Soon Gu Kwon

Argonne National Laboratory, Лимонт, Illinois, United States

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Publications (23)248.01 Total impact

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    ABSTRACT: Magnetite (Fe3O4) is one of the most actively studied materials with a famous metal-insulator transition (MIT), so-called the Verwey transition at around 123 K. Despite the recent progress in synthesis and characterization of Fe3O4 nanocrystals (NCs), it is still an open question how the Verwey transition changes on a nanometer scale. We herein report the systematic studies on size dependence of the Verwey transition of stoichiometric Fe3O4 NCs. We have successfully synthesized stoichiometric and uniform-sized Fe3O4 NCs with sizes ranging from 5 to 100 nm. These stoichiometric Fe3O4 NCs show the Verwey transition when they are characterized by conductance, magnetization, cryo-XRD, and heat capacity measurements. The Verwey transition is weakly size-dependent and becomes suppressed in NCs smaller than 20 nm before disappearing completely for less than 6 nm, which is a clear, yet highly interesting indication of a size effect of this well-known phenomena. Our current work will shed new light on this ages-old problem of Verwey transition.
    Nano Letters 06/2015; DOI:10.1021/acs.nanolett.5b00331 · 13.59 Impact Factor
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    ABSTRACT: To be able to control the functions of engineered multicomponent nanomaterials, a detailed understanding of heterogeneous nucleation at the nanoscale is essential. Here, by using in situ synchrotron X-ray scattering, we show that in the heterogeneous nucleation and growth of Au on Pt or Pt-alloy seeds the heteroepitaxial growth of the Au shell exerts high stress (∼2 GPa) on the seed by forming a core/shell structure in the early stage of the reaction. The development of lattice strain and subsequent strain relaxation, which we show using atomic-resolution transmission electron microscopy to occur through the slip of {111} layers, induces morphological changes from a core/shell to a dumbbell structure, and governs the nucleation and growth kinetics. We also propose a thermodynamic model for the nucleation and growth of dumbbell metallic heteronanostructures.
    Nature Material 11/2014; 14(2). DOI:10.1038/nmat4115 · 36.43 Impact Factor
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    ABSTRACT: In this paper, we present a simple method for scalable synthesis of uniform carbon nanoshell coated monodispersed iron oxide nanocrystals and report on the electrochemical performances of such nanocomposites. The structure and morphology of the resulting nanocomposites are investigated with X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Cyclic voltammetry (CV) and cyclic test of the nanocomposites as an anode material for lithium ion batteries are also studied. The magnetite-C nanostructured material have a very high specific capacity of 859 mAh g−1 in the initial cycle and high capacity retention out to 50 cycles at a constant current density of 100 mA/g, showing its potential as an anode material in lithium-ion batteries. The superior electrochemical characteristics are a result of the uniform distribution of magnetite nanoparticles, proper nanostructure, and good conductivity as well as the prevention of aggregation by the carbon nanoshell during cycling.
    Electrochimica Acta 08/2014; 136:47–51. DOI:10.1016/j.electacta.2014.05.081 · 4.50 Impact Factor
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    ABSTRACT: Diamond anvil cell (DAC), synchrotron X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS) techniques are used to probe the composition inside hollow γ-Fe3O4 nanoparticles (NPs). SAXS experiments on 5.2, 13.3, and 13.8 nm hollow-shell γ-Fe3O4 NPs, and 6 nm core/14.8 nm hollow-shell Au/Fe3O4 NPs, reveal the significantly high (higher than solvent) electron density of the void inside the hollow shell. In high-pressure DAC experiments using Ne as pressure-transmitting medium, formation of nanocrystalline Ne inside hollow NPs is not detected by XRD, indicating that the oxide shell is impenetrable. Also, FTIR analysis on solutions of hollow-shell γ-Fe3O4 NPs fragmented upon refluxing shows no evidence of organic molecules from the void inside, excluding the possibility that organic molecules get through the iron oxide shell during synthesis. High-pressure DAC experiments on Au/Fe3O4 core/hollow-shell NPs show good transmittance of the external pressure to the gold core, indicating the presence of the pressure-transmitting medium in the gap between the core and the hollow shell. Overall, our data reveal the presence of most likely small fragments of iron and/or iron oxide in the void of the hollow NPs. The iron oxide shell seems to be non-porous and impenetrable by gases and liquids.
    Journal of the American Chemical Society 02/2013; 135(7):2435–2438. DOI:10.1021/ja311926r · 11.44 Impact Factor
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    ABSTRACT: We present a rapid and reliable method to determine sizes and size distributions of < 5 nm-sized iron oxide nanocrystals using matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry (MS). MS data were readily converted to size information using a simple equation. The size distribution converted from the mass spectrum is well matched with the data from TEM which requires long and tedious analysis work. The size distribution from mass spectrum is highly resolved and can detect the size difference of only few angstrom. We used this mass spectrum technique to investigate the formation process of iron oxide nanocrystals which is not easy to monitor with other methods. From ex situ measurements, we observed the transition from molecular precursors to clusters, and then finally to nanocrystals.
    Journal of the American Chemical Society 01/2013; 135(7). DOI:10.1021/ja310030c · 11.44 Impact Factor
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    ABSTRACT: We systematically investigated the role of surface modification of nanoparticles catalyst in alkyne hydrogenation reactions and proposed the general explanation of effect of surface ligands on the selectivity and activity of Pt and Co/Pt nanoparticles (NPs) using experimental and computational approaches. We show that the proper balance between adsorption energetics of alkenes at the surface of NPs as compared to that of capping ligands defines the selectivity of the nanocatalyst for alkene in alkyne hydrogenation reaction. We report that addition of primary alkylamines to Pt and CoPt(3) NPs can drastically increase selectivity for alkene from 0 to more than 90% with ∼99.9% conversion. Increasing the primary alkylamine coverage on the NP surface leads to the decrease in the binding energy of octenes and eventual competition between octene and primary alkylamines for adsorption sites. At sufficiently high coverage of catalysts with primary alkylamine, the alkylamines win, which prevents further hydrogenation of alkenes into alkanes. Primary amines with different lengths of carbon chains have similar adsorption energies at the surface of catalysts and, consequently, the same effect on selectivity. When the adsorption energy of capping ligands at the catalytic surface is lower than adsorption energy of alkenes, the ligands do not affect the selectivity of hydrogenation of alkyne to alkene. On the other hand, capping ligands with adsorption energies at the catalytic surface higher than that of alkyne reduce its activity resulting in low conversion of alkynes.
    Nano Letters 09/2012; 12(10):5382-8. DOI:10.1021/nl3027636 · 13.59 Impact Factor
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    ABSTRACT: The dimension-controlled synthesis of CdS nanocrystals in the strong quantum confinement regime is reported. Zero-, one-, and two-dimensional CdS nanocrystals are selectively synthesized via low-temperature reactions using alkylamines as surface-capping ligands. The shape of the nanocrystals is controlled systematically by using different amines and reaction conditions. The 2D nanoplates have a uniform thickness as low as 1.2 nm. Furthermore, their optical absorption and emission spectra show very narrow peaks indicating extremely uniform thickness. It is demonstrated that 2D nanoplates are generated by 2D assembly of CdS magic-sized clusters formed at the nucleation stage, and subsequent attachment of the clusters. The stability of magic-sized clusters in amine solvent strongly influences the final shapes of the nanocrystals. The thickness of the nanoplates increases in a stepwise manner while retaining their uniformity, similar to the growth behavior of inorganic clusters. The 2D CdS nanoplates are a new type of quantum well with novel nanoscale properties in the strong quantum confinement regime.
    Small 08/2012; 8(15):2394-402. DOI:10.1002/smll.201200506 · 7.51 Impact Factor
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    ABSTRACT: Large scale for small size: We report a large-scale synthetic method to produce uniform and ultra-small-sized Ag nanoparticles with good productivity. This method is simple and efficient. It produces Ag nanoparticles within 20 min by heating a reaction mixture containing only three chemicals. The size of the nanoparticles is controlled by varying the heating rate.
    ChemPhysChem 07/2012; 13(10):2540-3. DOI:10.1002/cphc.201101035 · 3.36 Impact Factor
  • Soon Gu Kwon, Taeghwan Hyeon
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    ABSTRACT: Synthesis of uniform nanocrystals is very important because the size uniformity of an ensemble of nanocrystals is directly related to the homogeneity of their chemical and physical properties. Classical theory suggests that burst nucleation and diffusion-controlled growth are the most important factors for the control of the size distribution in colloidal synthesis. In the last two decades, the numerous reports on the synthesis of uniform nanocrystals have popularized two major synthetic methods, namely, hot-injection and heat-up, to obtain uniform nanocrystals of various materials including metals, semiconductors, and oxides. Mechanistic studies on how such uniform nanocrystals are obtained in those two methods are reviewed and theoretical explanations are provided in the current review.
    Small 10/2011; 7(19):2685-702. DOI:10.1002/smll.201002022 · 7.51 Impact Factor
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    ABSTRACT: 2D semiconductor quantum wells have been recognized as potential candidates for various quantum devices. In quantum wells, electrons and holes are spatially confined within a finite thickness and freely move in 2D space. Much effort has focused on shape control of colloidal semiconductor nanocrystals(NCs), and synthesis of 2D colloidal NCs has been achieved very recently. Here, recent advances in colloidal synthesis of uniform and ultrathin 2D CdSeNCs are highlighted. Structural and optical property characterization of these quantum-sized 2D CdSe NCs is discussed. Additionally, 2D CdSe NCs doped with Mn 2+ ions for dilute magnetic semiconductors (DMS) are presented.These 2D CdSe-based NCs can be used as model systems for studying quantum-well structures.
    Advanced Materials 09/2011; 23(28):3214-9. DOI:10.1002/chin.201139207 · 15.41 Impact Factor
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    ABSTRACT: Uniform and extremely small-sized iron oxide nanoparticles (ESIONs) of < 4 nm were synthesized via the thermal decomposition of iron-oleate complex in the presence of oleyl alcohol. Oleyl alcohol lowered the reaction temperature by reducing iron-oleate complex, resulting in the production of small-sized nanoparticles. XRD pattern of 3 nm-sized nanoparticles revealed maghemite crystal structure. These nanoparticles exhibited very low magnetization derived from the spin-canting effect. The hydrophobic nanoparticles can be easily transformed to water-dispersible and biocompatible nanoparticles by capping with the poly(ethylene glycol)-derivatized phosphine oxide (PO-PEG) ligands. Toxic response was not observed with Fe concentration up to 100 μg/mL in MTT cell proliferation assay of POPEG-capped 3 nm-sized iron oxide nanoparticles. The 3 nm-sized nanoparticles exhibited a high r(1) relaxivity of 4.78 mM(-1) s(-1) and low r(2)/r(1) ratio of 6.12, demonstrating that ESIONs can be efficient T(1) contrast agents. The high r(1) relaxivities of ESIONs can be attributed to the large number of surface Fe(3+) ions with 5 unpaired valence electrons. In the in vivo T(1)-weighted magnetic resonance imaging (MRI), ESIONs showed longer circulation time than the clinically used gadolinium complex-based contrast agent, enabling high-resolution imaging. High-resolution blood pool MR imaging using ESIONs enabled clear observation of various blood vessels with sizes down to 0.2 mm. These results demonstrate the potential of ESIONs as T(1) MRI contrast agents in clinical settings.
    Journal of the American Chemical Society 08/2011; 133(32):12624-31. DOI:10.1021/ja203340u · 11.44 Impact Factor
  • Soon Gu Kwon, Taeghwan Hyeon
    Nanoscale Materials in Chemistry, Second Edition, 10/2009: pages 127 - 153; , ISBN: 9780470523674
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    Angewandte Chemie International Edition 09/2009; 48(37):6861-4. DOI:10.1002/anie.200902791 · 11.26 Impact Factor
  • Soon Gu Kwon, Taeghwan Hyeon
    ChemInform 04/2009; 40(14). DOI:10.1002/chin.200914226
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    ABSTRACT: We present a method for assembling silicon nanowires (Si-NWs) in virtually general shape patterns using only conventional microfabrication facilities. In this method, silicon nanowires were functionalized with amine groups and dispersed in deionized water. The functionalized Si-NWs exhibited positive surface charges in the suspensions, and they were selectively adsorbed and aligned onto negatively charged surface regions on solid substrates. As a proof of concepts, we demonstrated transistors based on individual Si-NWs and long networks of Si-NWs.
    Nano Letters 01/2009; 8(12):4523-7. DOI:10.1021/nl802570m · 13.59 Impact Factor
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    ABSTRACT: We synthesized various hollow oxide nanoparticles from as-prepared MnO and iron oxide nanocrystals. Heating metal oxide nanocrystals dispersed in technical grade trioctylphosphine oxide (TOPO) at 300 degrees C for hours yielded hollow nanoparticles retaining the size and shape uniformity of the original nanocrystals. The method was highly reproducible and could be generalized to synthesize hollow oxide nanoparticles of various sizes, shapes, and compositions. Control experiments revealed that the impurities in technical grade TOPO, especially alkylphosphonic acid, were responsible for the etching of metal oxide nanocrystals to the hollow structures. Elemental mapping analysis revealed that the inward diffusion of phosphorus and the outward diffusion of metal took place in the intermediate stages during the etching process. The elemental analysis using XPS, EELS, and EDX showed that the hollow nanoparticles were amorphous metal oxides containing significant amount of phosphorus. The hollow nanoparticles synthesized from MnO and iron oxide nanocrystals were paramagnetic at room temperature and when dispersed in water showed spin relaxation enhancement effect for magnetic resonance imaging (MRI). Because of their morphology and magnetic property, the hollow nanoparticles would be utilized for multifunctional biomedical applications such as the drug delivery vehicles and the MRI contrast agents.
    Nano Letters 01/2009; 8(12):4252-8. DOI:10.1021/nl8019467 · 13.59 Impact Factor
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    ABSTRACT: Heterostructured nanoparticles composed of metals and Fe3O4 or MnO were synthesized by thermal decomposition of mixtures of metal−oleate complexes (for the oxide component) and metal−oleylamine complexes (for the metal component). The products included flowerlike-shaped nanoparticles of Pt−Fe3O4 and Ni−Fe3O4 and snowmanlike-shaped nanoparticles of Ag−MnO and Au−MnO. Powder X-ray diffraction patterns showed that these nanoparticles were composed of face-centered cubic (fcc)-structured Fe3O4 or MnO and fcc-structured metals. The relaxivity values of the Au−MnO and Au−Fe3O4 nanoparticles were similar to those of the MnO and Fe3O4 nanoparticles, respectively. Au−Fe3O4 heterostructured nanoparticles conjugated with two kinds of 12-base oligonucleotide sequences were able to sense a complementary 24-mer sequence, causing nanoparticle aggregation. This hybridization-mediated aggregation was detected by the overall size increase indicated by dynamic light scattering data, the red shift of the surface plasmon band of the Au component, and the enhancement of the signal intensity of the Fe3O4 component in T2-weighted magnetic resonance imaging.
    Journal of the American Chemical Society 10/2008; 130(46). DOI:10.1021/ja805311x · 11.44 Impact Factor
  • Soon Gu Kwon, Taeghwan Hyeon
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    ABSTRACT: Nanocrystals exhibit interesting electrical, optical, magnetic, and chemical properties not achieved by their bulk counterparts. Consequently, to fully exploit the potential of nanocrystals, the synthesis of nanocrystals must focus on producing materials with uniform size and shape. Top-down physical processes can produce large quantities of nanocrystals, but controlling the size is difficult with these methods. On the other hand, colloidal chemical synthetic methods can produce uniform nanocrystals with a controlled particle size. In this Account, we present our synthesis of uniform nanocrystals of various shapes and materials, and we discuss the kinetics of nanocrystal formation. We employed four different synthetic approaches including thermal decomposition, nonhydrolytic sol-gel reactions, thermal reduction, and use of reactive chalcogen reagents. We synthesized uniform oxide nanocrystals via heat-up methods. This method involved slowly heat-up reaction mixtures composed of metal precursors, surfactants, and solvents from room temperature to high temperature. We then held reaction mixtures at an aging temperature for a few minutes to a few hours. Kinetics studies revealed a three-step mechanism for the synthesis of nanocrystals through the heat-up method with size distribution control. First, as metal precursors thermally decompose, monomers accumulate. At the aging temperature, burst nucleation occurs rapidly; at the end of this second phase, nucleation stops, but continued diffusion-controlled growth leads to size focusing to produce uniform nanocrystals. We used nonhydrolytic sol-gel reactions to synthesize various transition metal oxide nanocrystals. We employed ester elimination reactions for the synthesis of ZnO and TiO(2) nanocrystals. Uniform Pd nanoparticles were synthesized via a thermal reduction reaction induced by heating up a mixture of Pd(acac)(2), tri-n-octylphosphine, and oleylamine to the aging temperature. Similarly, we synthesized nanoparticles of copper and nickel using metal(II) acetylacetonates. Ni/Pd core/shell nanoparticles were synthesized by simply heating the reaction mixture composed of acetylacetonates of nickel and palladium. Using alternative chalcogen reagents, we synthesized uniform nanocrystals of various metal chalcogenides. Uniform nanocrystals of PbS, ZnS, CdS, and MnS were obtained by heating reaction mixtures composed of metal chlorides and sulfur dissolved in oleylamine. In the future, a detailed understanding of nanocrystal formation kinetics and synthetic chemistry will lead to the synthesis of uniform nanocrystals with controlled size, shape, and composition. In particular, the synthesis of uniform nanocrystals of doped materials, core/shell materials, and multicomponent materials is still a challenge. We expect that these uniformly sized nanocrystals will find important applications in areas including information technology, biomedicine, and energy/environmental technology.
    Accounts of Chemical Research 09/2008; 41(12):1696-709. DOI:10.1021/ar8000537 · 24.35 Impact Factor
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    ABSTRACT: We studied the kinetics of the formation of iron oxide nanocrystals obtained from the solution-phase thermal decomposition of iron-oleate complex via the "heating-up" process. To obtain detailed information on the thermal decomposition process and the formation of iron oxide nanocrystals in the solution, we performed a thermogravimetric-mass spectrometric analysis (TG-MS) and in-situ magnetic measurements using SQUID. The TG-MS results showed that iron-oleate complex was decomposed at around 320 degrees C. The in-situ SQUID data revealed that the thermal decomposition of iron-oleate complex generates intermediate species, which seem to act as monomers for the iron oxide nanocrystals. Extensive studies on the nucleation and growth process using size exclusion chromatography, the crystallization yield data, and TEM showed that the sudden increase in the number concentration of the nanocrystals (burst of nucleation) is followed by the rapid narrowing of the size distribution (size focusing). We constructed a theoretical model to describe the "heating-up" process and performed a numerical simulation. The simulation results matched well with the experimental data, and furthermore they are well fitted to the well-known LaMer model that is characterized by the burst of nucleation and the separation of nucleation and growth under continuous monomer supply condition. Through this theoretical work, we showed that the "heating-up" and "hot injection" processes could be understood within the same theoretical framework in which they share the characteristics of nucleation and growth stages.
    Journal of the American Chemical Society 11/2007; 129(41):12571-84. DOI:10.1021/ja074633q · 11.44 Impact Factor
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    ABSTRACT: Much progress has been made over the past ten years on the synthesis of monodisperse spherical nanocrystals. Mechanistic studies have shown that monodisperse nanocrystals are produced when the burst of nucleation that enables separation of the nucleation and growth processes is combined with the subsequent diffusion-controlled growth process through which the crystal size is determined. Several chemical methods have been used to synthesize uniform nanocrystals of metals, metal oxides, and metal chalcogenides. Monodisperse nanocrystals of CdSe, Co, and other materials have been generated in surfactant solution by nucleation induced at high temperature, and subsequent aging and size selection. Monodisperse nanocrystals of many metals and metal oxides, including magnetic ferrites, have been synthesized directly by thermal decomposition of metal-surfactant complexes prepared from the metal precursors and surfactants. Nonhydrolytic sol-gel reactions have been used to synthesize various transition-metal-oxide nanocrystals. Monodisperse gold nanocrystals have been obtained from polydisperse samples by digestive-ripening processes. Uniform-sized nanocrystals of gold, silver, platinum, and palladium have been synthesized by polyol processes in which metal salts are reduced by alcohols in the presence of appropriate surfactants.
    Angewandte Chemie International Edition 08/2007; 46(25):4630-60. DOI:10.1002/anie.200603148 · 11.26 Impact Factor

Publication Stats

2k Citations
248.01 Total Impact Points

Institutions

  • 2012–2014
    • Argonne National Laboratory
      Лимонт, Illinois, United States
  • 2005–2014
    • Seoul National University
      • • Department of Chemical and Biological Engineering
      • • Institute of Chemical Processes
      Sŏul, Seoul, South Korea
  • 2011
    • Kyungpook National University
      • Department of Applied Chemistry
      Daikyū, Daegu, South Korea
  • 2009
    • Colorado School of Mines
      • Department of Chemistry and Geochemistry
      گلدن، کلرادو, Colorado, United States