Gun‐Do Lee

Diffusion, coalescence, and reconstruction of vacancy defects in graphene layers are investigated by tight-binding molecular dynamics (TBMD) simulations and by first principles total energy calculations. It is observed in the TBMD simulations that two single vacancies coalesce into a 5-8-5 double vacancy at the temperature of 3000 K, and it is further reconstructed into a new defect structure, the 555-777 defect, by the Stone-Wales type transformation at higher temperatures. First principles cal

We perform ab initio pseudopotential total-energy calculations for ZnS, ZnSe, and ZnTe. Unlike in most previous calculations, we include Zn 3d orbitals (and, in case of ZnTe, Te 4d orbitals as well) as part of the valence states in order to study the behavior of the d electrons and their influence on energy levels. The results for the structural and electronic properties are in better agreement with experimental data than in the case where d electrons are considered as part of the core states. T

The dynamics of multivacancy defects in a graphene layer is investigated by tight-binding molecular dynamics simulations and by first principles calculation. The simulations show that four single vacancies in the graphene layer first coalesce into two double vacancies, each consisting of a pentagon-heptagon-pentagon (5-8-5) defective structure. While one of the 5-8-5 defects further reconstructs into a 555-777 defect, which is composed of three pentagonal rings and three heptagonal rings, anothe

The reconstruction and evaporation at graphene nanoribbon (GNR) edges are investigated by tight-binding molecular-dynamics simulations and ab initio calculations. It is observed that reconstruction through the formation of pentagon-heptagon pairs can take place quickly along the zigzag edge and it is energetically favorable. At very high temperatures, the armchair edge is found to change into a zigzag edge structure, which further accelerates the evaporation of carbon atoms and leads to the form

Heat-induced structural transformation in nanodiamond of diameter approximately 1.4 nm is investigated by tight-binding molecular dynamics simulations using the environment-dependent tight-binding carbon potential. The nanodiamond is found to transform into a tube-shaped fullerene via annealing. Three interesting mechanisms for promoting inner carbon atoms of the nanodiamond into the surface carbon atoms of the tubular structure are observed. The "flow-out" mechanism prevails at temperatures low

We have performed ab initio pseudopotential calculations within the local-density approximation to investigate the structural phase transition of ZnTe under pressure. By calculating the total energy, atomic forces, and stress tensors, we theoretically determine the structural phase transition of ZnTe from the zinc-blende to the cinnabar to the orthorhombic structure under increasing pressure, which agrees well with experiment. We demonstrate that rotation of bonds toward lower-symmetry positions

The reconstruction process of vacancy hole in carbon nanotube is investigated by tight-binding molecular dynamics simulations and by ab initio total energy calculations. In the molecular dynamics simulation, a vacancy hole is found to reconstruct into two separated pentagon-heptagon pair defects. As the result of reconstruction, the radius of the carbon nanotube is reduced and the chirality of the tube is partly changed. During the vacancy hole healing process, the formation of pentagonal and he

We use time-dependent HRTEM to reveal that stable dislocation pairs in graphene are formed from an initial complex multi-vacancy cluster that undergoes multiple bond rotations and adatom incorporation. In the process, it is found that the transformation from the formed complex multi-vacancy cluster can proceed without the increase of vacancy because many atoms and dimers are not only evaporated but also actively adsorbed. In tight-binding molecular dynamics simulations, it is confirmed that adat

The diffusion pathways between the trough and the dimer row on the Si(100) surface are investigated by tight-binding molecular dynamics calculations using the environment-dependent tight-binding silicon potential and by ab initio calculations using the Car-Parrinello method. The studies discover a new diffusion pathway consisting of the rotation of the ad-dimer. The calculated energy barrier is in excellent agreement with experiment and is much more energetically favorable than other diffusion p

The formation and development processes of dislocation in graphene are investigated by performing tight-binding molecular dynamics (TBMD) simulation and ab initio total energy calculation. It is found that the coalescence of pentagon-heptagon (5-7) pairs with vacancy defects induces the formation of dislocation due to the separation of two 5-7 pairs. In TBMD simulations, adatoms are ejected and evaporated from graphene surface so that the dislocation is developed. It is observed that diffusing c

Atomistic processes of carbon nanotube semiconductor-metal intramolecular junction formation are investigated by tight-binding molecular dynamics simulations and first-principles total energy calculations. We show that the junctions can be formed by reconstruction of vacancy clusters through a series of generalized Stone-Wales transformations [Chem. Phys. Lett. 128, 501 (1986)]. Our simulations suggest a mechanism for synthesis of carbon nanotube semiconductor-metal intramolecular junctions with

The catalytic decomposition process in the initial stage of carbon nanotube (CNT) growth is investigated for the system of acetylene $({\mathrm{C}}_{2}{\mathrm{H}}_{2})$ on the Fe(001) surface through first-principles electronic structure calculation. In its most stable configuration, the C-C bond of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ is lying on the hollow site along the [110] direction of the Fe(001) surface. The formation of the CH fragments accompanied by C-C bond breaking is found to be an

Al-Zn-Mg alloys are widely used in the transportation industry owing to their high strength-to-weight ratio. In these alloys, the main strengthening mechanism is precipitation hardening that occurs because of the formation of nano-sized precipitates. Herein, an interfacial structure of η₄ precipitates, one of the main precipitates in these alloys, is revealed using aberration-corrected scanning transmission electron microscopy and first-principles calculations. These precipitates exhi

Amorphous carbon (a-C) films have attracted significant attention due to their reliable structures and superior mechanical, chemical and electronic properties, making them a strong candidate as an etch hard mask material for the fabrication of future integrated semiconductor devices. Density functional theory (DFT) calculations and <i>ab initio</i> molecular dynamics (AIMD) simulations were performed to investigate the energetics, structure, and mechanical properties of the a-C films with an inc

Vapor phase carbon (C)-reduction-based syntheses of C nanotubes and graphene, which are highly functional solid C nanomaterials, have received extensive attention in the field of materials science. This study suggests a revolutionary method for precisely controlling the C structures by oxidizing solid C nanomaterials into gaseous products in the opposite manner of the conventional approach. This gaseous nanocarving enables the modulation of inherent metal assembly in metal/C hybrid nanomaterials