Hyuck Mo Lee

CO Oxidation Mechanism on CeO2-Supported Au Nanoparticles

Density functional theory was used to study the CO oxidation catalytic activity of CeO(2)-supported Au nanoparticles (NPs). Experimental observations on CeO(2) show that the surface of CeO(2) is enriched with oxygen vacancies. We compare CO oxidation by a Au(13) NP supported on stoichiometric CeO(2) (Au(13)@CeO(2)-STO) and partially reduced CeO(2) with three vacancies (Au(13)@CeO(2)-3VAC). The structure of the Au(13) NP was chosen to minimize structural rearrangement during CO oxidation. We suggest three CO oxidation mechanisms by Au(13)@CeO(2): CO oxidation by coadsorbed O(2), CO oxidation by a lattice oxygen in CeO(2), and CO oxidation by O(2) bound to a Au-Ce(3+) anchoring site. Oxygen vacancies are shown to open a new CO oxidation pathway by O(2) bound to a Au-Ce(3+) anchoring site. Our results provide a design strategy for CO oxidation on supported Au catalysts. We suggest lowering the vacancy formation energy of the supporting oxide, and using an easily reducible oxide to increase the concentration of reduced metal ions, which act as anchoring sites for O(2) molecules.

Prediction of interface reaction products between Cu and various solder alloys by thermodynamic calculation

A new scheme to predict the intermetallic compound which forms first at the substrate/solder interface during the soldering process has been suggested. A local equilibrium was assumed at the interface between the substrate and the liquid solder. By calculating metastable equilibria between the substrate and the liquid solder phases and by comparing the calculated driving forces of formation for individual phases, the compound which forms first at the substrate/solder interface could be successfully predicted. The prediction of interface reactions between Cu substrate and Sn-Pb, Sn-Bi and Sn-Zn binary eutectic solder was in agreement with already known experimental results.

Steering Epitaxial Alignment of Au, Pd, and AuPd Nanowire Arrays by Atom Flux Change

We have synthesized epitaxial Au, Pd, and AuPd nanowire arrays in vertical or horizontal alignment on a c-cut sapphire substrate. We show that the vertical and horizontal nanowire arrays grow from half-octahedral seeds by the correlations of the geometry and orientation of seed crystals with those of as-grown nanowires. The alignment of nanowires can be steered by changing the atom flux. At low atom deposition flux vertical nanowires grow, while at high atom flux horizontal nanowires grow. Similar vertical/horizontal epitaxial growth is also demonstrated on SrTiO(3) substrates. This orientation-steering mechanism is visualized by molecular dynamics simulations.

Yellow-emitting γ-Ca 2 SiO 4 :Ce 3+ , Li + phosphor for solid-state lighting: luminescent properties, electronic structure, and white light-emitting diode application

A new yellow-emitting γ-Ca2SiO4:Ce3+,Li+ phosphor was synthesized via a solid-state reaction. The phosphor showed a strong yellow emission with a wide bandwidth of 135.4 nm under blue light excitation. Absorption and photoluminescence measurements and density functional theory calculations suggest that the luminescence of the phosphor can be attributed primarily to the transitions of 5d→4f (2F(7/2) and 2F(5/2)) of Ce3+ ions occupying Ca(1) sites in the host crystal. White light-emitting diodes (LEDs) were fabricated by combining this phosphor with a blue LED, and excellent white light with a high color rendering index of 86 was created owing to the wide emission bandwidth of the phosphor.

The effect of heat treatments and Si contents on B2 ordering reaction in high-silicon steels

The silicon content was increased up to 6.5% (by weight, unless specified otherwise) to reduce the power loss of the silicon steels. These steels were prepared by the conventional casting method or by spray forming and were investigated with the aid of light optical microscopy (LOM) and transmission electron microscopy (TEM). The difference in the casting method did not result in any difference in suppressing the B2 ordering. The D03 phase was observed only in the as-cast 6.5%Si steel. It was almost impossible to suppress the B2 ordered phase keeping the silicon level as high as 6.5% even after the heat treatment at 1000°C for 24 h or after hot rolling. It was necessary to change the Si level and control the cooling rate to suppress the ordering reaction, especially, in cooling after heat treatment. The silicon level of 5.87% was observed to be a critical value in suppressing the B2 ordering reaction.

Synthesis and characterization of low temperature Sn nanoparticles for the fabrication of highly conductive ink

To fabricate a low cost, highly conductive ink for inkjet printing, we synthesized a gram scale of uniformly sized Sn nanoparticles by using a modified polyol process and observed a significant size-dependent melting temperature depression from 234.1 °C for bulk Sn to 177.3 °C for 11.3 nm Sn nanoparticles. A 20 wt% of Sn nanoparticles was dispersed in the 50% ethylene glycol: 50% isopropyl alcohol mixed solvent for the appropriate viscosity (11.6 cP) and surface tension (32 dyn cm(-1)). To improve the electrical property, we applied the surface treatments of hydrogen reduction and plasma ashing. The two treatments had the effect of diminishing the sheet resistance from 1 kΩ/sq to 50 Ω/sq. In addition, conductive patterns (1 cm × 1 cm) were successfully drawn on the Si wafer using an inkjet printing instrument with conductive Sn ink. The maximum resistivity for an hour of sintering at 250 °C was 64.27 µΩ cm, which is six times higher than the bulk Sn resistivity (10.1 µΩ cm).

Interfacial microstructure and joint strength of Sn-3.5Ag-X (X = Cu, In, Ni) solder joint

This study has been supported by the Center for Electronic Packaging Materials (CEPM) of the Korea Science and Engineering Foundation (KOSEF).

The theoretical study on interaction of hydrogen with single-walled boron nitride nanotubes. I. The reactive force field ReaxFFHBN development

We present a new reactive force field ReaxFFHBN derived to accurately model large molecular and condensed phase systems of H, B, and N atoms. ReaxFFHBN has been tested against quantum calculation data for B–H, B–B, and B–N bond dissociations and for H–B–H, B–N–B, and N–B–N bond angle strain energies of various molecular clusters. The accuracy of the developed ReaxFFHBN for B–N–H systems is also tested for (i) H–B and H–B bond energies as a function of out of plane in H–B(NH2)3 and H–N(BH2)3, respectively, (ii) the reaction energy for the B3N3H6+H2-->B3N3H8, and (iii) crystal properties such as lattice parameters and equations of states for the hexagonal type (h-BN) with a graphite structure and for the cubic type (c-BN) with a zinc-blende structure. For all these systems, ReaxFFHBN gives reliable results consistent with those from quantum calculations as it describes well bond breaking and formation in chemical processes and physical properties. Consequently, the molecular-dynamics simulation based on ReaxFFHBN is expected to give a good description of large systems (>2000 atoms even on the one-CPU machine) with hydrogen, boron, and nitrogen atoms.

Theoretical study on interaction of hydrogen with single-walled boron nitride nanotubes. II. Collision, storage, and adsorption

Collision and adsorption of hydrogen with high incident kinetic energies on a single-walled boron nitride (BN) nanotube have been investigated. Molecular-dynamics (MD) simulations indicate that at incident energies below 14 eV hydrogen bounces off the BN nanotube wall. On the other hand, at incident energies between 14 and 22 eV each hydrogen molecule is dissociated at the exterior wall to form two hydrogen atoms, but only one of them goes through the wall. However, at the incident energies between 23 and 26 eV all of the hydrogen atoms dissociated at the exterior wall are found to be capable of going inside the nanotube and then to recombine to form hydrogen molecules inside the nanotube. Consequently, it is determined that hydrogen should have the incident energy >22 eV to go inside the nanotube. On the other hand, we find that the collisions using the incident energies >26 eV could result in damaging the nanotube structures. In addition our MD simulations find that hydrogen atoms dissociated at the wall cannot bind to either boron or nitrogen atoms in the interior wall of the nanotube.

Grain morphology of intermetallic compounds at solder joints

This study was supported by the Center for Electronic Packaging Materials of the Korea Science and Engineering Foundation.