Yong-Chae Chung

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.

Surface alloy formation of Co on Al surface: Molecular dynamics simulation

Control of the interface structure of atomic scale multilayers is a fundamental issue to improve the performance of spin electronic devices. Deposition behavior of Co on Al surface at 300 K was investigated by molecular dynamics simulation. The deposited Co resulted in the formation of CoAl surface alloy regardless of the Al surface orientation. Structure of the surface alloy was dependent on the substrate orientation. Crystalline B2 structure was formed on Al (001) surface. On the contrary, amorphous mixed layers were evolved on Al (011) and (111) surfaces. In the case of Al (001) surface, 4 ML of the surface alloy were observed, which is consistent with the previous experimental observation. The present work shows that the formation of CoAl surface alloy should be considered even at the low substrate temperature and the low incident energy of deposited atoms.

Achieving type I, II, and III heterojunctions using functionalized MXene.

In the present work, type I, II, and III heterostructures are constructed with the same base material using three representative functionalized monolayer scandium carbides (Sc2CF2, Sc2C(OH)2, and Sc2CO2) by first-principles calculations based on density functional theory. In contrast to general bilayer heterosystems composed of two-dimensional semiconductors, type I and III heterojunctions are obtained in one Sc2CF2/Sc2CO2 system and the remains of the functionalized Sc2C heterostructures, respectively. It is noteworthy that the same monolayer Sc2CF2 and Sc2CO2 constituents lead to dissimilar heterostructure types in the two Sc2CF2/Sc2CO2 systems by modifying the stacking interface. In addition, in the two Sc2CF2/Sc2CO2 systems, remarkable changes in the heterojunction type are induced by a strain, and two distinct type-II heterostructures are generated where one layer with the conduction band minimum state and the other layer including the valence band maximum level are different. The present work suggests an attractive direction to obtain all heterostructure types with the same base material for novel nanodevices in various fields such as photonics, electronics, and optoelectronics using only the two monolayer components Sc2CF2 and Sc2CO2.

Surface characteristics of epitaxially grown Ni layers on Al surfaces: Molecular dynamics simulation

The deposition behavior for Ni thin film growth on Al substrates of various orientations according to the incident energy of adatoms was investigated by molecular dynamics simulation. In spite of the low adatom incident energy of 0.1eV, Ni–Al intermixing occurred actively at the surfaces of Al(001), Al(011), and Al(111) at 80K and Ni atoms apparently favored the island growth mode irrespective of the Al surface orientation. The highest surface roughness was shown for the case of Al(111) surface. The steering effect, which results in rougher surface, was significantly observed at low incident energy. The steering effect was quantitatively investigated through the extensive measurement of the trajectory and deposition flux of atoms with the variation of incident energy near the artificially structured Ni step positioned on Al surfaces.

Stress evolution during the oxidation of silicon nanowires in the sub-10 nm diameter regime

Using a reactive molecular dynamics simulation, the oxidation of Si nanowires (Si-NWs) with diameters of 5, 10, and 20 nm was investigated. The compressive stress at the interface between the oxide and the Si core decreased with increasing curvature in the sub-10 nm regime of the diameter, in contrast to the theory of self-limiting oxidation where rigid mechanical constraint of the Si core was assumed. The Si core of the thinner Si-NW was deformed more with surface oxidation, resulting in a lower compressive stress at the interface. These results explain the experimental observation of full oxidation of very thin Si-NWs.

Molecular dynamics investigation of interfacial mixing behavior in transition metals (Fe, Co, Ni)-Al multilayer system

The interface and surface structure of transition metal (TM) (Fe, Co, and Ni) thin film deposited on Al(001) substrate at atomic level were investigated by molecular dynamics simulation. In spite of the low adatom incident energy of 0.1 eV, TM-Al intermixing occurred actively at the surface of Al(001) at room temperature. At the interface region of TM/Al(001), an intermetallic compound was formed and found to be of B2 structure. The Co/Al system showed different layer coverage and pair correlation function characteristics, in comparison with Fe/Al and Ni/Al systems. The different structural and intermixing characteristics at the interface were successfully explained in terms of lattice matching, cohesive energy, and local acceleration effect.

Calculation of the contribution to grain boundary diffusion in ionic systems that arises from enhanced defect concentrations adjacent to the boundary

The enhancement of the concentration of a defect in the space-charge region near a grain boundary in an intrinsic ionic system was used to specify the change in the corresponding diffusion coefficient as a function of distance, x, normal to the boundary. This zone of enhanced and continuously varying diffusivity serves as a vehicle for enhanced transport along the boundary in addition to the expected contribution arising from the different structure at the boundary core. A two-dimensional diffusion equation was established for concentration c(x,y,t) in the space-charged region, solved numerically using the Crank–Nicolson finite-difference method, and integrated normal to the boundary to provide the average concentration gradient c(y,t) along the boundary—the solute distribution usually measured by experiment. The present gradients were analyzed in the same fashion as experimental data using a recent solution to the conventional model for the grain–boundary diffusion problem in which the interface is trea...

Atomic Investigation of Al/Ni(001) by Molecular Dynamics Simulation

Using molecular dynamics simulation, interfacial features of the Ni–Al magnetic metallic multilayer system were quantitatively investigated with regard to substrate temperature and deposition rate. It was found that a mixture confined to a single atomic layer at the Ni(001) surface was clearly formed at relatively high temperatures, 800–1000 K, and that the degree of mixing at the interface was largely dependent on the deposition rate of adatoms. The degree of lateral displacement of Al adatoms on the Ni(001) surface, which is a key parameter for determining the growth mode, was observed to depend on the deposition rate below 700 K.

Ab-initio investigation of the early stage of nano-scale thin film growth: Co and Al adatoms on Co (111) surface

We considered the diffusion behavior which occurred in the initial stages of thin film deposition for fabrication of tunneling magneto-resistance (TMR) junction with the use of ab-initio density functional theory. Al adatom deposited on the Co (111) substrate had the most stable configuration when it is adsorbed on hcp site and the energy barrier for the diffusion to neighbor fcc or hcp site was low enough to move freely. The energy barrier for the Co on the Co substrate was about 3.8 times higher than Al on Co case. The energies for the step down (step energy barrier) of adatom from the top of the adatom trimer were estimated as 0.15 eVand 1.27 eV for Al/Al trimer/Co and Co/Co trimer/Co, respectively.

Design of amorphous magnetic materials for high frequency sensors based upon permalloy characteristics

Abstract Fabrication of harmonic sensor for the rapidly expanding technology of electronic article surveillance (EAS) is addressed. The focus is on the magnetic characteristics relevant to sensors in EAS. The principal characteristic of materials which make them suitable as harmonic tags is highly non-linear B – H characteristics stemming from near zero magnetic anisotropies. Specifically, field annealing as well as magnetic domain engineering is described for high response signals.