Jung-Hae Choi - Academia.edu (original) (raw)

Papers by Jung-Hae Choi

Research paper thumbnail of Equilibrium Shape of Internal Cavities in Sapphire

Journal of the American Ceramic Society, 1997

Page 1. J. Am. Ceram. Soc., 80 [1] 62–68 (1997) Equilibrium Shape of Internal Cavities in Sapphir... more Page 1. J. Am. Ceram. Soc., 80 [1] 62–68 (1997) Equilibrium Shape of Internal Cavities in Sapphire Jung-Hae Choi* and Doh-Yeon Kim* Department of Inorganic Materials Engineering, Seoul National University, Seoul, 151-742, Korea ...

Research paper thumbnail of Equilibrium Shape of Internal Cavities in Ruby and the Effect of Surface Energy Anisotropy on the Equilibrium Shape

Journal of the American Ceramic Society, 2004

Cavities formed in ruby (99.46Al 2 O 3 ⅐0.54Cr 2 O 3 ) by the healing and annealing of indentatio... more Cavities formed in ruby (99.46Al 2 O 3 ⅐0.54Cr 2 O 3 ) by the healing and annealing of indentation cracks at 1600°C are more equiaxed than similar cavities in sapphire. Surface energies for the observed facet planes (R, S, and A) relative to the surface energy of the basal plane, C, were ␥ A/C ‫؍‬ 1.00 ؎ 0.03, ␥ R/C ‫؍‬ 1.05 ؎ 0.07, and ␥ S/C ‫؍‬ 1.02 ؎ 0.04, with the uncertainty representing 95% confidence limits. Thus, the surface energies of all observed facets were statistically indistinguishable. Unlike sapphire, P-plane facets were not observed. The substantial rounding of the cavities in ruby indicated that portions of the Wulff shape were above the roughening transition temperature. Thus, even though Cr 2 O 3 and Al 2 O 3 form ideal solutions, Cr 3؉ ions are sufficiently surface active to modify the relative free energy of the surfaces.

Research paper thumbnail of Alternative interpretations for decreasing voltage with increasing charge in ferroelectric capacitors

Scientific reports, 2016

Recent claim on the direct observation of a negative capacitance (NC) effect from a single layer ... more Recent claim on the direct observation of a negative capacitance (NC) effect from a single layer epitaxial Pb(Zr0.2,Ti0.8)O3 (PZT) thin film was carefully reexamined, and alternative interpretations that can explain the experimental results without invoking the NC effect are provided. Any actual ferroelectric capacitor has an interfacial layer, and experiment always measures the sum of voltages across the interface layer and the ferroelectric layer. The main observation of decreasing ferroelectric capacitor voltage (VF) for increasing ferroelectric capacitor charge (QF), claimed to be the direct evidence for the NC effect, could be alternatively interpreted by either the sudden increase in the positive capacitance of a ferroelectric capacitor or decrease in the voltage across the interfacial layer due to resistance degradation. The experimental time-transient VF and QF could be precisely simulated by these alternative models that fundamentally assumes the reverse domain nucleation a...

Research paper thumbnail of Resistance switching behavior of atomic layer deposited SrTiO3 film through possible formation of Sr2Ti6O13 or Sr1Ti11O20 phases

Scientific reports, 2016

Identification of microstructural evolution of nanoscale conducting phase, such as conducting fil... more Identification of microstructural evolution of nanoscale conducting phase, such as conducting filament (CF), in many resistance switching (RS) devices is a crucial factor to unambiguously understand the electrical behaviours of the RS-based electronic devices. Among the diverse RS material systems, oxide-based redox system comprises the major category of these intriguing electronic devices, where the local, along both lateral and vertical directions of thin films, changes in oxygen chemistry has been suggested to be the main RS mechanism. However, there are systems which involve distinctive crystallographic phases as CF; the Magnéli phase in TiO2 is one of the very well-known examples. The current research reports the possible presence of distinctive local conducting phase in atomic layer deposited SrTiO3 RS thin film. The conducting phase was identified through extensive transmission electron microscopy studies, which indicated that oxygen-deficient Sr2Ti6O13 or Sr1Ti11O20 phase wa...

Research paper thumbnail of Effect of oxygen vacancy on the structural and electronic characteristics of crystalline Zn 2 SnO 4

J. Mater. Chem. C, 2014

ABSTRACT The effects of oxygen vacancies on the atomic and electronic structures in crystalline Z... more ABSTRACT The effects of oxygen vacancies on the atomic and electronic structures in crystalline Zn2SnO4 were examined by ab-initio calculations using screened hybrid density functional. The formation energy and the electronic structure indicated that the neutral oxygen vacancy was stable in the n-type region with forming deep level in the bandgap. The ionization energy of oxygen vacancy was calculated to be around 2.0 eV, which can be excited by visible photon energy. The atomic movements and the Bader charge analyses around the neutral oxygen vacancy showed that Sn played a dominant role in changing the electronic properties by forming the pair of Sn2+-VO0, which was accompanied by the reduction of the charge state of Sn due to its multiple oxidation states. On the contrary, the electronic state of Zn hardly changed despite of its more predominant atomic shifts than Sn around the oxygen vacancy.

Research paper thumbnail of Catalytic activity for oxygen reduction reaction on platinum-based core-shell nanoparticles: all-electron density functional theory

Nanoscale, 2015

Pt nanoparticles (NPs) in a proton exchange membrane fuel cell as a catalyst for an oxygen reduct... more Pt nanoparticles (NPs) in a proton exchange membrane fuel cell as a catalyst for an oxygen reduction reaction (ORR) fairly overbind oxygen and/or hydroxyl to their surfaces, causing a large overpotential and thus low catalytic activity. Realizing Pt-based core-shell NPs (CSNPs) is perhaps a workaround for the weak binding of oxygen and/or hydroxyl without a shortage of sufficient oxygen molecule dissociation on the surface. Towards the end, we theoretically examined the catalytic activity of NPs using density functional theory; each NP consists of one of 12 different 3d-5d transition metal cores (groups 8-11) and a Pt shell. The calculation results evidently suggest the enhancement of catalytic activity of CSNPs in particular when 3d transition metal cores are in use. The revealed trends in activity change upon the core metal were discussed with respect to the thermodynamic and electronic structural aspects of the NPs in comparison with the general d-band model. The disparity between the CSNP and the corresponding bilayer catalyst, which is the so-called size effect, was remarkable; therefore, it perhaps opens up the possibility of size-determined catalytic activity. Finally, the overpotential for all CSNPs was evaluated in an attempt to choose promising combinations of CSNP materials.

Research paper thumbnail of Growth, Quantitative Growth Analysis, and Applications of Graphene on γ-Al2O3 catalysts

Scientific reports, 2015

The possibilities offered by catalytic γ-Al2O3 substrates are explored, and the mechanism governi... more The possibilities offered by catalytic γ-Al2O3 substrates are explored, and the mechanism governing graphene formation thereon is elucidated using both numerical simulations and experiments. The growth scheme offers metal-free synthesis at low temperature, grain-size customization, large-area uniformity of electrical properties, single-step preparation of graphene/dielectric structures, and readily detachable graphene. We quantify based on thermodynamic principles the activation energies associated with graphene nucleation/growth on γ-Al2O3, verifying the low physical and chemical barriers. Importantly, we derive a universal equation governing the adsorption-based synthesis of graphene over a wide range of temperatures in both catalytic and spontaneous growth regimes. Experimental results support the equation, highlighting the catalytic function of γ-Al2O3 at low temperatures. The synthesized graphene is manually incorporated as a 'graphene sticker' into an ultrafast mode-lo...

Research paper thumbnail of Effect of surface hydroxyl coverage on platinum nanoparticles in the oxygen reduction reaction: All-electron density functional theory analysis

Chemical Physics Letters, 2014

ABSTRACT Catalysts in commercial proton exchange membrane fuel cells must be optimized for activi... more ABSTRACT Catalysts in commercial proton exchange membrane fuel cells must be optimized for activity and cost, but improvements are hampered for Pt nanoparticles by low activity resulting from extensive OH adsorption. A model system clearly elucidating the impact of OH coverage on Pt nanoparticles was therefore developed. Using first principles calculations, free energies due to changing OH coverage in the oxygen reduction reaction were predicted as functions of electrode potential and local surface structure. Free energy contour plots that considered surface OH coverage were developed. Therefrom, we theoretically predicted the optimum range of operating potentials, yielding design guidelines for catalytic surfaces.

Research paper thumbnail of First-principles study on the formation of a vacancy in Ge under biaxial compressive strain

Thin Solid Films, 2010

The effects of the biaxial compressive strain on the atomic relaxation and the formation energy o... more The effects of the biaxial compressive strain on the atomic relaxation and the formation energy of a neutral vacancy in Ge were investigated using first-principles calculations. Prior to this, the effects of the supercell size and Brillouin zone sampling were tested. The vacancy formation energy and atomic configuration around a vacancy are strongly affected by the inter-vacancy distance determined by

Research paper thumbnail of Variation in the threshold voltage of amorphous-In2Ga2ZnO7 thin-film transistors by ultrathin Al2O3 passivation layer

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2013

ABSTRACT

Research paper thumbnail of A case of primary intestinal T-cell lymphoma involving entire gastrointestinal tract: esophagus to rectum

The Korean journal of internal medicine, 2000

Primary intestinal T-cell lymphoma is a rare disease entity, which is approximately 10% to 25% of... more Primary intestinal T-cell lymphoma is a rare disease entity, which is approximately 10% to 25% of intestinal lymphomas, and most of the lymphomas occur in the small intestine. We report here a case of a 56-year-old woman who has been suffering from chronic diarrhea and weight loss for 6 months. Abdominal CT scan and small bowel series showed diffuse wall thickening of the small bowel. Gastroscopic examination showed diffuse erythematous lesions on the esophagus and small gastric ulcerations on the antrum of the stomach, and colonoscopic examination also showed multiple punched-out ulcerations and erosions on the entire colon, including the sigmoid colon to the terminal ileum. Diffuse infiltration of CD 3 positive lymphoma cells was found on biopsy. The patient was diagnosed as primary intestinal T-cell lymphoma with diffuse involvement of the entire gastrointestinal tracts from the esophagus to the rectum. Although the patient received systemic combination chemotherapy and achieved ...

Research paper thumbnail of Structure and mechanical properties of Ag-incorporated DLC films prepared by a hybrid ion beam deposition system

Thin Solid Films, 2007

Ag-incorporated diamond-like carbon films were prepared on Si(100) wafers using a hybrid depositi... more Ag-incorporated diamond-like carbon films were prepared on Si(100) wafers using a hybrid deposition system composed of an end-Hall-type hydrocarbon ion gun and a silver DC magnetron sputter source. The Ag concentration in the films was controlled by changing the fraction of Ar in the Ar and benzene reaction gas. The chemical composition, microstructure, atomic bond structure, and mechanical properties were investigated for Ag concentrations ranging from 0 to 9.7 at.%. When the Ag concentration was 0.1 at.%, the Ag atoms were fully dissolved in the amorphous carbon matrix without forming any second phase. Amorphous and crystalline silver particles appeared when the Ag concentration was 1.7 and N 6.8at.%, respectively. It was found that the hardness was not sensitive to the Ag concentration in this concentration range. On the other hand, the residual compressive stress was strongly dependent on the chemical state of the incorporated Ag atoms. The mechanical properties are discussed in terms of the changes in the microstructure and atomic bond structure induced by Ag incorporation.

Research paper thumbnail of Ab initio Calculations on the Atomic and Electronic Structures of Oxygen-Doped Hexagonal Ge$_{2}$Sb$_{2}$Te$_{5}$

Applied Physics Express, 2012

ABSTRACT The atomic configurations, energetics, and electronic structures of oxygen doped hexagon... more ABSTRACT The atomic configurations, energetics, and electronic structures of oxygen doped hexagonal Ge2Sb2Te5 were investigated by using ab-initio calculations. Oxygen atoms or molecules were added at the possible interstitial or substitutional sites. The formation energies showed that both oxygen atoms and molecules would be spontaneously incorporated into hexagonal Ge2Sb2Te5. Oxygen atoms prefer forming bonds with Ge and Sb than with Te. The doped oxygen molecule was immediately dissociated into two oxygen atoms. The spontaneous incorporation of oxygen and dissociation of oxygen molecules are distinctly opposite to the nitrogen doping behavior in hexagonal Ge2Sb2Te5.

Research paper thumbnail of Thermodynamic stability of various phases of zinc tin oxides from ab initio calculations

Journal of Materials Chemistry C, 2013

ABSTRACT Thermodynamic stabilities of various phases in ZnO–SnO2 systems were investigated based ... more ABSTRACT Thermodynamic stabilities of various phases in ZnO–SnO2 systems were investigated based on the Gibbs energy obtained from density functional theory (DFT) calculations. The pressure–temperature (p–T) phase diagram was determined; the coexistence of ZnO and SnO2 was the most stable phase in the low temperature region at zero external pressure, while Zn2SnO4 with the inverse spinel structure and ZnSnO3 with the lithium niobate structure were stable at the high temperature and high pressure region. Various octahedral configurations of the inverse spinel structures of Zn2SnO4 were considered. The calculated results showed feasible agreement with experimental data on the phase stability and explained well the experimental observation of the mixed state of Zn2SnO4, ZnO and SnO2 at mid-range temperatures and pressures. Considering the atomic structures, bulk moduli and thermodynamic stabilities, the local density approximation calculations were found to describe experimental observations more accurately than the generalized gradient approximation calculations. The phase transitions in the ZnO–SnO2 system were found to be dominated by the changes in both the Zn–O bond length and the coordination number of Zn, rather than changes in the bond length of Sn–O and the coordination number of Sn.

Research paper thumbnail of The Impact of Carbon Concentration on the Crystalline Phase and Dielectric Constant of Atomic Layer Deposited HfO2 Films on Ge Substrate

Ecs Journal of Solid State Science and Technology, 2012

ABSTRACT The effect of the carbon concentration on the crystalline phase and dielectric constant ... more ABSTRACT The effect of the carbon concentration on the crystalline phase and dielectric constant (k) of atomic layer deposited HfO2 films on Ge substrate was investigated. After annealing, the HfO2 films grown at 200 degrees C and 280 degrees C were crystallized to the tetragonal (t) and monoclinic (m) phases, respectively, which was related to the carbon contents within the films and grain boundary energy. To clarify this, the energy difference between a t- and a m-phases (Delta E-tetra) was calculated by first principles calculations. The higher k value of t-HfO2 compared to amorphous and monoclinic HfO2 was experimentally confirmed.

Research paper thumbnail of Strain evolution of each type of grains in poly-crystalline (Ba,Sr)TiO3 thin films grown by sputtering

Scientific Reports, 2012

The strain states of [111]-, [110]-, and [002]-oriented grains in poly-crystalline sputtered (Ba,... more The strain states of [111]-, [110]-, and [002]-oriented grains in poly-crystalline sputtered (Ba,Sr)TiO(3) thin films on highly [111]-oriented Pt electrode/Si substrates were carefully examined by X-ray diffraction techniques. Remarkably, [002]-oriented grains respond more while [110]- and [111]-oriented grains do less than the theoretically estimated responses, which is understandable from the arrangement of the TiO(6) octahedra with respect to the stress direction. Furthermore, such mechanical responses are completely independent of the degree of crystallization and film thickness. The transition growth temperature between the positive and negative strains was also different depending on the grain orientation. The unstrained lattice parameter for each type of grain was different suggesting that the oxygen vacancy concentration for each type of grain is different, too. The results reveal that polycrystalline (Ba,Sr)TiO(3) thin films are not an aggregation of differently oriented grains which simply follow the mechanical behavior of single crystal with different orientations.

Research paper thumbnail of Improved properties of Pt–HfO2 gate insulator–ZnO semiconductor thin film structure by annealing of ZnO layer

Thin Solid Films, 2010

Metal-insulator-semiconductor capacitors were fabricated with sputtered ZnO and atomic layer depo... more Metal-insulator-semiconductor capacitors were fabricated with sputtered ZnO and atomic layer deposited HfO2 as the semiconductor and gate dielectric layers, respectively. From the capacitance–voltage measurements, it was confirmed that pre-deposition annealing of the ...

Research paper thumbnail of First-Principles Study of the Interfaces between Fe and Transition Metal Carbides

The Journal of Physical Chemistry C, 2013

ABSTRACT The interface energies and electronic structures of the interfaces between BCC Fe and tr... more ABSTRACT The interface energies and electronic structures of the interfaces between BCC Fe and transition metal carbides have been investigated using first-principles calculations based on density functional theory. The effects of the composition and configuration of the carbides on the interface properties have been determined. It was shown that the Fe/TiC interface has the highest interface energy and the formation of complex carbides leads to a significant decrease in the interface energy. The complex carbide of (Ti0.5Mo0.5)C, which has Mo present at the interface, has been found to be the most stable. From the analysis of the density of states, the stability of the Mo-segregated (Ti0.5Mo0.5)C carbides has been revealed to be due to the hybridization of the d-orbitals in the t2g local symmetry between Mo and its first nearest neighboring Fe atoms.

Research paper thumbnail of Migration of nitrogen in hexagonal Ge2Sb2Te5: An ab-initio study

physica status solidi (RRL) - Rapid Research Letters, 2012

Kim, S.-J., Lee, J., Lee, S.-C., Park, C., Hwang, CS and Choi, J.-H.(2012), Migration of nitrogen... more Kim, S.-J., Lee, J., Lee, S.-C., Park, C., Hwang, CS and Choi, J.-H.(2012), Migration of nitrogen in hexagonal Ge 2 Sb 2 Te 5: An ab-initio study. physica status solidi (RRL)-Rapid Research Letters. doi: 10.1002/pssr. 201105516

Research paper thumbnail of Predictions on atomic structures of Ti1−xMoxC using combined approach of first-principles calculation and the cluster expansion method

Metals and Materials International, 2009

Recently, binary transition metal carbides such as (Ti1-xMox)C have attracted much attention for ... more Recently, binary transition metal carbides such as (Ti1-xMox)C have attracted much attention for use in increasing the strength of steels. This study aims to understand the ground state structure of various compositions of that carbide. Using a combined approach of first-principles calculation and the cluster expansion method, ground state structures of the compositions of TiC and MoC were determined. Only

Research paper thumbnail of Equilibrium Shape of Internal Cavities in Sapphire

Journal of the American Ceramic Society, 1997

Page 1. J. Am. Ceram. Soc., 80 [1] 62–68 (1997) Equilibrium Shape of Internal Cavities in Sapphir... more Page 1. J. Am. Ceram. Soc., 80 [1] 62–68 (1997) Equilibrium Shape of Internal Cavities in Sapphire Jung-Hae Choi* and Doh-Yeon Kim* Department of Inorganic Materials Engineering, Seoul National University, Seoul, 151-742, Korea ...

Research paper thumbnail of Equilibrium Shape of Internal Cavities in Ruby and the Effect of Surface Energy Anisotropy on the Equilibrium Shape

Journal of the American Ceramic Society, 2004

Cavities formed in ruby (99.46Al 2 O 3 ⅐0.54Cr 2 O 3 ) by the healing and annealing of indentatio... more Cavities formed in ruby (99.46Al 2 O 3 ⅐0.54Cr 2 O 3 ) by the healing and annealing of indentation cracks at 1600°C are more equiaxed than similar cavities in sapphire. Surface energies for the observed facet planes (R, S, and A) relative to the surface energy of the basal plane, C, were ␥ A/C ‫؍‬ 1.00 ؎ 0.03, ␥ R/C ‫؍‬ 1.05 ؎ 0.07, and ␥ S/C ‫؍‬ 1.02 ؎ 0.04, with the uncertainty representing 95% confidence limits. Thus, the surface energies of all observed facets were statistically indistinguishable. Unlike sapphire, P-plane facets were not observed. The substantial rounding of the cavities in ruby indicated that portions of the Wulff shape were above the roughening transition temperature. Thus, even though Cr 2 O 3 and Al 2 O 3 form ideal solutions, Cr 3؉ ions are sufficiently surface active to modify the relative free energy of the surfaces.

Research paper thumbnail of Alternative interpretations for decreasing voltage with increasing charge in ferroelectric capacitors

Scientific reports, 2016

Recent claim on the direct observation of a negative capacitance (NC) effect from a single layer ... more Recent claim on the direct observation of a negative capacitance (NC) effect from a single layer epitaxial Pb(Zr0.2,Ti0.8)O3 (PZT) thin film was carefully reexamined, and alternative interpretations that can explain the experimental results without invoking the NC effect are provided. Any actual ferroelectric capacitor has an interfacial layer, and experiment always measures the sum of voltages across the interface layer and the ferroelectric layer. The main observation of decreasing ferroelectric capacitor voltage (VF) for increasing ferroelectric capacitor charge (QF), claimed to be the direct evidence for the NC effect, could be alternatively interpreted by either the sudden increase in the positive capacitance of a ferroelectric capacitor or decrease in the voltage across the interfacial layer due to resistance degradation. The experimental time-transient VF and QF could be precisely simulated by these alternative models that fundamentally assumes the reverse domain nucleation a...

Research paper thumbnail of Resistance switching behavior of atomic layer deposited SrTiO3 film through possible formation of Sr2Ti6O13 or Sr1Ti11O20 phases

Scientific reports, 2016

Identification of microstructural evolution of nanoscale conducting phase, such as conducting fil... more Identification of microstructural evolution of nanoscale conducting phase, such as conducting filament (CF), in many resistance switching (RS) devices is a crucial factor to unambiguously understand the electrical behaviours of the RS-based electronic devices. Among the diverse RS material systems, oxide-based redox system comprises the major category of these intriguing electronic devices, where the local, along both lateral and vertical directions of thin films, changes in oxygen chemistry has been suggested to be the main RS mechanism. However, there are systems which involve distinctive crystallographic phases as CF; the Magnéli phase in TiO2 is one of the very well-known examples. The current research reports the possible presence of distinctive local conducting phase in atomic layer deposited SrTiO3 RS thin film. The conducting phase was identified through extensive transmission electron microscopy studies, which indicated that oxygen-deficient Sr2Ti6O13 or Sr1Ti11O20 phase wa...

Research paper thumbnail of Effect of oxygen vacancy on the structural and electronic characteristics of crystalline Zn 2 SnO 4

J. Mater. Chem. C, 2014

ABSTRACT The effects of oxygen vacancies on the atomic and electronic structures in crystalline Z... more ABSTRACT The effects of oxygen vacancies on the atomic and electronic structures in crystalline Zn2SnO4 were examined by ab-initio calculations using screened hybrid density functional. The formation energy and the electronic structure indicated that the neutral oxygen vacancy was stable in the n-type region with forming deep level in the bandgap. The ionization energy of oxygen vacancy was calculated to be around 2.0 eV, which can be excited by visible photon energy. The atomic movements and the Bader charge analyses around the neutral oxygen vacancy showed that Sn played a dominant role in changing the electronic properties by forming the pair of Sn2+-VO0, which was accompanied by the reduction of the charge state of Sn due to its multiple oxidation states. On the contrary, the electronic state of Zn hardly changed despite of its more predominant atomic shifts than Sn around the oxygen vacancy.

Research paper thumbnail of Catalytic activity for oxygen reduction reaction on platinum-based core-shell nanoparticles: all-electron density functional theory

Nanoscale, 2015

Pt nanoparticles (NPs) in a proton exchange membrane fuel cell as a catalyst for an oxygen reduct... more Pt nanoparticles (NPs) in a proton exchange membrane fuel cell as a catalyst for an oxygen reduction reaction (ORR) fairly overbind oxygen and/or hydroxyl to their surfaces, causing a large overpotential and thus low catalytic activity. Realizing Pt-based core-shell NPs (CSNPs) is perhaps a workaround for the weak binding of oxygen and/or hydroxyl without a shortage of sufficient oxygen molecule dissociation on the surface. Towards the end, we theoretically examined the catalytic activity of NPs using density functional theory; each NP consists of one of 12 different 3d-5d transition metal cores (groups 8-11) and a Pt shell. The calculation results evidently suggest the enhancement of catalytic activity of CSNPs in particular when 3d transition metal cores are in use. The revealed trends in activity change upon the core metal were discussed with respect to the thermodynamic and electronic structural aspects of the NPs in comparison with the general d-band model. The disparity between the CSNP and the corresponding bilayer catalyst, which is the so-called size effect, was remarkable; therefore, it perhaps opens up the possibility of size-determined catalytic activity. Finally, the overpotential for all CSNPs was evaluated in an attempt to choose promising combinations of CSNP materials.

Research paper thumbnail of Growth, Quantitative Growth Analysis, and Applications of Graphene on γ-Al2O3 catalysts

Scientific reports, 2015

The possibilities offered by catalytic γ-Al2O3 substrates are explored, and the mechanism governi... more The possibilities offered by catalytic γ-Al2O3 substrates are explored, and the mechanism governing graphene formation thereon is elucidated using both numerical simulations and experiments. The growth scheme offers metal-free synthesis at low temperature, grain-size customization, large-area uniformity of electrical properties, single-step preparation of graphene/dielectric structures, and readily detachable graphene. We quantify based on thermodynamic principles the activation energies associated with graphene nucleation/growth on γ-Al2O3, verifying the low physical and chemical barriers. Importantly, we derive a universal equation governing the adsorption-based synthesis of graphene over a wide range of temperatures in both catalytic and spontaneous growth regimes. Experimental results support the equation, highlighting the catalytic function of γ-Al2O3 at low temperatures. The synthesized graphene is manually incorporated as a 'graphene sticker' into an ultrafast mode-lo...

Research paper thumbnail of Effect of surface hydroxyl coverage on platinum nanoparticles in the oxygen reduction reaction: All-electron density functional theory analysis

Chemical Physics Letters, 2014

ABSTRACT Catalysts in commercial proton exchange membrane fuel cells must be optimized for activi... more ABSTRACT Catalysts in commercial proton exchange membrane fuel cells must be optimized for activity and cost, but improvements are hampered for Pt nanoparticles by low activity resulting from extensive OH adsorption. A model system clearly elucidating the impact of OH coverage on Pt nanoparticles was therefore developed. Using first principles calculations, free energies due to changing OH coverage in the oxygen reduction reaction were predicted as functions of electrode potential and local surface structure. Free energy contour plots that considered surface OH coverage were developed. Therefrom, we theoretically predicted the optimum range of operating potentials, yielding design guidelines for catalytic surfaces.

Research paper thumbnail of First-principles study on the formation of a vacancy in Ge under biaxial compressive strain

Thin Solid Films, 2010

The effects of the biaxial compressive strain on the atomic relaxation and the formation energy o... more The effects of the biaxial compressive strain on the atomic relaxation and the formation energy of a neutral vacancy in Ge were investigated using first-principles calculations. Prior to this, the effects of the supercell size and Brillouin zone sampling were tested. The vacancy formation energy and atomic configuration around a vacancy are strongly affected by the inter-vacancy distance determined by

Research paper thumbnail of Variation in the threshold voltage of amorphous-In2Ga2ZnO7 thin-film transistors by ultrathin Al2O3 passivation layer

Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2013

ABSTRACT

Research paper thumbnail of A case of primary intestinal T-cell lymphoma involving entire gastrointestinal tract: esophagus to rectum

The Korean journal of internal medicine, 2000

Primary intestinal T-cell lymphoma is a rare disease entity, which is approximately 10% to 25% of... more Primary intestinal T-cell lymphoma is a rare disease entity, which is approximately 10% to 25% of intestinal lymphomas, and most of the lymphomas occur in the small intestine. We report here a case of a 56-year-old woman who has been suffering from chronic diarrhea and weight loss for 6 months. Abdominal CT scan and small bowel series showed diffuse wall thickening of the small bowel. Gastroscopic examination showed diffuse erythematous lesions on the esophagus and small gastric ulcerations on the antrum of the stomach, and colonoscopic examination also showed multiple punched-out ulcerations and erosions on the entire colon, including the sigmoid colon to the terminal ileum. Diffuse infiltration of CD 3 positive lymphoma cells was found on biopsy. The patient was diagnosed as primary intestinal T-cell lymphoma with diffuse involvement of the entire gastrointestinal tracts from the esophagus to the rectum. Although the patient received systemic combination chemotherapy and achieved ...

Research paper thumbnail of Structure and mechanical properties of Ag-incorporated DLC films prepared by a hybrid ion beam deposition system

Thin Solid Films, 2007

Ag-incorporated diamond-like carbon films were prepared on Si(100) wafers using a hybrid depositi... more Ag-incorporated diamond-like carbon films were prepared on Si(100) wafers using a hybrid deposition system composed of an end-Hall-type hydrocarbon ion gun and a silver DC magnetron sputter source. The Ag concentration in the films was controlled by changing the fraction of Ar in the Ar and benzene reaction gas. The chemical composition, microstructure, atomic bond structure, and mechanical properties were investigated for Ag concentrations ranging from 0 to 9.7 at.%. When the Ag concentration was 0.1 at.%, the Ag atoms were fully dissolved in the amorphous carbon matrix without forming any second phase. Amorphous and crystalline silver particles appeared when the Ag concentration was 1.7 and N 6.8at.%, respectively. It was found that the hardness was not sensitive to the Ag concentration in this concentration range. On the other hand, the residual compressive stress was strongly dependent on the chemical state of the incorporated Ag atoms. The mechanical properties are discussed in terms of the changes in the microstructure and atomic bond structure induced by Ag incorporation.

Research paper thumbnail of Ab initio Calculations on the Atomic and Electronic Structures of Oxygen-Doped Hexagonal Ge$_{2}$Sb$_{2}$Te$_{5}$

Applied Physics Express, 2012

ABSTRACT The atomic configurations, energetics, and electronic structures of oxygen doped hexagon... more ABSTRACT The atomic configurations, energetics, and electronic structures of oxygen doped hexagonal Ge2Sb2Te5 were investigated by using ab-initio calculations. Oxygen atoms or molecules were added at the possible interstitial or substitutional sites. The formation energies showed that both oxygen atoms and molecules would be spontaneously incorporated into hexagonal Ge2Sb2Te5. Oxygen atoms prefer forming bonds with Ge and Sb than with Te. The doped oxygen molecule was immediately dissociated into two oxygen atoms. The spontaneous incorporation of oxygen and dissociation of oxygen molecules are distinctly opposite to the nitrogen doping behavior in hexagonal Ge2Sb2Te5.

Research paper thumbnail of Thermodynamic stability of various phases of zinc tin oxides from ab initio calculations

Journal of Materials Chemistry C, 2013

ABSTRACT Thermodynamic stabilities of various phases in ZnO–SnO2 systems were investigated based ... more ABSTRACT Thermodynamic stabilities of various phases in ZnO–SnO2 systems were investigated based on the Gibbs energy obtained from density functional theory (DFT) calculations. The pressure–temperature (p–T) phase diagram was determined; the coexistence of ZnO and SnO2 was the most stable phase in the low temperature region at zero external pressure, while Zn2SnO4 with the inverse spinel structure and ZnSnO3 with the lithium niobate structure were stable at the high temperature and high pressure region. Various octahedral configurations of the inverse spinel structures of Zn2SnO4 were considered. The calculated results showed feasible agreement with experimental data on the phase stability and explained well the experimental observation of the mixed state of Zn2SnO4, ZnO and SnO2 at mid-range temperatures and pressures. Considering the atomic structures, bulk moduli and thermodynamic stabilities, the local density approximation calculations were found to describe experimental observations more accurately than the generalized gradient approximation calculations. The phase transitions in the ZnO–SnO2 system were found to be dominated by the changes in both the Zn–O bond length and the coordination number of Zn, rather than changes in the bond length of Sn–O and the coordination number of Sn.

Research paper thumbnail of The Impact of Carbon Concentration on the Crystalline Phase and Dielectric Constant of Atomic Layer Deposited HfO2 Films on Ge Substrate

Ecs Journal of Solid State Science and Technology, 2012

ABSTRACT The effect of the carbon concentration on the crystalline phase and dielectric constant ... more ABSTRACT The effect of the carbon concentration on the crystalline phase and dielectric constant (k) of atomic layer deposited HfO2 films on Ge substrate was investigated. After annealing, the HfO2 films grown at 200 degrees C and 280 degrees C were crystallized to the tetragonal (t) and monoclinic (m) phases, respectively, which was related to the carbon contents within the films and grain boundary energy. To clarify this, the energy difference between a t- and a m-phases (Delta E-tetra) was calculated by first principles calculations. The higher k value of t-HfO2 compared to amorphous and monoclinic HfO2 was experimentally confirmed.

Research paper thumbnail of Strain evolution of each type of grains in poly-crystalline (Ba,Sr)TiO3 thin films grown by sputtering

Scientific Reports, 2012

The strain states of [111]-, [110]-, and [002]-oriented grains in poly-crystalline sputtered (Ba,... more The strain states of [111]-, [110]-, and [002]-oriented grains in poly-crystalline sputtered (Ba,Sr)TiO(3) thin films on highly [111]-oriented Pt electrode/Si substrates were carefully examined by X-ray diffraction techniques. Remarkably, [002]-oriented grains respond more while [110]- and [111]-oriented grains do less than the theoretically estimated responses, which is understandable from the arrangement of the TiO(6) octahedra with respect to the stress direction. Furthermore, such mechanical responses are completely independent of the degree of crystallization and film thickness. The transition growth temperature between the positive and negative strains was also different depending on the grain orientation. The unstrained lattice parameter for each type of grain was different suggesting that the oxygen vacancy concentration for each type of grain is different, too. The results reveal that polycrystalline (Ba,Sr)TiO(3) thin films are not an aggregation of differently oriented grains which simply follow the mechanical behavior of single crystal with different orientations.

Research paper thumbnail of Improved properties of Pt–HfO2 gate insulator–ZnO semiconductor thin film structure by annealing of ZnO layer

Thin Solid Films, 2010

Metal-insulator-semiconductor capacitors were fabricated with sputtered ZnO and atomic layer depo... more Metal-insulator-semiconductor capacitors were fabricated with sputtered ZnO and atomic layer deposited HfO2 as the semiconductor and gate dielectric layers, respectively. From the capacitance–voltage measurements, it was confirmed that pre-deposition annealing of the ...

Research paper thumbnail of First-Principles Study of the Interfaces between Fe and Transition Metal Carbides

The Journal of Physical Chemistry C, 2013

ABSTRACT The interface energies and electronic structures of the interfaces between BCC Fe and tr... more ABSTRACT The interface energies and electronic structures of the interfaces between BCC Fe and transition metal carbides have been investigated using first-principles calculations based on density functional theory. The effects of the composition and configuration of the carbides on the interface properties have been determined. It was shown that the Fe/TiC interface has the highest interface energy and the formation of complex carbides leads to a significant decrease in the interface energy. The complex carbide of (Ti0.5Mo0.5)C, which has Mo present at the interface, has been found to be the most stable. From the analysis of the density of states, the stability of the Mo-segregated (Ti0.5Mo0.5)C carbides has been revealed to be due to the hybridization of the d-orbitals in the t2g local symmetry between Mo and its first nearest neighboring Fe atoms.

Research paper thumbnail of Migration of nitrogen in hexagonal Ge2Sb2Te5: An ab-initio study

physica status solidi (RRL) - Rapid Research Letters, 2012

Kim, S.-J., Lee, J., Lee, S.-C., Park, C., Hwang, CS and Choi, J.-H.(2012), Migration of nitrogen... more Kim, S.-J., Lee, J., Lee, S.-C., Park, C., Hwang, CS and Choi, J.-H.(2012), Migration of nitrogen in hexagonal Ge 2 Sb 2 Te 5: An ab-initio study. physica status solidi (RRL)-Rapid Research Letters. doi: 10.1002/pssr. 201105516

Research paper thumbnail of Predictions on atomic structures of Ti1−xMoxC using combined approach of first-principles calculation and the cluster expansion method

Metals and Materials International, 2009

Recently, binary transition metal carbides such as (Ti1-xMox)C have attracted much attention for ... more Recently, binary transition metal carbides such as (Ti1-xMox)C have attracted much attention for use in increasing the strength of steels. This study aims to understand the ground state structure of various compositions of that carbide. Using a combined approach of first-principles calculation and the cluster expansion method, ground state structures of the compositions of TiC and MoC were determined. Only