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Now showing items 81 - 96 of 106

  • Atomistic Simulation Protocol for Improved Design of Si-O-C Hybrid Nanostructures as Li-Ion Battery Anodes:ReaxFF Reactive Force Field

    Yeo, Byung Chul   Jung, Hyun   Lee, Hong Woo   Yun, Kang-Seop   Kim, Hyungjun   Lee, Kwang-Ryeol   Han, Sang Soo  

    Recently, silicon oxygen carbon hybrid nanostructures have received great attention as a promising anode material for Li-ion batteries, for which their diverse structures can be synthesized. Here,. using molecular dynamics (MD) simulations with a reactive force field (ReaxFF), we studied the atomistic lithiation behaviors of sp(2) carbon coated Si and SiOx nanostructures, such as nanowires (NWs) and nanoparticles (NPs), in which various kinds and sizes of carbonaceous coating layers were explored. The introduction of an sp(2) carbonaceous coating layer to Si-based anodes makes Li diffusion more facile, which leads to improved battery performances such as faster charge/discharge rates. Moreover, the carbonaceous coating layer can also provide a buffer effect to volume changes during lithiation along with the well-known functions of preventing the loss of electrical continuity and increasing electrical conductivity of Si-based anodes. However, a thick carbonaceous coating layer can strongly suppress the volume expansion behavior of Si-based nanostructures and thus prevent Li penetration into the nanostructures, leading to a very low Li capacity. According to our ReaxFF-MD simulations, the critical size of the carbonaceous coating layer that can act as a buffer layer is approximately C/Si =3D 2.4, which is the circumference ratio of the carbonaceous coating layer over the Si NWs. For a coating layer that has a higher ratio, Li cannot penetrate into the Si NWs; instead, they exist only on and in the sp(2) coating layers including in the spaces between two graphene layers. Moreover, the shape of the Si nanostructures (e.g., NW and NP) does little to affect the anode properties, such as Li capacity and volume change, although Si NP confined in a carbon nanotube shows anisotropic volume expansion behavior during lithiation. We expect that the ReaxFF will provide a useful protocol for designing Si-O-C hybrid anodes to obtain better performing Li-ion batteries.
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  • Chemically-induced liquid film migration with low lattice diffusivity relative to the migration rate in MoNi(W)

    Lee, Kwang-Ryeol   Yoon, Duk N.  

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  • A comparative first-principles study of the lithiation,sodiation,and magnesiation of black phosphorus for Li-,Na-,and Mg-ion batteries

    Hembram, K. P. S. S.   Jung, Hyun   Yeo, Byung Chul   Pai, Sung Jin   Lee, Heon Ju   Lee, Kwang-Ryeol   Han, Sang Soo  

    Using first-principles calculations, we describe and compare atomistic lithiation, sodiation, and magnesiation processes in black phosphorous with a layered structure similar to graphite for Li-, Na-, and Mg-ion batteries because graphite is not considered to be an electrode material for Na- and Mg-ion batteries. The three processes are similar in that an intercalation mechanism occurs at low Li/Na/Mg concentrations, and then further insertion of Li/Na/Mg leads to a change from the intercalation mechanism to an alloying process. Li and Mg show a columnar intercalation mechanism and prefer to locate in different phosphorene layers, while Na shows a planar intercalation mechanism and preferentially localizes in the same layer. In addition, we compare the mechanical properties of black phosphorous during lithiation, sodiation, and magnesiation. Interestingly, lithiation and sodiation at high concentrations (Li2P and Na2P) lead to the softening of black phosphorous, whereas magnesiation shows a hardening phenomenon. In addition, the diffusion of Li/Na/Mg in black phosphorus during the intercalation process is an easy process along one-dimensional channels in black phosphorus with marginal energy barriers. The diffusion of Li has a lower energy barrier in black phosphorus than in graphite.
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  • Tribological performance of hydrophilic diamond-like carbon coatings on Ti-6Al-4V in biological environment RID A-4224-2010 RID E-1242-2011

    Anil, Midathada   Ahmed, Sk Faruque   Yi, Jin Woo   Moon, Myoung-Woon   Lee, Kwang-Ryeol   Kim, Yu Chan   Seok, Hyun Kwang   Han, Seung Hee  

    Tribological performance of diamond-like carbon (DLC) and Si doped DLC (Si-DLC) films on Ti-6Al-4V under bovine serum as well as water and ambient air condition has been studied in terms of surface modification with O(2) plasma treatment for superhydrophilic surface. A tribo-test revealed that bovine serum significantly enhanced the tribological performance on all DLC surfaces in comparison with those under water or in air medium. Especially, O(2) plasma treated Si-DLC coatings with superhydrophilic nature were found to lower the wear and increase coating stability associated with macromolecules especially proteins in the bovine serum and their interactions with surfaces. (C) 2009 Elsevier B.V. All rights reserved.
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  • Transformation of amorphous carbon to graphene on low-index Ni surfaces during rapid thermal processing:a reactive molecular dynamics study

    Li, Xiaowei   Wang, Aiying   Lee, Kwang-Ryeol  

    The transformation of amorphous carbon to graphene on different Ni surfaces during rapid thermal processing was explored using reactive molecular dynamics simulation. Due to the difference in activation energy, Ni surfaces affected the diffusion behavior of C into Ni and thus modulated the remnant number of C atoms, dominating the formation and quality of graphene, which accorded with the developed empirical equation.
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  • Transformation of amorphous carbon to graphene on low-index Ni surfaces during rapid thermal processing: a reactive molecular dynamics study

    Li, Xiaowei   Wang, Aiying   Lee, Kwang-Ryeol  

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  • Unexpected Roles of Interstitially Doped Lithium in Blue and Green Light Emitting Y2O3:Bi3+:A Combined Experimental and Computational Study

    Khan, Sovann   Choi, Heechae   Lee, Seung Yong   Lee, Kwang-Ryeol   Ntwaeaborwa, Odireleng Martin   Kim, Seungchul   Cho, So-Hye  

    To enhance the photoluminescence of lanthanide oxide, a cleat understanding of its defect chemistry is necessary. In particular, when yttrium oxide, a widely used phosphor,,,. undergoes doping, several of its atomic structures may be coupled with point defects that are difficult to understand through experimental results alone. Here, we report the Strong enhancement of the photoluminescence (PL) of Y2O3:Bi3+ via codoping with Li+ ions and suggest a plausible mechanism for that enhancement using both experimental and computational studies. The codoping of Li+ ions into the Y2O3:Bi3+ phosphor was found to cause significant changes in its structural and optical properties. Interestingly, unlike previous reports on codoping with several other phosphors, we found that Li+ ions preferentially occupy interstitial sites of the Y2O3:Bi3+ phosphor. Computational insights based on density functional theory calculations also indicate that Li+ is energetically more stable in the interstitial sites than in the substitutional sites. In addition, interstitially doped Li+ was found to favor the vicinity of Bi3+ by an energy difference of 0.40 eV in comparison to isolated sites. The calculated DOS showed the formation of a shallow level directly above the unoccupied 6p orbital of Bi3+ as the result of interstitial Li+ doping, which may be responsible for the enhanced PL. Although the crystallinity of the host materials increased with the addition of Li salts, the degree of increase was minimal when the Li+ content was low (<1 ma%) where major PL enhancement was observed. Therefore, we reason that the enhanced PL mainly results from the shallow levels created by the interstitial Li+.
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  • Defect effect on tribological behavior of diamond-like carbon films deposited with hydrogen diluted benzene gas in aqueous environment RID A-4224-2010

    Yi, Jin Woo   Park, Se Jun   Moon, Myoung-Woon   Lee, Kwang-Ryeol   Kim, Seock-Sam  

    This study examined the friction and wear behavior of diamond-like carbon (DLC) films deposited from a radio frequency glow discharge using a hydrogen diluted benzene gas mixture. The DLC films were deposited on Si (1 0 0) and polished stainless steel substrates by radio frequency plasma-assisted chemical vapor deposition (r.f.-PACVD) at hydrogen to benzene ratios, or the hydrogen dilution ratio, ranging from 0 to 2.0. The wear test was carried out in both ambient and aqueous environments using a homemade ball-on-disk type wear rig. The stability of the DLC coating in an aqueous environment was improved by diluting the benzene precursor gas with hydrogen, suggesting that hydrogen dilution during the deposition of DLC films suppressed the initiation of defects in the film and improved the adhesion of the coating to the interface. (c) 2009 Elsevier B.V. All rights reserved.
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  • Simulation Protocol for Prediction of a Solid-Electrolyte Interphase on the Silicon-based Anodes of a Lithium-Ion Battery:ReaxFF Reactive Force Field

    Yun, Kang-Seop   Pai, Sung Jin   Yeo, Byung Chul   Lee, Kwang-Ryeol   Kim, Sun-Jae   Han, Sang Soo  

    We propose the ReaxFF reactive force field as a simulation protocol for predicting the evolution of solid-electrolyte interphase (SEI) components such as gases (C2H4, CO, CO2, CH4, and C2H6), and inorganic (Li2CO3, Li2O, and LiF) and organic (ROLi and ROCO2Li: R =3D -CH3 or -C2H5) products that are generated by the chemical reactions between the anodes and liquid electrolytes. Re-IYFF was developed from ab initio results, and a molecular dynamics simulation with ReaxFF realized the prediction of SEI formation under real experimental conditions and with a reasonable computational cost. We report the effects on SEI formation of different kinds of Si anodes (pristine Si and SiOx), of the different types and compositions of various carbonate electrolytes, and of the additives. From the results, we expect that ReaxFF will be very useful for the development of novel electrolytes or additives and for further advances in Li-ion battery technology.
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  • The Morphology and Mechanical Properties of Polycarbonate/Acrylonitrile Butadiene Styrene Modified by Ar Ion Beam Irradiation RID A-4224-2010 RID E-1242-2011

    Ahmed, Sk. Faruque   Yi, Jin Woo   Moon, Myoung-Woon   Jang, Yong-Jun   Park, Bong-Hyun   Lee, Seong-Hoon   Lee, Kwang-Ryeol  

    This study examined the surface morphological evolution of polycarbonate and acrylonitrile butadiene styrene (PC/ABS) irradiated with an Ar ion beam using an ion beam system. PC was not affected by the Ar ion beam treatment at a lower ion beam treatment times but the ABS portion formed a foam-like nanostructure at the surface. On the other hand, both PC and ABS formed nanostructures with length of 100 to 120 nm and a mean diameter of approximate to 3 5 nm at longer beam treatment times. The Raman and FTIR spectra revealed polymer chain scissioning. Nanoindentation showed that the hardness and elastic modulus of the PC/ABS decreases from 0.22 to 0.18 GPa and from 3.39 to 2.98 GPa, respectively with increasing Ar ion beam treatment time due to the surface nano-structures formed by Ar ion beam.
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  • Band-gap sensitive adsorption of fluorine molecules on sidewalls of carbon nanotubes: an ab initio study RID H-4499-2011 RID G-2017-2011 RID E-1242-2011

    Choi, Woon Ih   Park, Sohee   Kim, Tae-Eun   Park, Noejung   Lee, Kwang-Ryeol   Lee, Young Hee   Ihm, Jisoon   Han, Seungwu  

    We report from ab initio calculations that the band-gap sensitive side-wall functionalization of a carbon nanotube is feasible with the fluorine molecule (F-2), which can provide a route to the extraction of semiconducting nanotubes by etching away metallic ones. In the small diameter cases like (11, 0) and (12, 0), the nanotubes are easily functionalized with F-2 regardless of their electronic properties. As the diameter becomes larger, however, the fluorination is favoured on metallic CNTs with smaller activation barriers than those of semiconducting ones. Our results suggest that low-temperature exposure to F-2 molecules in the gas phase can make a dominant portion of fluorinated metallic nanotubes and unfluorinated semiconducting ones. This is consistent with recent experimental reports.
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  • Experimental and simulation study to identify current-confined path in Cu-Al space layer for CPP-GMR spin-valve application RID G-9233-2011 RID E-1242-2011

    Soh, Joon-Young   Kim, Sang-Pil   Kim, Young Keun   Lee, Kwang-Ryeol   Chung, Yong-Chae   Kawasaki, S.   Miyake, K.   Doi, M.   Sahashi, M.  

    To understand the mechanism of current-confined-path formation for the current-perpendicular-to-plane type of giant magnetoresistive devices, we have investigated the evolution of an Al monolayer on the Cu (111) surface both by in situ scanning tunneling microscopy and by molecular dynamics simulation. Ultrathin Al nano-clusters were formed on the plateaus and step (or plateau) edges of the Cu surface in the as-deposited state. Upon annealing at 300 degrees C, Al atoms migrated toward the step edges by surface diffusion. As a consequence, nanometer-sized Cu channels not covered by Al clusters can be formed. These channels could serve as current-confined paths if subsequent mild Al oxidation is provided.
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  • Asymmetric surface intermixing during thin-film growth in the Co-Al system: Role of local acceleration of the deposited atoms RID G-9233-2011 RID E-1242-2011

    Kim, Sang-Pil   Lee, Seung-Cheol   Lee, Kwang-Ryeol   Chung, Yong-Chae  

    Surface intermixing behavior during thin-film deposition in the Co-Al system was investigated on the atomic scale by three-dimensional classical molecular dynamics simulation. Asymmetry of the surface intermixing was observed: Al deposition on a Co substrate resulted in an Al thin-film with an atomically sharp interface, while a Co thin-film deposited on an Al substrate had an interfacial intermixing layer of B2 structure. This phenomenon is discussed in terms of the kinetics of atomic intermixing on the surface. A kinetic criterion for the atomic intermixing is whether the increased kinetic energy of the deposited atom near the surface is larger than the energy barrier to atomic intermixing on the surface. Local acceleration of the deposited atoms near the surface provides an explanation of the puzzling phenomenon of the significant intermixing under low-energy deposition conditions such as thermal evaporation or molecular beam epitaxy. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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  • Investigation of the microstructure, mechanical properties and tribological behaviors of Ti-containing diamond-like carbon films fabricated by a hybrid ion beam method

    Dai, Wei   Ke, Peiling   Moon, Myoung-Woon   Lee, Kwang-Ryeol   Wang, Aiying  

    Diamond-like carbon (DLC) films with various titanium contents were investigated using a hybrid ion beam system comprising an anode-layer linear ion beam source and a DC magnetron sputtering unit. The film composition and microstructure were characterized carefully by X-ray photoelectron spectroscopy, transmission electron microscopy and Raman spectroscopy, revealing that the doped Ti atoms had high solubility in the DLC films. The maximum solubility was found to lie between about 7 and 13 at.%. When the Ti content was lower than this solubility, the doped Ti atoms dissolved in the DLC matrix and the films exhibited the typical features of the amorphous DLC structure and displayed low compressive stresses, friction coefficients and wear rates. However, as the doped content exceeded the solubility, Ti atoms bonded with C atoms, resulting in the formation of carbide nano-particles embedded in the DLC matrix. Although the emergence of the carbide nano-particles promoted graphitizing due to a catalysis effect, the film hardness was enhanced to a great extent. On the other hand, the hard carbides particles caused abrasive wear behavior, inducing a high friction coefficient and wear rate. (C) 2012 Elsevier B.V. All rights reserved.
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  • Molecular dynamics simulation study of the growth of a rough amorphous carbon film by the grazing incidence of energetic carbon atoms RID A-4224-2010 RID E-1242-2011

    Joe, Minwoong   Moon, Myoung-Woon   Oh, Jungsoo   Lee, Kyu-Hwan   Lee, Kwang-Ryeol  

    The morphological evolution of an amorphous carbon film deposited by energetic carbon atoms of 75 eV with various angles of incidence was investigated by molecular dynamics simulation. Normal or near-normal incidence of carbon atoms resulted in a smooth surface of the deposited film. In contrast, a bump-like surface structure emerged and led to rough surfaces at grazing incidences, in agreement with the experiments. The bifurcated growth mode was explained by the impact-induced transport of atoms on the growing surface. The downhill transport of atoms on a sloping surface dominates at normal incidence, which suppresses the evolution of surface irregularities to form a rough surface. However, the dominance of uphill transport at a grazing incidence made the surface irregularities grow to a seed structure, which provided the shadowing effect during carbon deposition. This mechanism mediates initial seed formation and subsequent roughening together with shadowing effects under grazing incidence. (C) 2011 Elsevier Ltd. All rights reserved.
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  • THE PROPHYLAXIS OF DIPHTHERIA.

    Lee, Kwang-Ryeol   Baik, Young-Joon   Eun, Kwang-Yong  

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