Lijun Meng

Molecular dynamics study of ripples in graphene nanoribbons on 6H-SiC(0001): Temperature and size effects

Using the classical molecular dynamics and the simulated annealing techniques, we show that monolayer graphene nanoribbons (GNRs) on 6H-SiC(0001) surface form atomic scale rippled structures. From the analysis of atomic configurations, two different types of rippled structures in GNRs can be identified, namely, the periodic rippled structure at room temperature or even at lower temperatures and random ripples at high temperatures. The dependence of microscopic roughness of the ripples on temperature and size are studied through analyzing the covalent bonding inhomogeneities in bond-length and bond-angle distributions. Our results provide atomic-level information about the rippled GNRs on SiC substrate, which is useful not only for understanding the structure and stability of monolayer GNRs but also for future applications of GNRs in nanoelectronics.

Growth of graphene structure on 6H-SiC(0001) : Molecular dynamics simulation

The growth process of graphene structure on 6H-SiC(0001) surface has been studied using the classical molecular dynamics simulation and the simulated annealing technique. Effects of annealing temperature and coverage of carbon atoms on the formation of graphene have been investigated. We found that two layers of carbon atoms of the 6H-SiC(0001) subsurface after sublimation of Si atoms undergo a transformation from a diamondlike phase to a graphenelike structure at annealing temperature above 1500K. This transformation temperature is in good agreement with experimental observations. We also found that the formation of graphene structure strongly depends on the number of carbon layers. Two layers of carbon atoms result in large graphene clusters and four layers of carbon lead to the formation of graphene bilayer sheets. However, a single layer of carbon only forms chainlike and ringlike clusters without the hexagonal ordering. Our results provide atomic-level information about the graphitization of the 6H-S...

Direct and quasi-direct band gap silicon allotropes with remarkable stability

In our present work, five previously proposed sp(3) carbon crystals were suggested as silicon allotropes and their stabilities, electronic and optical properties were investigated using the first-principles method. We find that these allotropes with direct or quasi-direct band gaps in a range of 1.2-1.6 eV are very suitable for applications in thin-film solar cells. They display strong adsorption coefficients in the visible range of sunlight in comparison with diamond silicon. These five silicon allotropes are confirmed to possess positive dynamical stability and remarkable themodynamical stability close to that of diamond silicon. In particular, the direct band gap M585-silicon possessing energy higher than diamond silicon only 25 meV per atom is expected to be experimentally produced for thin-film solar cells.

Structural phase transitions of FeCo and FeNi nanoparticles: A molecular dynamics study

We have investigated the structural phase transition of FeCo and FeNi nanoparticles by molecular dynamics (MD) simulation using the generalized embedded atom potential (GEAM). It is found that the phase varies with the atomic compositions and annealing processes. By using the Honeycutt and Andersen index (HA index), bond order parameters (BOP) and pair correlation function (PCF), we found that a BCC to defective icosahedra phase transition occurs in the FeCo nanoparticle when Co composition is increased to about 60 at %. In the FeNi nanoparticle, three phases have been identified, namely, the BCC phase, the mixed BCC/FCC phase, and the multilayer defective icosahedral phase, which correspond to the Ni compositions of 0–20 at %, 20–70 at %, and 70–100 at %, respectively. Our simulations have well reproduced the phase transition points and most of the phases observed in recent experiments.

Surface reconstruction and core distortion of silicon and germanium nanowires

We report the results of molecular dynamics simulations for structures of pristine silicon nanowires and germanium nanowires with bulk cores oriented along the [110] direction and bounded by the (100) and (110) surfaces in the lateral direction. We found that the (100) surfaces for both silicon and germanium nanowires undergo 2 ? 1 dimerization while their (110) surfaces do not show reconstruction. The direction of the dimer rows is either parallel or perpendicular to the wire axis depending on the orientation of the surface dangling bonds. The dimer length for Si is in good agreement with the result obtained by first-principles calculations. However, the geometry of Si dimers belongs to the symmetrical 2 ? 1 reconstruction rather than the asymmetrical buckled dimers. We also show that surface reconstruction of a small nanowire induces significant change in the lattice spacing for the atoms not on the (100) surface, resulting in severe structural distortion of the core of the nanowire.

Two-dimensional topological insulators with tunable band gaps: Single-layer HgTe and HgSe

Two-dimensional (2D) topological insulators (TIs) with large band gaps are of great importance for the future applications of quantum spin Hall (QSH) effect. Employing ab initio electronic calculations we propose a novel type of 2D topological insulators, the monolayer (ML) low-buckled (LB) mercury telluride (HgTe) and mercury selenide (HgSe), with tunable band gap. We demonstrate that LB HgTe (HgSe) monolayers undergo a trivial insulator to topological insulator transition under in-plane tensile strain of 2.6% (3.1%) due to the combination of the strain and the spin orbital coupling (SOC) effects. Furthermore, the band gaps can be tuned up to large values (0.2 eV for HgTe and 0.05 eV for HgSe) by tensile strain, which far exceed those of current experimentally realized 2D quantum spin Hall insulators. Our results suggest a new type of material suitable for practical applications of 2D TI at room-temperature.

New candidate for the simple cubic carbon sample shock-synthesized by compression of the mixture of carbon black and tetracyanoethylene

Traditionally, all superhard carbon phases including diam ond are electric insulators and all conductive carbon phases including graphite are mechanically soft. Ba sed on first-principles calculation results, we report a superhard but conductive carbon phase C21-sc which can be o btained through increasing the sp 3 bonds in the previously proposed soft and conductive phase C20-sc ( Phys. Rev. B 74, 172101 2006). We also show that further increase of sp 3 bonds in C21-sc results in a superhard and insulating phase C 22-sc with sp3 bonds only. With C20-sc, C21-sc, C22-sc and graphite, the X-ray di ffraction peaks from the unidentified carbon material synthesized by compressing the mixture of tetracyanoethyl ne and carbon black ( Carbon, 41, 1309, 2003) can be understood. In view of its positive stability, superhard n conductive features, and the strong possibility of existence in previous experiments, C21-sc is a promising mu lti-functional material with potential applications in extreme conditions.

Dewetting and detachment of Pt nanofilms on graphitic substrates: A molecular dynamics study

We have investigated the dynamics of dewetting and detachment of nanoscale platinum (Pt) films on graphitic substrates using molecular dynamics (MD). For the thinner Pt nanofilms ( 0.6 nm), nanodroplets are formed directly. Interestingly, the nanodroplets can detach from the substrate and the detachment velocity (vd) increases and then decreases as the film gets thicker. We have analyzed the dependence of the detachment velocity on the thickness of the nanofilm by considering the conversion of surface energy to the kinetic energy of a droplet. In addition, the effect of temperature on the dewetting and detachment behavior of the Pt films is also discussed. Our results show that vd increases monotonically with temperature. These results are important for understanding the dewetting and detachment dynamics of metal films o...

A quasicore-shell structure of FeCo and FeNi nanoparticles

Based on semiempirical generalized embedded atom method (GEAM), we carried out molecular dynamics and Monte Carlo simulations to study the structural properties of FeCo and FeNi nanoparticles. It is found that these two kinds of nanoparticles possess a new stable quasicore-shell structure, no matter whether they are in molten or condensed state and whether they are prepared by annealing or quenching. In FeCo (FeNi) nanoparticles of various sizes and atom compositions, the quasicore-shell structure is always preferred, with the shell formed only by Fe atoms and the core formed by randomly distributed Co(Ni) and Fe atoms. We have also investigated the formation mechanism of the quasicore-shell structure by energy difference analysis of the pure and doped icosahedra structure of FeCo and FeNi nanoparticles.

Small Si clusters on surfaces of carbon nanotubes

Structures of small Si clusters, Sin, on surfaces of carbon nanotubes have been studied by molecular dynamics simulation. We show that the lowest-energy structures of Sin are three-dimensional clusters rather than thin Si sheets covering the surface of a nanotube. As n increases from 10 to 30, Sin undergoes structural transitions from a tentlike structure (with nanotube surface as its base) to a cagelike structure (without interior atoms) and further to a spherical compact structure (with interior atoms). Our results are different from the structures of small Si clusters found in a free space without Si-nanotube interaction.