Hongxia Bu

Isoelectronic Doping of Graphdiyne with Boron and Nitrogen: Stable Configurations and Band Gap Modification

Graphdiyne, consisting of sp- and sp(2)-hybridized carbon atoms, is a new member of carbon allotropes which has a natural band gap ~1.0 eV. Here, we report our first-principles calculations on the stable configurations and electronic structures of graphdiyne doped with boron-nitrogen (BN) units. We show that BN unit prefers to replace the sp-hybridized carbon atoms in the chain at a low doping rate, forming linear BN atomic chains between carbon hexagons. At a high doping rate, BN units replace first the carbon atoms in the hexagons and then those in the chains. A comparison study indicates that these substitution reactions may be easier to occur than those on graphene which composes purely of sp(2)-hybridized carbon atoms. With the increase of BN component, the band gap increases first gradually and then abruptly, corresponding to the transition between the two substitution motifs. The direct-band gap feature is intact in these BN-doped graphdiyne regardless the doping rate. A simple tight-binding model is proposed to interpret the origin of the band gap opening behaviors. Such wide-range band gap modification in graphdiyne may find applications in nanoscaled electronic devices and solar cells.

Graphdiyne: A promising anode material for lithium ion batteries with high capacity and rate capability

We have carried out first-principles calculations to explore the energetics and dynamics of Li in graphdiyne monolayers. The porous structure of graphdiyne enables both in-plane and out-plane diffusion of Li ions with moderate barriers, 0.35–0.52 eV. A unique Li occupation pattern named as a triangular pattern is identified, with Li atoms occupying three symmetric sites in the triangular-like pores. Based on this occupation pattern, the Li storage capacity of single-layer graphdiyne can be as high as LiC3, which is twice the capacity of commonly used graphite (LiC6). With high Li mobility and high storage capacity, this experimentally available porous carbon material is expected to find applications in efficient lithium storage.

Ultra-high hydrogen storage capacity of Li-decorated graphyne: A first-principles prediction

Graphyne, consisting of sp- and sp2-hybridized carbon atoms, is a new member of carbon allotropes which has a natural porous structure. Here, we report our first-principles calculations on the possibility of Li-decorated graphyne as a hydrogen storage medium. We predict that Li-doping significantly enhances the hydrogen storage ability of graphyne compared to that of pristine graphyne, which can be attributed to the polarization of H2 molecules induced by the charge transfer from Li atoms to graphyne. The favorite H2 molecules adsorption configurations on a single side and on both sides of a Li-decorated graphyne layer are determined. When Li atoms are adsorbed on one side of graphyne, each Li can bind four H2 molecules, corresponding to a hydrogen storage capacity of 9.26 wt. %. The hydrogen storage capacity can be further improved to 15.15 wt. % as graphyne is decorated by Li atoms on both sides, with an optimal average binding energy of 0.226 eV/H2. The results show that the Li-decorated graphyne can s...

First-principles prediction of a new Dirac-fermion material: silicon germanide monolayer.

From first-principles calculations, we proposed a silicon germanide (SiGe) analog of silicene. This SiGe monolayer is stable and free from imaginary frequency in the phonon spectrum. The electronic band structure near the Fermi level can be characterized by Dirac cones with the Fermi velocity comparable to that of silicene. The Ge and Si atoms in SiGe monolayer exhibit different tendencies in binding with hydrogen atoms, making sublattice-selective hydrogenation and consequently electron spin-polarization possible.

Is yne-diamond a super-hard material?

Yne-diamond —a new carbon allotrope constructed by inserting two carbon atoms into the carbon-carbon bonds of diamond was expected to have super-hardness comparable to diamond because of the three-dimensional network of strong sp-sp3 and sp-sp bonds. However, from a theoretical point of view, this idea has never been validated carefully. Based on first-principles calculations, we present the first theoretical evidence that, in contrast to the early expectation, yne-diamond possesses low ideal tensile strength, low shear strength and small Pughs modulus ratio, owning to the large void network in the covalent bond skeleton. Combined with a simple model, we predict that the yne-diamond family will not be a super-hard material family. This provides a new understanding of the mechanical properties of carbon allotropes containing sp-hybridized carbon atoms.

A metallic carbon allotrope with superhardness: a first-principles prediction

Carbon has abundant allotropes with superhardness, but few of them are metallic. From first-principles calculations, we propose a stable metallic carbon allotrope (Hex-C24) phase with superhardness. The Hex-C24 can be thought of as a superlattice of carbon nanotubes and graphene nanoribbons composed of sp2- and sp3-hybridized carbon atoms. A possible synthetic route towards Hex-C24 from graphyne multilayers is evaluated by calculating the transition states between the two phases. Our calculations show that at a uniaxial pressure of around 25 GPa, the energy barrier of this endothermic transition is estimated to be 0.04 eV per atom, while at a pressure of 34 GPa, the transition is barrierless for specific initial configurations. The cohesive energy, elastic constants, and phonon frequencies unambiguously confirm the structural stability. The hardness of the Hex-C24 is estimated to exceed 44.54 GPa, which is 1/2 that of diamond. The Hex-C24 phase is metallic with several bands across the Fermi level. Both mechanical and metallic properties of Hex-C24 are anisotropic.

Formation and annealing behaviors of qubit centers in 4H-SiC from first principles

Inspired by finding that the nitrogen-vacancy center in diamond is a qubit candidate, similar defects in silicon carbide (SiC) have drawn considerable interest. However, the generation and annealing behaviors of these defects remain unclear. Using first-principles calculations, we describe the equilibrium concentrations and annealing mechanisms based on the diffusion of silicon vacancies. The formation energies and energy barriers along different migration paths, which are responsible for the formation rates, stability, and concentrations of these defects, are investigated. The effects on these processes of charge states, annealing temperature, and crystal orientation are also discussed. These theoretical results are expected to be useful in achieving controllable generation of these defects in experiments.

Adsorption and diffusion of gold adatoms on boron nitride nanoribbons: A first-principles study

We have carried out first-principles calculations to explore the adsorption and diffusion of Au adatoms on boron nitride nanoribbons (BNNRs). We found that Au adatoms prefer to locate at the edge B site of the ribbons for both armchair (A-) and zigzag (Z-) BNNRs. Different diffusion paths, such as diffusion from central region to edge site, along the subedge sites or along the edge sites, are considered. The unique atomic arrangement and electronic structures of Z-BNNRs make the Au adatom tend to migrate only to B edge site rather than to the both edges. Different from the cases of graphene nanoribbons, the energy barriers for A-BNNRs are higher than those of the corresponding paths for Z-BNNRs. The electronic structure calculations indicate the wide-band-gap features are preserved in the Au-doped BNNRs as the Au concentration is low. With the increase of Au concentration, the Au adatoms form an atomic chain along the B zigzag edge, resulting in band gap closure. These results are expected to provide usef...

Electronic and mechanical properties of C/Si phases with sp2 and sp3 hybridization: A first-principles study

A first-principles approach is utilized to systematically investigate the electronic and mechanical properties of SiC3/Si3C phases with sp2 and sp3 hybridization. In the SiC3 phases, electronic states around the Fermi level mainly originate from the C-2p orbitals, whereas in the case of Si3C phases, it is the C-2p and Si-3p orbitals. Cm-SiC3 and Cmc21-SiC3 show metallic properties arising from sp2-hybridized components. P4¯m2-Si3C exhibits good ductility and metallic properties due to the formation of conductive sublattices as a result of the distribution of valence electrons in three-dimensional C and Si frameworks. Furthermore, the semiconducting P4¯m2-SiC3 phase is a superhard material with a remarkable hardness of 47.14 GPa. In general, SiC3 phases exhibit higher brittleness due to sp3-hybridized C atoms while Si3C phases are more ductile.A first-principles approach is utilized to systematically investigate the electronic and mechanical properties of SiC3/Si3C phases with sp2 and sp3 hybridization. In the SiC3 phases, electronic states around the Fermi level mainly originate from the C-2p orbitals, whereas in the case of Si3C phases, it is the C-2p and Si-3p orbitals. Cm-SiC3 and Cmc21-SiC3 show metallic properties arising from sp2-hybridized components. P4¯m2-Si3C exhibits good ductility and metallic properties due to the formation of conductive sublattices as a result of the distribution of valence electrons in three-dimensional C and Si frameworks. Furthermore, the semiconducting P4¯m2-SiC3 phase is a superhard material with a remarkable hardness of 47.14 GPa. In general, SiC3 phases exhibit higher brittleness due to sp3-hybridized C atoms while Si3C phases are more ductile.

Electron spin-polarization and spin-gapless states in an oxidized carbon nitride monolayer

Electron spin-polarization in metal-free organic materials is currently drawing considerable attention due to their applications in organic electronics. Using first-principles calculations, we propose a stable two-dimensional (2D) honeycomb lattice oxidized carbon nitride material, C2NO. The energetic favorability, phonon spectrum and molecular dynamics simulation confirm the stability and plausibility of the C2NO material. The electronic structure of the metal-free organic material is spin-polarized, yielding magnetic moments of 1.0 μB in one primitive cell. More interestingly, the spin-polarized electronic band lines have a zero band gap at the Fermi level, exhibiting spin-gapless features. These unique properties are promising for spin detectors and generators for electromagnetic radiation over a wide range of wavelengths based on the spin photoconductivity.