Daejeon, South Korea
Daejeon, South Korea

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Kim S.H.,Sogang University | Choi C.H.,Sogang University | Jung M.-H.,Sogang University | Yoon J.-B.,Nano Materials Research Team | And 6 more authors.
Journal of Applied Physics | Year: 2010

The magnetofluctuation conductivity, called excess conductivity, originated from the forming of the superconducting droplet near to the mean-field transition temperature, was measured for the optimally doped BaFe1.8 Co0.2 As2 single crystals with a critical temperature, Tc, of 24.6 K. This measurement of the excess conductivity for magnetic fields up to 9 T was compared with the thermodynamic scaling theory in the critical region, in which not only the Gaussian fluctuation but also fourth order terms of the order parameter are included. An analysis of the excess conductivity showed that the superconductivity followed three-dimensional scaling rather than two-dimensional scaling even though the sample had a layered structure. © 2010 American Institute of Physics.


Kim S.-I.,KAIST | Yoon H.,KAIST | Lee H.,KAIST | Lee S.,KAIST | And 4 more authors.
Journal of Materials Chemistry C | Year: 2015

Highly oriented single-crystalline ferromagnetic Co nanowire (NW) arrays were synthesized on sapphire substrates via a single-step chemical vapor deposition (CVD) method. On an m-cut sapphire substrate, Co NWs were vertically grown in epitaxial relationship with the substrate without using any catalysts or templates. On an r-cut sapphire substrate, Co NWs were horizontally grown in two perpendicular directions. Furthermore, we report that the Co NWs were transformed into Co3O4 nanotubes by thermal annealing under dilute O2 conditions. Such formation of hollow structures is ascribed to favored outward diffusion of Co ions. The present vertically aligned arrays of single-crystalline Co NWs could be utilized for advanced magnetic memory applications owing to their uniform orientations. © 2015 The Royal Society of Chemistry.


Bagkar N.,KAIST | Seo K.,KAIST | Yoon H.,KAIST | In J.,KAIST | And 3 more authors.
Chemistry of Materials | Year: 2010

We report synthesis of vertical Ni3Co nanowires on a c-plane sapphire substrate by chemical vapor transport method. A vapor - solid - solid mechanism was proposed for Ni3Co nanowire growth, which was catalyzed by Ni3Co2 nanoparticle at the tip. As-synthesized vertically aligned singlecrystalline Ni3Co nanowires exhibit strongly ferromagnetic properties. Such single-crystalline ferromagnetic alloy nanowires could be important in the application, of spintronics such as nonvolatile memory devices. We could also control the composition of the nanowire by adjusting experimental conditions, producing Ni, Ni3Co, and NiCo nanowires. © 2010 American Chemical Society.


Seo K.,KAIST | Bagkar N.,KAIST | Kim S.-I.,KAIST | In J.,KAIST | And 3 more authors.
Nano Letters | Year: 2010

We report fabrication of Heusler alloy Fe3Si nanowires by a diffusion-driven crystal structure transformation method from paramagnetic FeSi nanowires. Magnetic measurements of the Fe3Si nanowire ensemble show high-temperature ferromagnetic properties with Tc ≫ 370 K. This methodology is also successfully applied to Co2Si nanowires in order to obtain metal-rich nanowires (Co) as another evidence of the structural transformation process. Our newly developed nanowire crystal transformation method would be valuable as a general method to fabricate metal-rich silicide nanowires that are otherwise difficult to synthesize. © 2010 American Chemical Society.


Lee S.,KAIST | Lee S.,Center for Electricity and Magnetism | In J.,KAIST | Yoo Y.,KAIST | And 7 more authors.
Nano Letters | Year: 2012

A recent theoretical study suggested that Ag 2Te is a topological insulator with a highly anisotropic Dirac cone. Novel physics in the topological insulators with an anisotropic Dirac cone is anticipated due to the violation of rotational invariance. From magnetoresistance (MR) measurements of Ag 2Te nanowires (NWs), we have observed Aharanov-Bohm (AB) oscillation, which is attributed to the quantum interference of electron phase around the perimeter of the NW. Angle and temperature dependences of the AB oscillation indicate the existence of conducting surface states in the NWs, confirming that Ag 2Te is a topological insulator. For Ag 2Te nanoplates (NPLs), we have observed high carrier mobility exceeding 22 000 cm 2/(V s) and pronounced Shubnikov-de Haas (SdH) oscillation. From the SdH oscillation, we have obtained Fermi state parameters of the Ag 2Te NPLs, which can provide valuable information on Ag 2Te. Understanding the basic physics of the topological insulator with an anisotropic Dirac cone could lead to new applications in nanoelectronics and spintronics. © 2012 American Chemical Society.


Jeong H.-S.,Kangwon National University | Noh J.H.,Nano Materials Research Team | Hwang C.-G.,Nano Materials Research Team | Kim S.H.,Nano Materials Research Team | Lee S.-Y.,Kangwon National University
Macromolecular Chemistry and Physics | Year: 2010

As a new approach to improve the thermal stability of separator membranes crucially affecting the internal short-circuit failures of lithium-ion batteries, we develop a new composite separator membrane. The composite separator membrane is prepared by introducing microporous composite coating layers onto both sides of a polyethylene (PE) separator membrane. The composite coating layers consist of silica (SiO2) nanoparticles and gel-type polymer electrolytes (PVdF-HFP, polyvinylidene fluoride-hexafluoropropylene). The microporous morphology of composite coating layers is determined by controlling the phase inversion, more specifically the solvent-nonsolvent miscibility in the coating solutions. To induce the phase inversion, three different nonsolvents are chosen in the decreasing order of solubility parameter (d) difference against the solvent (acetone, δ=20 MPa1/2); the nonsolvents are water (δ=48 MPa1/2), ethanol (δ=26 MPa1/2), and isopropyl alcohol (δ=24 MPa1/2). The microporous structure of composite coating layers becomes more developed with the increase of not only the nonsolvent content, but also the solubility parameter difference between acetone and the nonsolvent. Based on this understanding of the phase inversion, the influence of the morphological variation on the thermal shrinkage and electrochemical performance of the composite separator membranes is quantitatively identified. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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