Chunxiao Zhang

Magnetic Properties of Single Transition-Metal Atom Absorbed Graphdiyne and Graphyne Sheet from DFT+U Calculations

The electronic and magnetic properties of single 3d transition-metal (TM) atom (V, Cr, Mn, Fe, Co, and Ni) adsorbed graphdiyne (GDY) and graphyne (GY) are systematically studied using density functional theory (DFT). It is found that the electronic structures of TM-GDY/GY are sensitive to the value of the on-site Coulomb energy for the TM 3d orbital. It is crucial to use DFT+U method and accurately account for the electron correlation in the calculations. By using linear response method, we are able to determine the Ueff value for all TM adatom. We find that the adsorption of TM atom not only efficiently modulates the electronic structures of GDY/GY system but also introduces excellent magnetic properties, such as spin-polarized half-semiconductor. Such modulation originates from the charge transfer between TM adatom and GDY/GY sheet as well as the electron redistribution of the TM intra-atomic s, p, and d orbitals. Our results indicate that the TM adsorbed GDY/GY are excellent candidates for spintronics.

The structural, electronic and magnetic properties of bi-layered MoS2 with transition-metals doped in the interlayer

We have carried out first-principles calculations and theoretical analysis to explore the structural, spin-polarized electronic and magnetic properties of bi-layered MoS2 with transition-metal (TM) atoms (Cr, Mn, Fe, Co and Ni) doped in the interlayer. The charge density distribution indicates that the doping TM atoms and the nearest S atoms in the lower and upper planes display a clear covalent-bonding feature. The local moments of the doping TM atoms are smaller than the magnetic moments of their free states. Also, the spin polarization is found to be 100% at the Fermi level or HOMO level for interlayer doping with Cr, Mn, Fe and Co.

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.

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.

Low energy three-dimensional hydrocarbon crystal from cold compression of benzene.

We demonstrate an interesting phase transition from cold compressed benzene to a fully saturated three-dimensional hydrocarbon crystal, Hex-CH. The very low transition point pressure, remarkable energetic stability and positive dynamical stability indicate that Hex-CH is a promising three-dimensional hydrocarbon crystal. As a transparent insulator and potential hard hydrocarbon material, Hex-CH is expected to be of general interest in organic chemistry, condensed matter physics and material science.

Electronic structures and optical properties of hexagonal boron nitride under hydrostatic pressures

Electronic structures and optical properties of hexagonal boron nitride (h-BN) under hydrostatic pressure are studied using density functional theory calculations. Charge density difference, density of states, band structures, and dielectric functions are calculated to reveal the evolution of the electronic structures, band-gap, and optical properties of five typical stackings of h-BN corresponding to the hydrostatic pressures. The band-gap of h-BN decreases with the increasing hydrostatic pressure. The band-gap of h-BN with AA and AF stacking decreases faster than that of the others. The positions of their valence band maximum and the conduction band minimum shift in the Brillouin Zone corresponding to the external hydrostatic pressure, depending on the different stackings. In particular, the band structure of AA becomes direct at 9.19 GPa, which does not occur in other stackings. The band-edge optical absorption thresholds of AA, AD, AE, and AF show the redshift as the pressure increases, except for tha...

Five low energy phosphorene allotropes constructed through gene segments recombination

Based on the crystal structures of the previously proposed low energy η-P and θ-P, five new phosphorene allotropes were predicted through gene segments recombination method. These five new phosphorene allotropes are confirmed dynamically stable and energetically more favorable than their parents (η-P and θ-P). Especially, the XX-XX type G1-P is confirmed energetically more favorable than most of all the previously proposed phosphorene allotropes, including black phosphorene and blue phosphorene, which is highly expected to be synthesized in future experiment through vapor deposition or epitaxial growth method like blue β-P. The calculated results also show that such a new promising phosphorene allotrope G1-P is a potential candidate for application in nano-electronics according to its middle band gap of about 1.491 eV from DFT-HSE06 calculation.

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.

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.

Effect of hydrogen passivation on the decoupling of graphene on SiC(0001) substrate: First-principles calculations

Intercalation of hydrogen is important for understanding the decoupling of graphene from SiC(0001) substrate. Employing first-principles calculations, we have systematically studied the decoupling of graphene from SiC surface by H atoms intercalation from graphene boundary. It is found the passivation of H atoms on both graphene edge and SiC substrate is the key factor of the decoupling process. Passivation of graphene edge can weaken the interaction between graphene boundary and the substrate, which reduced the energy barrier significantly for H diffusion into the graphene-SiC interface. As more and more H atoms diffuse into the interface and saturate the Si dangling bonds around the boundary, graphene will detach from substrate. Furthermore, the energy barriers in these processes are relatively low, indicating that these processes can occur under the experimental temperature.