Kaiwang Zhang

Melting and premelting of carbon nanotubes

We report the results of molecular dynamics simulations of melting and premelting of single-walled carbon nanotubes (SWNTs). We found that the traditional critical Lindemann parameter for the melting of bulk crystals is not valid for SWNTs. Using the much smaller critical Lindemann parameter for the melting of nanoparticles as a criterion, we show that the melting temperature of perfect SWNTs is around 4800 K. We further show that Stone–Wales defects in a SWNT significantly reduce the melting temperature of atoms around the defects, resulting in the premelting of SWNTs at 2600 K.

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.

Magnetic Exchange Coupling and Anisotropy of 3d Transition Metal Nanowires on Graphyne

Applying two-dimensional monolayer materials in nanoelectronics and spintronics is hindered by a lack of ordered and separately distributed spin structures. We investigate the electronic and magnetic properties of one-dimensional zigzag and armchair 3d transition metal (TM) nanowires on graphyne (GY), using density functional theory plus Hubbard U (DFT + U). The 3d TM nanowires are formed on graphyne (GY) surfaces. TM atoms separately and regularly embed within GY, achieving long-range magnetic spin ordering. TM exchange coupling of the zigzag and armchair nanowires is mediated by sp-hybridized carbon, and results in long-range magnetic order and magnetic anisotropy. The magnetic coupling mechanism is explained by competition between through-bond and through-space interactions derived from superexchange. These results aid the realization of GY in spintronics.

Structure, stability and electronic properties of tricycle type graphane

We propose a new allotrope of graphane, named as tricycle graphane, with a 4up/2down UUUDUD hydrogenation in each hexagonal carbon ring, which is different from previously proposed allotropes with UUDUUD (boat-1) and UUUUDD (boat-2) types of hydrogenation. Its stability and electronic structures are systematically studied using first-principles method. We find that the tricycle graphane is a stable phase in between the previously proposed chair and stirrup allotropes. Its electronic properties are very similar to those of chair, stirrup, boat-1 , boat-2, and twist-boat allotropes. The negative Gibbs free energy of tricycle graphane is -91 meV/atom, which very close to that of the most stable chair one (-103 meV/atom). Thus, this new two-dimensional hydrocarbon may be produced in the process of graphene hydrogenation with a relative high probability compared to other conformers.

Size and boundary scattering controlled contribution of spectral phonons to the thermal conductivity in graphene ribbons

Although graphene holds great promise in thermal applications owing to its superior thermal conductivity, an intriguing question remains as to which polarizations and frequencies are dominant in its heat conduction. In this work, by incorporating the direction-dependent phonon-boundary scattering and the special selection rule for three-phonon scattering into the linearized phonon Boltzmann transport equation, we systematically investigate the relative contributions from longitudinal-acoustic, transverse-acoustic, and out-of-plane acoustic (ZA) branches to the thermal conductivity of graphene ribbons, focusing on the effects of their size and temperature. We find that the relative contribution from ZA branch to heat conduction increases with decreasing the size, specularity parameter, and temperature of graphene ribbons. Our analysis reveals that this change arises from the huge difference in the phonon dispersion and in the phonon mean free path of Umklapp process between in-plane and out-of-plane branches.

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.

Modulation effect of hydrogen and fluorine decoration on the surface work function of BN sheets

Using first-principles calculations within the framework of density-functional theory, we studied the modulation effect of hydrogen/fluorine chemical decoration on the surface work function of BN sheets. We found that the difference in the work function (ΔWBN) between two surfaces of the chair structure varies with the different decoration. Geometric distortion and chemical effects cause opposite modulation effects, and the chemical effect plays a leading role by inducing charge redistribution in the system.

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.

Prediction of two planar carbon allotropes with large meshes

Two novel two-dimensional (2D) carbon allotropes named C(y) and C(z) with large meshes are predicted based on first-principles calculations. Their formation energies are lower than that of graphdiyne, which was recently synthesized in an experiment. Molecular dynamics simulations indicate that C(y) and C(z) are stable even when the temperature is over 1000 K. The calculated Poissons ratios of C(y) and C(z) show their anisotropic mechanical properties. The electronic structure calculations indicate that C(y) is a metal, while C(z) behaves as a semiconductor. Moreover, C(z) shows conductive anisotropy suggesting its potential in nanoelectronic devices. Meanwhile, their well-defined mesh structures are suitable for molecular sieves.