0.25–0.5) as a charge carrier to the composition of glass-ceramic Li7+xP3S11. Structural analysis clarified that the structural changes caused by the ratio of ortho-thiophosphate tetrahedra PS43− and pyro-thiophosphate ditetrahedra P2S74− affect the Li-ion conductivity. The ratio of PS43− and P2S74− varies depending on x and the highest Li-ion conductivity (2.5 × 10−3 S cm−1) at x = 0.25. All-solid-state LiNi0.8Co0.15Al0.05O2/Li7.25P3S11/In-metal cell exhibits the discharge capacity of 106.2 mAh g−1. This ion conduction enhancement from excess Li is expected to contribute to the future design of sulfide-type electrolytes.

Quantitative Analysis of Microstructures and Reaction Interfaces on Composite Cathodes in All-Solid-State Batteries Using a Three-Dimensional Reconstruction Technique

The composite cathode of an all-solid-state battery composed of various solid-state components requires a dense microstructure and a highly percolated solid-state interface different from that of a conventional liquid-electrolyte-based Li-ion battery. Indeed, the preparation of such a system is particularly challenging. In this study, quantitative analyses of composite cathodes by three-dimensional reconstruction analysis were performed beyond the existing qualitative analysis, and their microstructures and reaction interfaces were successfully analyzed. Interestingly, various quantitative values of structure properties (such as the volume ratio, connectivity, tortuosity, and pore formation) associated with material optimization and process development were predicted, and they were found to result in limited electrochemical charge/discharge performances. We also verified that the effective two-phase boundaries were significantly suppressed to ∼23% of the total volume because of component dispersion and packing issues.

Strain-Induced Tailoring of Oxygen-Ion Transport in Highly Doped CeO2 Electrolyte: Effects of Biaxial Extrinsic and Local Lattice Strain

We explored oxygen-ion transport in highly doped CeO2 through density-functional theory calculations. By applying biaxial strain to 18.75 mol % CeO2:Gd, we predicted the average migration-barrier energy with six different pathways, with results in good agreement with those of experiments. Additionally, we found that the migration-barrier energy could be lowered by increasing the tetrahedron volume, including the space occupied by the oxygen vacancy. Our results indicate that the tetrahedron volume can be expanded by larger codopants, as well as biaxial tensile strain. Thus, the combination of thin-film structure and codoping could offer a new approach to accelerate oxygen-ion transport.

Ion Assisted Reaction In Polymer And Ceramics

Ion assisted reaction (IAR), which was firstly presented in 1995 MRS Fall meeting, has been reviewed for the surface modifications of polymer and ceramics. The reaction is assisted by energetic ions from 0.5 to 1.5 keV, doses 10 14 to 10 17 ions/cm 2 , and blowing rate of oxygen 0 ∼ 8 ml/min. Hydrophilic surfaces of polymers (wetting angle ° and surface energy 60 ∼ 70 erg/cm 2 ) have been accomplished by the reaction, and an improvement of wettability and an increment of the surface energy are mainly due to the polar force and hydrophilic functional groups such as C=O, (C=O)-O, C-O, etc., without surface damage. The IAR was also applied on aluminum nitride in an O 2 environment and AMON on AIN is formed by the Ar + irradiation. The improvement of bond strength of Cu films on the AIN surface resulted from the interface bonds between Cu and the surface layers. Comparisons between the conventional surface treatments and the IAR are described in terms of physical bombardment, surface damage, functional group, and chain mobility in polymer.