Jong-Young Kim

Unoxidized Graphene/Alumina Nanocomposite: Fracture- and Wear-Resistance Effects of Graphene on Alumina Matrix

It is of critical importance to improve toughness, strength, and wear-resistance together for the development of advanced structural materials. Herein, we report on the synthesis of unoxidized graphene/alumina composite materials having enhanced toughness, strength, and wear-resistance by a low-cost and environmentally benign pressure-less-sintering process. The wear resistance of the composites was increased by one order of magnitude even under high normal load condition (25 N) as a result of a tribological effect of graphene along with enhanced fracture toughness (KIC) and flexural strength (σf) of the composites by ~75% (5.60 MPa·m1/2) and ~25% (430 MPa), respectively, compared with those of pure Al2O3. Furthermore, we found that only a small fraction of ultra-thin graphene (0.25–0.5 vol%, platelet thickness of 2–5 nm) was enough to reinforce the composite. In contrast to unoxidized graphene, graphene oxide (G-O) and reduced graphene oxide (rG-O) showed little or less enhancement of fracture toughness due to the degraded mechanical strength of rG-O and the structural defects of the G-O composites.

Incorporation of uranium(VI) into metal–organic framework solids, [UO2(C4H4O4)]·H2O, [UO2F(C5H6O4)]·2H2O, and [(UO2)1.5(C8H4O4)2]2[(CH3)2NCOH2]·H2O

Uranium–organic framework solids, in which uranium building units are connected by bidentate dicarboxylate anions such as succinate, glutarate, and isophthalate, were synthesized; the glutarate and isophthalate compounds contain cavities, in which occluded water and organic templates reside.

Structurally Nanocrystalline-Electrically Single Crystalline ZnO-Reduced Graphene Oxide Composites

ZnO, a wide bandgap semiconductor, has attracted much attention due to its multifunctionality, such as transparent conducting oxide, light-emitting diode, photocatalyst, and so on. To improve its performances in the versatile applications, numerous hybrid strategies of ZnO with graphene have been attempted, and various synergistic effects have been achieved in the ZnO-graphene hybrid nanostructures. Here we report extraordinary charge transport behavior in Al-doped ZnO (AZO)-reduced graphene oxide (RGO) nanocomposites. Although the most challenging issue in semiconductor nanocomposites is their low mobilities, the AZO-RGO nanocomposites exhibit single crystal-like Hall mobility despite the large quantity of nanograin boundaries, which hinder the electron transport by the scattering with trapped charges. Because of the significantly weakened grain boundary barrier and the proper band alignment between the AZO and RGO, freely conducting electrons across the nanograin boundaries can be realized in the nanocomposites. This discovery of the structurally nanocrystalline-electrically single crystalline composite demonstrates a new route for enhancing the electrical properties in nanocomposites based on the hybrid strategy.

The first organically templated thorium compounds, [C4N2H12]0.5[ThF5] and [C5N2H14][ThF6]·0.5H2O

Organically templated thorium compounds were synthesized for the first time under hydrothermal conditions; the piperazine containing compound consists of 2-D layers, while the 2-methylpiperazine phase contains unprecedented 1-D chains of face-sharing ThF9 polyhedra.

Soft XAFS study on the 4d electronic structure of ruthenium in complex perovskite oxide

Abstract The bond covalency effect on the electronic structure of the perovskite oxides, La 2 MRuO 6 (M=Zn, Mg, and Li) and Ba 2 YRuO 6 , has been investigated by the complementary methods of X-ray absorption spectroscopy and X-ray diffraction. Ru K-edge and Ru L III -edge XANES analyses clarify that the ruthenium ions with oxidation states of IV ( d 4 ) and V ( d 3 ) are stabilized in a nearly regular RuO 6 octahedron. Comparison of L-edge XANES spectra for the compounds with isovalent ruthenium ion has shown that the t 2g level is mainly influenced by the A site cation, whereas the e g level is mainly affected by the neighboring B site cation, which can be understood on the basis of the bond competition scheme. The present Ru K-edge EXAFS analysis results reveal that the first neighbor coordination (Ru–O shell) is nearly isotropic even in monoclinic perovskites, which is consistent with the XRD and XANES results. The experimental EXAFS spectra in the range R ≤∼4.5 A are well reproduced by ab initio calculation based on crystallographic data, which supports the long-range structure presented by Rietveld refinement. The EXAFS curve-fitting analyses allow us to distinguish the differences in (Ru–O) bond lengths for isovalent ruthenates, which is supported by the XANES spectral features and the chemical interpretation of bond covalency.

Nanograined thermoelectric Bi2Te2.7Se0.3 with ultralow phonon transport prepared from chemically exfoliated nanoplatelets

Herein, we report on a scalable synthesis of surfactant-free Bi2Te2.7Se0.3 nanocrystals by chemical exfoliation and subsequent spark plasma sintering to fabricate nanostructured thermoelectric bulk materials. The exfoliated n-type Bi2Te2.7Se0.3 nanoplatelets were shown to transform into nanoscroll-type crystals (∼5 nm in diameter, ∼50 nm in length) by ultrasonication. The thermoelectric performance of the Bi2Te2.7Se0.3 nanocrystals was found to be recoverable by minimizing surface oxides by chemical reduction of the exfoliated suspensions. Nanostructured bulk materials, composed of plate-like grains with ∼50 nm thickness, were prepared by sintering of the ultrasonicated sample using a spark plasma sintering technique. The resulting compound showed drastic reduction of lattice thermal conductivity (0.31 W m−1 K−1 @ 400 K) due to enhanced phonon scattering at highly dense grain boundaries without deterioration of the power factor (21.0 × 10−4 W m−1 K−2 @ 400 K). The peak ZT value of the present compound (∼0.8 @400 K) is comparable to that of n-type single crystalline Bi2(Te,Se)3, which is one of the highest among the reported values for n-type materials synthesized by a soft chemical route.

X-Ray absorption spectroscopic study on LaPdO3

Pd L-edge XANES and Pd K-edge XANES/EXAFS have been studied to investigate the electronic configuration and local structure of Pd in a new perovskite LaPdO3, which was prepared under high pressure. A white line in the Pd L-edge XANES for LaPdO3 is positioned just in the middle of those for divalent (PdIIO) and tetravalent (Zn2PdIVO4) palladates. The peak intensity for LaPdO3 is also in-between those for PdO and Zn2PdO4. Pd K-edge XANES spectra for PdO is characterized by a shoulder in the threshold edge due to the planar square coordination of Pd. The absence of such a shoulder in Pd K-edge XANES spectra for LaPdO3 and Zn2PdIVO4 suggests a nearly regular octahedral symmetry of (PdO6) in both structures. These XANES results underline that the palladium ions with an oxidation state of III are stabilized with the electronic configuration, t2g6σ*1 in LaPdO3. The Pd K-edge EXAFS analysis confirms that (PdO6) octahedra in LaPdO3 are nearly isotropic and the average PdIII–O bond distance is 2.064 A, which is compatible with the average PdIII–F bond distance (2.00 A) estimated from a recent EXAFS study on trivalent palladium fluoride.

Highly transparent and conducting graphene-embedded ZnO films with enhanced photoluminescence fabricated by aerosol synthesis.

Graphene/inorganic hybrid structures have attracted increasing attention in research aimed at producing advanced optoelectronic devices and sensors. Herein, we report on aerosol synthesis of new graphene-embedded zinc oxide (ZnO) films with higher optical transparency (>80% at visible wavelengths), improved electrical conductivity (>2 orders of magnitude, ∼ 20 kΩ/□), and enhanced photoluminescence (∼ 3 times), as compared to bare ZnO film. The ZnO/graphene composite films, in which reduced graphene oxide nanoplatelets (∼ 4 nm thick) are embedded in nanograined ZnO (∼ 50 nm in grain size), were fabricated from colloidal suspensions of graphene oxide with an aqueous zinc precursor. These new luminescent ZnO/graphene composites, with high optical transparency and improved electrical conductivity, are promising materials for use in optoelectronic devices.

Electron transport in modulation-doped GaAs v-groove quantum wires

Abstract We report the growth of modulation-doped GaAs/Al x Ga 1− x As v-groove quantum wires and structural, electrical and optical investigations of their electronic states and transport properties. By using alternative group III precursors on partially SiO 2 masked pre-patterned GaAs substrates, samples have been fabricated which permit electrical measurements of single isolated wire structures without the need for additional electron-beam lithography. Magneto-transport was measured as a function of tilt angle of the incident magnetic field to identify the formation of low-dimensional electron gases in different parts of the structure. Photoluminescence investigations reveal 1D and 2D confined states which show different carrier heating when electric fields are applied along the wire structure.

Fe-Doping Effect on Thermoelectric Properties of p-Type Bi0.48Sb1.52Te3

The substitutional doping approach has been shown to be an effective strategy to improve ZT of Bi2Te3-based thermoelectric raw materials. We herein report the Fe-doping effects on electronic and thermal transport properties of polycrystalline bulks of p-type Bi0.48Sb1.52Te3. After a small amount of Fe-doping on Bi/Sb-sites, the power factor could be enhanced due to the optimization of carrier concentration. Additionally, lattice thermal conductivity was reduced by the intensified point-defect phonon scattering originating from the mass difference between the host atoms (Bi/Sb) and dopants (Fe). An enhanced ZT of 1.09 at 300 K was obtained in 1.0 at% Fe-doped Bi0.48Sb1.52Te3 by these synergetic effects.