Sangkyu Lee

Hanyang University, Sŏul, Seoul, South Korea

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Publications (24)79.95 Total impact

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    ABSTRACT: Si-Ge composites have recently been explored as an anode material for lithium-ion batteries due to their stable cycle performance and excellent rate capability. Although previous reports show the benefits of Si-Ge composites on electrochemical performance, the specific mechanism and structural effects have been overlooked. Here, the structural effect of Si-Ge heterogeneous nanostructures on both mechanics and kinetics is systematically studied through theoretical analysis and detailed experimental results. Si-Ge and Ge-Si core–shell nanowires are employed for this study. The Si-Ge core–shell nanowires show a much improved electrochemical performance, especially cycle performance and rate capability, when compared to those of the Ge-Si core–shell nanowires electrode. On the basis of the detailed experimental results and associated theoretical analysis, its is demonstrated that the strain distribution and Li diffusivity and/or diffusion path are significantly affected by the Si-Ge heterostructure, which induce different mechanics and kinetics associated with lithium.
    Advanced Functional Materials 03/2014; 24(10). · 9.77 Impact Factor
  • Advanced Energy Materials 01/2014; 4(1). · 10.04 Impact Factor
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    ABSTRACT: Nitridated Si–Ti–Ni (STN) ternary alloy as an anode for lithium ion batteries is prepared by the NH3 nitridation of STN alloy. The nitridated STN electrode shows 14% increased reversible capacity and 4 times higher rate capability at 3 C compared to those of pristine STN electrode. These improvements are attributed to the formation of nitride compounds (such as SiNx/SiNxOy, TiNx/TiNxOy and NiNx/NiNxOy) on STN alloy, which benefits both mechanics and kinetics.
    Journal of Power Sources 01/2014; 253:282–286. · 4.68 Impact Factor
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    ABSTRACT: The nanoweb structure of TiO2 anode, cross-linked between electrospun nanofibers, is directly fabricated on the current collector by utilizing the fluidity of low glass transition temperature polymer at room temperature. This characteristic enables us to fabricate the nanoweb structure by direct electrosspinning on the current collector, followed by uniaxial pressing. This proposed structure facilitates electron transport through the direct conducting pathways between TiO2 active materials and current collector as well as provides strong adhesion strength to the current collector without polymeric binders. Consequently, we could achieve stable cycle performance up to 100 cycles and the excellent rate capability of ~ 60 % at high rate charge/discharge condition of 10 C.
    ACS Applied Materials & Interfaces 11/2013; · 5.01 Impact Factor
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    ABSTRACT: The development of various technologies has led to the advent of a variety of deformable devices. Despite significant technological advancement in this area, it is still challenging to integrate different devices due to limitations such as substrate issues and differences among growth and deposition conditions. Creating an interconnection between two different devices currently requires the use of metallic wires/lines to build electrical connections. Here, we demonstrate a LEGO-like assembly of the free-standing film of individually operable components encapsulated in a polymer overcoat layer, leading to the production of an integrated architecture without additional electrical connections. The free-standing components are produced by the peeling-off process. The sticky nature of the polymer layer enables the construction of supercapacitor arrays and simple RLC circuits by interlocking the individual components. We expect that this approach will enable the fabrication of a variety of custom-built devices using a LEGO-like assembly method.
    NPG Asia Materials. 10/2013; 5(10).
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    ABSTRACT: A TiO2 nanofiber anode is glued using a sol-gel reaction to improve cycle performance and Coulombic efficiencies. The hydrolysis-condensation process produces TiO2 nanoglues consisting of TiO2 nanoparticles, providing strong adhesion between nanofibers and the current collector as well as between nanofibers, rendering favourable transporting properties of electrons and Li ions in the TiO2 nanofiber electrode, and eventually leading to the excellent performance of Li ion batteries exhibiting an excellent initial Coulombic efficiency (∼85%), cycle performance up to 100 cycles, and high rate capability (60% at 5 C).
    Nanoscale 03/2013; · 6.23 Impact Factor
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    ABSTRACT: This paper explores transport in transparent thin film transistors formed using a liquid precursor to indium zinc oxide, delivered to target substrates by electrohydrodynamic jet (e-jet) printing. Under optimized conditions, we observe field effect mobilities as high as 32 cm2V−1s−1, with on/off current ratios of 103 and threshold voltages of 2 V. These results provide evidence that material manipulated in fine-jet, electric field induced liquid flows can yield semiconductor devices without any adverse effects of residual charge or unintentional doping. E-jet printing methods provide levels of resolution (∼1.5 μm) that provide a path to printed transistors with small critical dimensions.
    Applied Physics Letters 03/2012; 100(10). · 3.79 Impact Factor
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    ABSTRACT: Nearly all research in micro- and nanofabrication focuses on the formation of solid structures of materials that perform some mechanical, electrical, optical, or related function. Fabricating patterns of charges, by contrast, is a much less well explored area that is of separate and growing interesting because the associated electric fields can be exploited to control the behavior of nanoscale electronic and mechanical devices, guide the assembly of nanomaterials, or modulate the properties of biological systems. This paper describes a versatile technique that uses fine, electrified liquid jets formed by electrohydrodynamics at micro- and nanoscale nozzles to print complex patterns of both positive and negative charges, with resolution that can extend into the submicrometer and nanometer regime. The reported results establish the basic aspects of this process and demonstrate the capabilities through printed patterns with diverse geometries and charge configurations in a variety of liquid inks, including suspensions of nanoparticles and nanowires. The use of printed charge to control the properties of silicon nanomembrane transistors provides an application example.
    Nano Letters 02/2010; 10(2):584-91. · 13.03 Impact Factor
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    ABSTRACT: We report a novel one-step method for the preparation of hierarchically patterned Au nanoparticles in a conducting polymer matrix by controlling the interface properties between Au nanoparticles and the conducting polymer matrix. The terminal group of capping molecules for the Au nanoparticles was modified to change the interface properties, not to change the size of the Au nanoparticles which affects their intrinsic properties. By modulating the interface properties, it is possible to construct Au nanoparticle-conducting polymer composites with two different structures: one presents a triple layer in which the conducting polymer layer is sandwiched between Au nanoparticle layers at the top and bottom; the other exhibits a form like a raisin cake in which Au nanoparticles are homogeneously organized in the conducting polymer matrix. High-resolution transmission electron microscopy was used to study the morphology and patterning of Au nanoparticles in the conducting polymer matrix.
    Nanotechnology 02/2008; 19(7):075606. · 3.84 Impact Factor
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    Ungyu Paik, Sangkyu Lee, Jea-Gun Park
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    ABSTRACT: The effect of physicochemical properties of solvents on the microstructure of polyvinyl carbazole (PVK) film for non-volatile polymer memory was investigated. For the solubilization of PVK molecules and the preparation of PVK films, four solvents with different physicochemical properties of the Hildebrand solubility parameter and vapor pressure were considered: chloroform, tetrahydrofuran (THF), 1,1,2,2-tetrachloroethane (TCE), and N,N- dimehtylformamide (DMF). The solubility of PVK molecules in the solvents was observed by ultraviolet- visible spectroscopy. PVK molecules were observed to be more soluble in chloroform, with a low Hildebrand solubility parameter, than solvents with higher values. The aggregated size and micro-/nano-topographical properties of PVK films were characterized using optical and atomic force microscopes. The PVK film cast from chloroform exhibited enhanced surface roughness compared to that from TCE and DMF. It was also confirmed that the microstructure of PVK film has an effect on the performance of non-volatile polymer memory.
    01/2008;
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    ABSTRACT: Transparent ZrO2–polydimethylsiloxane (PDMS) nanocomposites with high refractive index were prepared by dispersing ZrO2 nanoparticles in a PDMS matrix via ligand molecule engineering. The ligand molecule consists of a diamine head group that adsorbs strongly onto the ZrO2 nanoparticles, and a siloxane tail group with a double-tailed structure that can be easily stretched within the PDMS matrix and yields a greater steric hindrance compared to single-tailed structure. The transmission electron micrograph shows that the designed Si-based ligand molecule has a role to play in keeping ZrO2 nanoparticles fully-dispersed even after the ZrO2 nanoparticles were transferred into the PDMS matrix. UV-vis spectra indicate that ZrO2–PDMS nanocomposite had a high transparency of 93.3% in the whole visible range. The refractive index of the ZrO2–PDMS nanocomposite could be varied from 1.39 to 1.65 simply by increasing the ZrO2 content from 0 to 20.8% v/v.
    Journal of Materials Chemistry 01/2008; 18(15). · 5.97 Impact Factor
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    ABSTRACT: The preparation of BaMgAl10O17:Eu2+ (BAM) blue phosphor layer for a plasma display panel by the addition of a newly designed energetic dispersant (hereafter referred to as SAIT7) and its resulting photoluminescence (PL) efficiency is investigated. The addition of SAIT7 increases the maximum solids loading of BAM phosphor paste from 27.91 to 35.87 vol% and yields a highly packed microstructure of the phosphor layer. The PL intensity is increased by 7.57% compared to the BAM blue phosphor layer prepared without SAIT7. In conclusion, the addition of SAIT7 increased the packing density of the phosphor layer and resulted in improved luminescent properties of the phosphor layer.
    Journal of Materials Research. 11/2007; 22(12):3309 - 3315.
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    ABSTRACT: A dispersion technology for Ni particles suspended in a non-aqueous medium based on the quantitative evaluation of surface acid-base properties of Ni particles is described. A quantitative analysis of surface acid-base properties of Ni particles was performed using non-aqueous titration. Dimethylamino ethanol and acetic acid were used as probe molecules to detect surface acid-base amounts of Ni particles. The dispersion system was designed on the basis of the amounts of surface acid-base sites on the Ni particle surface. Rheological behavior and agglomerate particle size data demonstrate that the dispersion stability of the designed Ni suspension is markedly improved, as expected. Therefore, the design strategy to improve the dispersion stability of Ni particles was successful. This strategy is expected to be applicable to dispersion systems of other particles suspended in a non-aqueous medium.
    Journal of Colloid and Interface Science 09/2007; 312(2):265-71. · 3.17 Impact Factor
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    ABSTRACT: The interaction of a dispersant and a binder at the Ni particle surface is investigated to identify their influence on the dispersion stability of Ni particles in dihydroterpineol (DHT). Six different commercial dispersants are used to obtain a highly dispersed Ni suspension, and ethyl cellulose (EC) is used as a binder for the preparation of a Ni paste. The effectiveness of the dispersants on the dispersion stability of the Ni particles in DHT is evaluated by measuring the rheological behavior of the Ni suspension. Hypermer KD3 and RE610, among six different commercial dispersants, are found to be an effective dispersant for the Ni particles in DHT without EC. However, upon the addition of EC, different rheological behaviors of the Ni suspensions are shown according to the added dispersants. EC does not affect the rheological behavior of the RE610-stabilized Ni suspension. On the other hand, the addition of EC increases the apparent and relative viscosities of the Hypermer KD3-stabilized Ni suspension and causes a transition in rheological behavior from shear thinning behavior to shear thickening behavior. The adsorption results reveal that the deterioration in the stability of the Hypermer KD3-stabilized Ni suspension with the addition of EC is due to the partial displacement of the preadsorbed Hypermer KD3 by the EC. The influence of the interactions between the dispersant and binder on the dispersion stability of the Ni pastes is also confirmed via observations of the variations in the agglomerated particle size with the addition of EC.
    Journal of the American Ceramic Society 08/2006; 89(10):3050 - 3055. · 2.11 Impact Factor
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    ABSTRACT: An attempt was made to improve the conventional lamination process in the multilayered ceramic capacitors (MLCCs) fabrication by adapting a solvent treatment on the BaTiO3 green sheets. During the lamination process, additional particle rearrangement in the BaTiO3 green block was stimulated by solvent treatment. This rearrangement resulted in a more compact structure of the BaTiO3 green sheets compared to that obtained through the conventional lamination process. The overall thickness of BaTiO3/Ni/BaTiO3 laminates was also reduced after solvent treatment. It is suggested that miniaturization in the fabrication of MLCCs can be improved by adapting a solvent treatment during the lamination process.
    Journal of Electroceramics 01/2006; 17(2):351-354. · 1.14 Impact Factor
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    ABSTRACT: The consolidation behavior of ultrahigh-molecular-weight polyethylene (UHMWPE) in BaTiO3/ UHMWPE suspension was investigated by the control of the non-adsorbed organic additives. The suspensions were characterized by adsorption isotherm, rheology, sedimentation behavior, and microstructural observation. The co-dispersion stability of the BaTiO3/UHMWPE particulates was correlated with the uniformity of the green microstructure. These studies can be helpful for the successful fabrication of ultra thin dielectric layer of MLCC using newly improved extrusion method.
    Journal of Electroceramics 01/2006; 17(2):345-349. · 1.14 Impact Factor
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    ABSTRACT: An attempt was made to retard the dissolution of Ba ions from BaTiO3 by coating Y compounds on the BaTiO3 particle surface. A hysteresis in the electrokinetic behavior of as-received BaTiO3 occurred during sequential acid and base titrations, while there was no hysteresis in the Y-coated BaTiO3. In the bare BaTiO3, dissolved Ba ions are adsorbed and/or precipitated onto the surface during sequent base titration to yield a more positively charged Ba-rich phase on top of the Ba-depleted lattice. On the contrary, the dynamic mobility of Y-coated BaTiO3 during base titration closely followed the corresponding acid titration, providing that the starting surface of Y-coated BaTiO3 and the surface after acid-base treatment was the same. It is suggested that Y compounds on the BaTiO3 particles retard the dissolution of Ba ions from BaTiO3.
    Journal of Electroceramics 01/2006; 17(2):355-358. · 1.14 Impact Factor
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    ABSTRACT: Aqueous-based natural graphite particulates for fabrication of lithium-ion battery anodes are investigated with emphasis on chemical control of suspension component interactions among graphite particulates, sodium carboxymethyl cellulose (CMC), and emulsified styrene butadiene rubber (SBR). The chemical stability and dispersion properties of the natural graphite particles are characterized using electroacoustic, flow behaviour and green microstructural observations, as well as by measurement of pore size. Correlation is made between the dispersion characteristics and the electrochemical performance of the particles. The dispersion stability of the graphite suspension is improved by charge development when both SBR and CMC are incorporated into the graphite suspension, compared with an unstable graphite suspension prepared with CMC alone. A method to improve the dispersion and homogeneity of the suspension component based on the use of SBR and CMC is proposed. Electrochemical experiments using a Li–organic electrolyte–as-cast natural graphite half-cell and 750-mAh lithium-ion cells show an initial discharge capacity above 340mAhg−1, improved charge–discharge efficiency, and excellent rate capability.
    Journal of Power Sources 01/2005; 147(1):249-255. · 4.68 Impact Factor
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    ABSTRACT: We report findings on the electrokinetic and solubility behaviors of (Ba,Ca)(Ti,Zr)O3 (BCTZ) powders having three different [Ba + Ca]/[Ti + Zr] ratios: 0.995, 1.000, and 1.005. Electrokinetic and solubility properties of BCTZ powders in aqueous media are phenomenologically similar to BaTiO3. Ba and Ca ions, occupying primarily A-sites on the perovskite lattice, dissolve during acid titration, which results in surface depletion of A-site cations in the surface region of BCTZ particles. The electrokinetics of colloidal BCTZ powders reflects changes in the surface chemistry that occur as a result of dissolution and adsorption/reprecipitation of surface ions. An increase in [Ba + Ca]/[Ti + Zr] ratio results in an increase in the dynamic mobility at all pH values, an increase in the titration hysteresis, and an increase in the isoelectric pH. Each of these effects can be attributed to Ba and Ca in the near-surface region of BCTZ.
    Journal of the American Ceramic Society 12/2004; 86(6):1034 - 1036. · 2.11 Impact Factor
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    ABSTRACT: Hysteresis in the electrokinetic behavior of colloidal hydrothermal BaTiO3 occurs during sequential acid and base titrations. Ba dissolution during acid titration results in an oxide-rich surface. When the acid-treated BaTiO3 is titrated back to pH 10, dissolved Ba is specifically adsorbed and/or precipitated onto the particle surface. The combined effects of dissolution and subsequent adsorption–precipitation results in titration hysteresis. Most of the labile Ba can be removed by multiple acid treatments, which result in a TiO2-like surface layer composition. Barium dissolution increases with decreasing pH but levels off below pH 4 due to diffusion through the surface oxide layer as predicted previously. A phenomenological model is offered to explain the electrokinetic behavior as a function of pH. It is suggested that inherent BaCO3 contamination is not the primary source of dissolved Ba from hydrothermal BaTiO3 in acidic solution.
    Journal of the American Ceramic Society 12/2004; 86(10):1662 - 1668. · 2.11 Impact Factor