• Carbon letters
• /
• v.16 no.3
• /
• pp.198-202
• /
• 2015

Carbon-supported Pt catalyst systems containing defect adsorption sites on the anode of direct methanol fuel cells were investigated, to elucidate the mechanisms of H₂ dissociation and carbon monoxide (CO) poisoning. Density functional theory calculations were carried out to determine the effect of defect sites located neighboring to or distant from the Pt catalyst on H₂ and CO adsorption properties, based on electronic properties such as adsorption energy and electronic band gap. Interestingly, the presence of neighboring defect sites led to a reduction of H₂ dissociation and CO poisoning due to atomic Pt filling the defect sites. At distant sites, H₂ dissociation was active on Pt, but CO filled the defect sites to form carbon π-π bonds, thus enhancing the oxidation of the carbon surface. It should be noted that defect sites can cause CO poisoning, thereby deactivating the anode gradually.

• Proceedings of the IEEK Conference
• /
• 2003.07b
• /
• pp.731-734
• /
• 2003

An atomistic process modeling, Kinetic Monte Carlo simulation, has the advantage of being both conceptually simple and extremely powerful. Instead of diffusion equations, it is based on the definitions of the interactions between individual atoms and defects. Those interactions can be derived either directly from molecular dynamics, first principles calculations, or from experiment. In this paper, as a simple illustration of the kinetic Monte Carlo we simulate defects (self-interstitials and vacancies) diffusion after ion implantation in Si crystalline.

• Coupled systems mechanics
• /
• v.5 no.4
• /
• pp.305-314
• /
• 2016

We utilize first-principles calculations within density-functional theory to investigate the possibility of strain engineering in the tuning of the band structure of two-dimensional $MoS_2$. We find that the band structure of $MoS_2$ monolayers transits from direct to indirect when mechanical strain is applied. In addition, we discuss the change in the band gap energy and the critical stains for the direct-to-indirect transition under various strains such as uniaxial, biaxial, and pure shear. Biaxial strain causes a larger change, and the pure shear stain causes a small change in the electronic band structure of the $MoS_2$ monolayer. We observe that the change in the interaction between molecular orbitals due to the mechanical strain alters the band gap type and energy.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2010.08a
• /
• pp.56-56
• /
• 2010

Using first-principles density-functional theory calculations, we find dramatically different electronic states in the C chains generated on the H-terminated C(111) surface, depending on their length and parity. The infinitely long chain has $\pi$ electrons completely delocalized over the chain, yielding an equal C-C bond length. As the chain length becomes finite, such delocalized $\pi$ electrons are transformed into localized ones. As a result, even-numbered chains exhibit a strong charge-lattice coupling, leading to a bond-alternated structure, while odd-numbered chains show a ferrimagnetic spin ordering with a solitonlike structure. These geometric and electronic features of infinitely and finitely long chains are analogous to those of the closed (benzene) and open (polyacetylene) chains of hydrocarbons, respectively.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2014.02a
• /
• pp.415.1-415.1
• /
• 2014

Carrying out first-principles calculations, we study N-doped capped carbon nanotube (CNT) electrodes applied to DNA sequencing. While we obtain for the face-on nucleobase junction configurations a conventional conductance ordering where the largest signal results from guanine according to its high highest occupied molecular orbital (HOMO) level, we extract for the edge-on counterparts a distinct conductance ordering where the low-HOMO thymine provides the largest signal. The edge-on mode is shown to operate based on a novel molecular sensing mechanism that reflects the chemical connectivity between N-doped CNT caps that can act both as electron donors and electron acceptors and DNA functional groups that include the hyperconjugated thymine methyl group[1].

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
• /
• 2001.05a
• /
• pp.341-344
• /
• 2001

We investigate the electronic structure of perovskite PbTiO$_3$ and the microscopic origin of the ferroelectric lattice instability through first-principles pseudopotential calculations. We examine pseudo Jahn-Teller effect to discuss the lattice instability. The JT effect is caused by the hybridization of the p-orbitals of O atoms and d-orbital of Ti atom. We find the JT effect is most significant at Brillouin zone renter.

• Proceedings of the Korean Vacuum Society Conference
• /
• 2011.02a
• /
• pp.314-314
• /
• 2011

Using first principles calculations, we investigated the interface structure effects on the magnetic properties of the Fe/BaTiO3 system. On the BaO-terminated surface, a Fe monolayer is formed as two Fe atoms are adsorbed on the top sites of Ba and O in the ($1{\times}1$) surface unit and a Fe ML is formed on the TiO2-terminated surface as two Fe atoms are adsorbed on the two O top sites. The magnetic anisotropy energy of Fe was higher on the TiO2?-erminated surface (1.5 eV) than on the BaO-terminated surface (0.5 eV). The decomposed electron density of the states showed that the stronger hybridization of Fe with the TiO2 layer than with the BaO layer is the most important reason for the higher magnetic anisotropy energy.

• Proceeding of EDISON Challenge
• /
• 2015.03a
• /
• pp.367-370
• /
• 2015

Monolayer transition metal dicalcogenide (TMDC) materials are currently attracting extensive attention due to their distinctive electronic, transport, and optical properties. For example, monolayer $MoS_2$ exhibits a direct band gap in the visible frequency range, which makes it an attractive candidate for the photocatalytic water splitting. For the photoelectrochemical water splitting, the appropriate band edge positions that overlap with the water redox potential are necessary. Similarly, appropriate band level alignments will be crucial for the light emitting diode and photovoltaic applications utlizing heterojunctions between two TMDC materials. Carrying out first-principles calculations, we here investigate how the band edges of $MoS_2$ can be adjusted by surface ligand functionalization. This study will provide useful information for the realization of ligand-based band engineering of monolayer $MoS_2$ for various electronic, energy, and bio device applications.

• Journal of Magnetics
• /
• v.16 no.4
• /
• pp.332-336
• /
• 2011

We studied the effect on the electronic and magnetic properties of the N defect in clean and Cu-doped wurtzite III-nitrides by using the first-principles calculations. When it is doped two Cu atoms in the nearest neighboring sites, the system of AlN, GaN, or InN with the N vacancy is energetically more favorable than that without the N vacancy site. When the Cu concentration increases, the total magnetic moment of a supercell becomes small. The ferromagnetism of Cu atom is very low due to the weak 3d-3d coupling. It is noticeable that the spin-exchange interaction between the Cu-3d and N defect states is important.

• Proceedings of the Korean Magnestics Society Conference
• /
• 2013.12a
• /
• pp.29-29
• /
• 2013

We predict agigantic perpendicular magnetocrystalline anisotropy (MCA) in Fe (001) capped by 5d transition metal (TM) overlayers by using first principles calculations. Analysis of atom-by-atom contribution to MCA reveals that gigantic MCA as large as 11 meV/TM originates not from Fe atoms but from the 5d TMs through the strong spin-orbit coupling. More specifically, it is the hybridization between TM and Fe d orbitals that also induces non-negligible magnetic moments in TM. Furthermore, spin-channel decompositions of MCA matrix with and without the presence of Fe substrate identify the electronic origin of the perpendicular MCA that the down-down channel contribution plays the most crucial role for the sign changes of MCA of TM overlayers upon the hybridization with Fe-3d.