C.Z. Wang

Electronic and optical properties of novel carbon allotropes

Abstract The vibrational properties, electronic structures and optical properties of novel carbon allotropes, such as monolayer penta-graphene (PG), double-layer PG and T12-carbon, were studied by first-principles calculations. Results of phonon calculations demonstrate that these exotic carbon allotropes are dynamically stable. The bulk T12 phase is an indirect-gap semiconductor having a quasiparticle (QP) bandgap of ∼5.19xa0eV. When the bulk material transforms to a two-dimensional (2D) phase, the monolayer and double-layer PG become quasi-direct gap semiconductors with smaller QP bandgaps of ∼4.48xa0eV and ∼3.67xa0eV, respectively. Furthermore, the partial charge density analysis indicates that the 2D phases retain part of the electronic characteristics of the T12 phase. The linear photon energy-dependent dielectric functions and related optical properties including refractive index, extinction coefficient, absorption spectrum, reflectivity, and energy-loss spectrum were also computed and discussed. Additionally, the chemical stability of monolayer PG and the electronic and optical properties of double-side hydrogenated monolayer PG were also investigated. The results obtained from our calculations are beneficial to practical applications of these exotic carbon allotropes in optoelectronics and electronics.

Relative stability of C78 isomers

Abstract The structural and electronic properties of the five C78 isolated-pentagon isomers are studied using all-electron local-density functional calculations with complete geometry optimization. The results are compared with available experimental data, and suggest that kinetic factors play an important role in the formation of large fullerenes.

Medium-sized silicon oxide clusters by Si3O3-ring assembly

Abstract Using Si 2 O 2 rhombuses and Si 3 O 3 rings as building blocks, we have investigated the structures of medium-sized silicon oxide clusters. Total energies of a series of Si 8 O n and Si 12 O n clusters with different growth motifs have been studied using first-principles calculations. Our studies discover a new motif for medium-sized silicon oxide clusters in a form of wheel-like structure composed of Si 3 O 3 rings.

Cooling rate dependence of structural order in Al90Sm10 metallic glass

The atomic structure of Al90Sm10 metallic glass is studied using molecular dynamics simulations. By performing a long sub-Tg annealing, we developed a glass model closer to the experiments than the models prepared by continuous cooling. Using the cluster alignment method, we found that “3661” cluster is the dominating short-range order in the glass samples. The connection and arrangement of “3661” clusters, which define the medium-range order in the system, are enhanced significantly in the sub-Tg annealed sample as compared with the fast cooled glass samples. Unlike some strong binary glass formers such as Cu64.5Zr35.5, the clusters representing the short-range order do not form an interconnected interpenetrating network in Al90Sm10, which has only marginal glass formability.

Solute–solute correlations responsible for the prepeak in structure factors of undercooled Al-rich liquids: a molecular dynamics study

We have performed molecular dynamics simulations on a typical Al-based alloy Al90Sm10. The short-range and medium-range correlations of the system are reliably produced by ab initio calculations, whereas the long-range correlations are obtained with the assistance of a semi-empirical potential well-fitted to ab initio data. Our calculations show that a prepeak in the structure factor of this system emerges well above the melting temperature, and the intensity of the prepeak increases with increasing undercooling of the liquid. These results are in agreement with x-ray diffraction experiments. The interplay between the short-range order of the system originating from the large affinity between Al and Sm atoms, and the intrinsic repulsion between Sm atoms gives rise to a stronger correlation in the second peak than the first peak in the Sm-Sm partial pair correlation function (PPCF), which in turn produces the prepeak in the structure factor.

Effects of strontium impurity on the structure and dynamics of Al88Si12 liquid

The effects of strontium modification on the structure and dynamics of Al88Si12 liquid are studied by means of ab initio molecular dynamics simulations. By replacing 0.5% and 4.0% of Al with Sr, we show that the addition of Sr lowers the self-diffusion of Al and Si of the liquid and reduces the nearest-neighbor correlation between Si atoms. The simulation provides an explanation for the change in morphology of the eutectic phases observed in rapidly solidified Al-Si alloys modified with Sr.

Electronic and spin transport properties of graphene nanoribbon mediated by metal adatoms: a study by the QUAMBO-NEGF approach

Structural and electronic properties, including deformation, magnetic moment, Mulliken population, bond order, as well as electronic transport properties, of zigzag graphene nanoribbon (ZGNR) with Co adatoms on hollow sites are investigated by quasi-atomic minimal basis orbits (QUAMBOs), a first-principles tight binding (TB) scheme based on density functional theory (DFT), combined with a non-equilibrium Greens function. For electronic transport, below the Fermi level the transmission is strongly suppressed and spin dependent as a result of magnetism by Co adatom adsorption, while above the Fermi level the transmission is slightly distorted and spin independent. Due to the local environment dependence of QUAMBOs-TB parameters, we construct QUAMBOs-TB parameters of ZGNR leads and ZGNR with Co adatoms on hollow center sites by a divide-and-conquer approach, and accurately reproduce the electronic transmission behavior. Our QUAMBO-NEGF method is a new and promising way of examining electronic transport in large-scale systems.

Discovery of a metastable Al20Sm4 phase

We present an efficient genetic algorithm, integrated with experimental diffraction data, to solve a nanoscale metastable Al20Sm4 phase that evolves during crystallization of an amorphous magnetron sputtered Al90Sm10 alloy. The excellent match between calculated and experimental X-ray diffraction patterns confirms an accurate description of this metastable phase. Molecular dynamic simulations of crystal growth from the liquid phase predict the formation of disordered defects in the devitrified crystal.

Simulation of alnico coercivity

Micromagnetic simulations of alnico show substantial deviations from Stoner-Wohlfarth behavior due to the unique size and spatial distribution of the rod-like Fe-Co phase formed during spinodal decomposition in an external magnetic field. The maximum coercivity is limited by single-rod effects, especially deviations from ellipsoidal shape, and by interactions between the rods. Both the exchange interaction between connected rods and magnetostatic interaction between rods are considered, and the results of our calculations show good agreement with recent experiments. Unlike systems dominated by magnetocrystalline anisotropy, coercivity in alnico is highly dependent on size, shape, and geometric distribution of the Fe-Co phase, all factors that can be tuned with appropriate chemistry and thermal-magnetic annealing.

Structural and electronic properties of large fullerenes

We present the results of a first-principles calculation for the geometrical and electronic structure of C78 and C84 fullerene isomers. Our study confirms that for C84 two isomers withD2 andD2d symmetries are the most energetically favorable, and for C78 aC2v isomer is the ground-state configuration. Results from the present calculation are compared with recent experimental observations.