Sung Beom Cho

Tunable Indirect to Direct Band Gap Transition of Monolayer Sc2CO2 by the Strain Effect

MXene has not yet been investigated in optical applications because it is a newly suggested two-dimensional material. In the present work, the first investigation of the prospects of MXene as a novel optical nanodevice was done by applying strain to monolayer Sc2CO2 using first-principles density-functional theory. This single-layer material experiences an indirect to direct band gap transition with variation of the band gap size at a relatively small critical strain of about 2%. The present work emphasizes that monolayer MXene can become a promising material for an optical nanodevice by modulating the band gap properties using strain engineering.

Electronic Properties of Transition‐Metal‐Decorated Silicene

The electronic properties of 3d transition metal (TM)-decorated silicene were investigated by using density functional calculations in an attempt to replace graphene in electronic applications, owing to its better compatibility with Si-based technology. Among the ten types of TM-doped silicene (TM-silicene) studied, Ti-, Ni-, and Zn-doped silicene became semiconductors, whereas Co and Cu doping changed the substrate to a half-metallic material. Interestingly, in cases of Ti- and Cu-doped silicene, the measured band gaps turned out to be significantly larger than the previously reported band gap in silicene. The observed band-gap openings at the Fermi level were induced by breaking the sublattice symmetry caused by two structural changes, that is, the Jahn-Teller distortion and protrusion of the TM atom. The present calculation of the band gap in TM-silicene suggests useful guidance for future experiments to fabricate various silicene-based applications such as a field-effect transistor, single-spin electron source, and nonvolatile magnetic random-access memory.

Interface-Dependent Spin-Reorientation Energy Barrier in Fe/MgO(001) Thin Film

Using the density-functional-theory-based atomic modeling, the stable interface structure and the resultant magnetocrystalline anisotropy (MCA) of the Fe/MgO(001) for magnetic random access memory have been studied. The most stable surface structure of Fe/MgO(001) thin-film system was found to be either defect free or possessing oxygen vacancies in a c(2 ×1) periodicity. The formation of the oxygen vacancies in c(2 ×1) periodicity on MgO(001) surface reduced the MCA of Fe layer from 1.38 to 0.31 meV/atom. The reduced MCA is originated from the filling of the minority states of the Fe orbital below Fermi level.

Effects of uniaxial strains on the magnetic properties and the electronic structures of Fe/graphene system: An ab initio study

Using ab initio calculations, we investigated the changes of the magnetic moment and electronic structures of Fe adatoms on strained graphene sheets. By the uniaxial tensile strains in armchair and zig-zag directions on graphene sheets, the amounts of charge transfers from graphene 2pz orbital to Fe adatom 3d orbitals were linearly increased. The magnetic moments of Fe, however, show the tendency of linear decrements with the uniaxial tensile strains. The increased Fe magnetic moments by uniaxialy graphene compressions resulted from the shifting of spin-minority states of electrons while the decreased Fe magnetic moments were due to the reduction in the spin-majority states of 3dxy-orbitals of the Fe adatom.

Oxygen Vacancy Chain Formation in TiO2 under External Strain for Resistive Switching Memory

The electronic structure and vacancy formation energy of rutile TiO2 with ordered oxygen vacancies were calculated using the density functional theory with on-site Coulomb corrections between Ti 3d orbital and O 2p orbital (LDA+Ud+Up). The calculated band gaps are about 3 eV, using LDA+Ud+Up, and a hybrid functional proposed by Heyd–Scuseria–Ernzerhog. The ordered oxygen vacancies were introduced along the [001] direction within a 3×3×4 supercell of rutile TiO2-x that consisted of 72 Ti and 136 O atoms. Biaxial strain was induced in the rutile TiO2 along the x- and y-directions up to ±5%. The lowest formation energy of ordered oxygen vacancies was found in 5% compressive strain and deemed as a thermodynamically favorable structure.

Configuration Dependency of Attached Epoxy Groups on Graphene Oxide Reduction: A Molecular Dynamics Simulation

The atomic behavior of epoxy groups on a graphene oxide sheet was observed during high thermal heat annealing using a reactive force-field based on molecular dynamics simulations. We found the oxygen-containing functional groups interplay with each other and desorbed from the graphene oxide sheet by a form of O2 gas if they were initially in close distance. Through comparing reduction results of graphene oxide with different densities of the nearest neighboring epoxy pairs, we confirmed that the amount of released O2 gas has a clear tendency to increase with a higher density of epoxy pairs in close distance on a graphene oxide sheet.