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Search Results: 1 - 10 of 7511 matches for " Woo Seok Choi "
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Band gap tuning in ferroelectric Bi4Ti3O12 by alloying LaTMO3 (TM = Ti, V, Cr, Mn, Co, Ni, and Al)
Woo Seok Choi,Ho Nyung Lee
Physics , 2012, DOI: 10.1063/1.3697645
Abstract: We fabricated ferroelectric Bi4Ti3O12 (BiT) single crystalline thin films site-specifically substituted with LaTMO3 (TM = Al, Ti, V, Cr, Mn, Co, and Ni) on SrTiO3 substrates by pulsed laser epitaxy. When transition metals are incorporated into a certain site of the BiT, some of BiT-LaTMO3 showed a substantially decreased band gap, coming from the additional optical transition between oxygen 2p and TM 3d states. Specifically, all alloys with Mott insulators revealed a possibility of band gap reduction. Among them, BiT-LaCoO3 showed the largest band gap reduction by ~1 eV, positioning itself as a promising material for highly efficient opto-electronic devices.
Strain tuning of electronic structure in Bi4Ti3O12-LaCoO3 epitaxial thin films
Woo Seok Choi,Ho Nyung Lee
Physics , 2015, DOI: 10.1103/PhysRevB.91.174101
Abstract: We investigated the crystal and electronic structures of ferroelectric Bi4Ti3O12 (BiT) single crystalline thin films site-specifically substituted with LaCoO3 (LCO). The epitaxial films were grown by pulsed laser epitaxy on NdGaO3 and SrTiO3 substrates to vary the degree of strain. With increasing the LCO substitution, we observed a systematic increase in the c-axis lattice constant of the Aurivillius phase related with the modification of pseudo-orthorhombic unit cells. These compositional and structural changes resulted in a systematic decrease in the band gap, i.e., the optical transition energy between the oxygen 2p and transition metal 3d states, based on a spectroscopic ellipsometry study. In particular, the Co 3d state seems to largely overlap with the Ti t2g state, decreasing the band gap. Interestingly, the applied tensile strain facilitates the band gap narrowing, demonstrating that epitaxial strain is a useful tool to tune the electronic structure of ferroelectric transition metal oxides.
Understanding of Ultrasonic Assisted Machining with Diamond Grinding Tool  [PDF]
Kyung-Hee Park, Yun-Hyuck Hong, Kyeong-Tae Kim, Seok-Woo Lee, Hon-Zong Choi, Young-Jae Choi
Modern Mechanical Engineering (MME) , 2014, DOI: 10.4236/mme.2014.41001

In this work, machining test was carried out in various machining conditions using ultrasonic vibration capable CNC machine. For work material, alumina ceramic (Al2O3) was used while for tool material diamond electroplated grinding wheel was used. To evaluate ultrasonic vibration effect, grinding test was performed with and without ultrasonic vibration in same machining condition. In ultrasonic mode, ultrasonic vibration of 20 kHz was generated by HSK 63 ultrasonic actuator. On the other hand, grinding forces were measured by KISTLER dynamometer. And an optimal sampling rate for grinding force measurement was obtained by a signal processing and frequency analysis. The surface roughness of the ceramic was also measured by using stylus type surface roughness instrument and atomic force microscope (AFM). Besides, the scanning electron microscope (SEM) was used for observation of surface integrality.

Thermopower enhancement by fractional layer control in 2D oxide superlattices
Woo Seok Choi,Hiromichi Ohta,Ho Nyung Lee
Physics , 2014, DOI: 10.1002/adma.201401676
Abstract: We have investigated two-dimensional thermoelectric properties in transition metal oxide heterostructures. In particular, we adopted an unprecedented approach to direct tuning of the 2D carrier density using fractionally {\delta}-doped oxide superlattices. By artificially controlling the carrier density in the 2D electron gas that emerges at a LaxSr1-xTiO3 {\delta}-doped layer, we demonstrate that a thermopower as large as 408 {\mu}V K-1 can be reached. This approach also yielded a power factor of the 2D carriers 117 {\mu}Wcm-1K-2, which is one of the largest reported values from transition metal oxide based materials. The promising result can be attributed to the anisotropic band structure in the 2D system, indicating that {\delta}-doped oxide superlattices can be a good candidate for advanced thermoelectrics.
Polaron transport and thermoelectric behavior in La-doped SrTiO3 thin films with elemental vacancies
Woo Seok Choi,Hyang Keun Yoo,Hiromichi Ohta
Physics , 2015, DOI: 10.1002/adfm.201403023
Abstract: Electrodynamic properties of La-doped SrTiO3 thin films with controlled elemental vacancies have been investigated using optical spectroscopy and thermopower measurement. In particular, we observed a correlation between the polaron formation and thermoelectric properties of the transition metal oxide (TMO) thin films. With decreasing oxygen partial pressure during the film growth (P(O2)), a systematic lattice expansion was observed along with the increased elemental vacancy and carrier density, experimentally determined using optical spectroscopy. Moreover, we observed an absorption in the mid-infrared photon energy range, which is attributed to the polaron formation in the doped SrTiO3 system. Thermopower of the La-doped SrTiO3 thin films could be largely modulated from -120 to -260 {\mu}V K-1, reflecting an enhanced polaronic mass of ~3 < mpolron/m < ~4. The elemental vacancies generated in the TMO films grown at various P(O2) influences the global polaronic transport, which governs the charge transport behavior, including the thermoelectric properties.
Pb concentrations and isotopic compositions in the soil and sediments around the abandoned mine in southwest of Korea  [PDF]
Jong-Woo Choi, Eun-Jin Yoo, Jee-young Kim, Jong-Yeon Hwang, Khanghyun Lee, Won-Seok Lee, Jin-Seok Han, Kyu-Seung Lee
Journal of Agricultural Chemistry and Environment (JACEN) , 2013, DOI: 10.4236/jacen.2013.24015
Abstract: This study was undertaken to investigate the influence of Pb contaminant dispersal from an abandoned mine. For the study, 23 samples of sediment and soil were collected around the abandoned mine located in southwest of Korea. Pb concentrations and isotope ratios of sediment samples were analyzed through ICP-OES (Optima 5300 DV, Perkin Elmer) and MC-ICP-MS (Plasma II, Nu), respectively. The sediment samples collected in the main stream from the mine entrance showed high Pb concentrations of 139 - 1079 mg/kg, while Pb concentrations of the other sites were 19 - 29 mg/kg. Pb isotope ratios of the main stream sediments were 2.4854 - 2.4883 for 208Pb/207Pb and 1.1693 - 1.1740 for 206Pb/207Pb. On the other hand, the Pb isotopic compositions of sediments collected from the other sites were clearly different from those of ore deposits. The Pb isotope ratios versus inverse concentrations plot showed high linear correlation between the main stream, the tributary stream and one of the downstream sites representing the two end member mixing system between these sites. According to the binary mixing equation, the relative contribution of Pb pollutants from the abandoned mine to the downstream area were approximately 33% - 36%.
Clinical course of sub-centimeter-sized nodules detected during surveillance for hepatocellular carcinoma
Yang Won Min,Geum-Youn Gwak,Min Woo Lee,Moon Seok Choi
World Journal of Gastroenterology , 2012, DOI: 10.3748/wjg.v18.i21.2654
Abstract: AIM: To evaluate the outcome of sub-centimeter-sized nodules (SCSNs) detected during surveillance for hepatocellular carcinoma (HCC) in patients at risk. METHODS: We retrospectively analyzed a total of 142 patients with liver cirrhosis or chronic hepatitis B or C without a prior history of HCC in whom a SCSN was detected during HCC surveillance. We calculated the rate of HCC development from SCSNs in the study population and analyzed the differences in the baseline clinical characteristics and imaging features between the patients with SCSNs that eventually developed into HCC and patients with SCSNs that did not develop into HCC. RESULTS: During 667 person-years of follow-up, HCC developed in 33 patients. The calculated HCC development rate was 4.9% per year. The cumulative one-, two-, three- and five-year HCC development rates were 5.6%, 10.6%, 14.1% and 20.4%, respectively. Upon baseline comparison, the HCC group was older (54.4 ± 8.3 years vs 48.9 ± 9.4 years; P = 0.003) and had lower albumin levels (3.56 ± 0.58 g/dL vs 3.84 ± 0.55 g/dL; P = 0.012) and higher baseline alpha-fetoprotein (AFP) levels (8.5 ng/mL vs 5.4 ng/mL; P = 0.035) compared to the non-HCC group. Nodule pattern and initial radiologic diagnosis also differed between the two groups. Multivariate analysis revealed that age [P = 0.012, odds ratio (OR) =1.075, 95% confidence interval (CI) =1.016-1.137], sex (P = 0.009, OR = 3.969, 95% CI: 1.403-11.226), and baseline AFP level (P = 0.024, OR = 1.039, 95% CI: 1.005-1.073) were independent risk factors for developing HCC. CONCLUSION: The overall risk of HCC development in patients with SCSNs is similar to that in liver cirrhosis patients. Patients with these risk factors need to be closely monitored during follow-up.
Radiogenic and stable isotope measurement using a MC-ICP-MS
Min Seok Choi,Jong-Sik Ryu,Sin-Woo Lee,Chang-Sik Cheong
Journal of Analytical Science & Technology , 2011,
Abstract: In this study, we presented newly developed analytical methods for radioactive, radiogenic, and stable isotopes using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), where the elemental purification, and precision and accuracy were highlighted.
Dimensional crossover of polaron dynamics in Nb:SrTiO3/SrTiO3 superlattices: Possible mechanism of thermopower enhancement
Woo Seok Choi,Hiromichi Ohta,Soon Jae Moon,Yun Sang Lee,Tae Won Noh
Physics , 2009, DOI: 10.1103/PhysRevB.82.024301
Abstract: Using optical spectroscopy, we investigated the electrodynamic properties of Nb:SrTiO3/SrTiO3 superlattices. In these superlattices, a large enhancement of the Seebeck coefficient (S) has been reported with decreasing Nb:SrTiO3 layer thickness [refer to H. Ohta et al., Nature Mater. 6, 129 (2007)]. By analyzing the optical spectra, we found that the polaron plays an important role in determining the electrodynamic properties of the superlattices. With decreasing Nb:SrTiO3 layer thickness from eleven to one unit cell, we observed a threefold enhancement of the polaron effective mass and relaxation time. Such increases were attributed to a dimensional crossover of polaron from 3D to quasi-2D. Moreover, the modified nature of the polaron at low dimensions enhanced the thermoelectric properties of the oxide superlattice, by simultaneously increasing S and preventing the decrease of carrier mobility. Our results indicate that strong electron-phonon coupling can provide an alternative pathway in searching efficient thermoelectric materials.
Resonant tunneling in a quantum oxide superlattice
Woo Seok Choi,Sang A. Lee,Jeong Ho You,Suyoun Lee,Ho Nyung Lee
Physics , 2015, DOI: 10.1038/ncomms8424
Abstract: Resonant tunnelling is a quantum mechanical process that has long been attracting both scientific and technological attention owing to its intriguing underlying physics and unique applications for high-speed electronics. The materials system exhibiting resonant tunnelling, however, has been largely limited to the conventional semiconductors, partially due to their excellent crystalline quality. Here we show that a deliberately designed transition metal oxide superlattice exhibits a resonant tunnelling behaviour with a clear negative differential resistance. The tunnelling occurred through an atomically thin, lanthanum {\delta}-doped SrTiO3 layer, and the negative differential resistance was realized on top of the bipolar resistance switching typically observed for perovskite oxide junctions. This combined process resulted in an extremely large resistance ratio (~10^5) between the high and low-resistance states. The unprecedentedly large control found in atomically thin {\delta}-doped oxide superlattices can open a door to novel oxide-based high-frequency logic devices.
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