Using systematic evolutionary structure searching we propose a new carbon allotrope, phagraphene [fæ'græfi:n], standing for penta-hexa-hepta-graphene, because the structure is composed of 5-6-7 carbon rings. This two-dimensional (2D) carbon structure is lower in energy than most of the predicted 2D carbon allotropes due to its sp(2)-binding features and density of atomic packing comparable to graphene. More interestingly, the electronic structure of phagraphene has distorted Dirac cones. The direction-dependent cones are further proved to be robust against external strain with tunable Fermi velocities.

Using first-principles calculations, we show that the band gap and electron effective mass (EEM) of graphene/boron nitride heterobilayers (C/BN HBLs) can be modulated effectively by tuning the interlayer spacing and stacking arrangement. The HBLs have smaller EEM than that of graphene bilayers (GBLs), and thus higher carrier mobility. For specific stacking patterns, the nearly linear band dispersion relation of graphene monolayer can be preserved in the HBLs accompanied by a small band-gap opening. The tunable band gap and high carrier mobility of these C/BN HBLs are promising for building high-performance nanodevices.

We have carried out first-principles calculations to explore the energetics and dynamics of Li in graphyne, a novel carbon allotrope consisting of spÀsp 2 hybridized carbon atoms, relevant for anode lithium intercalation in rechargeable Li-ion batteries. In contrast to graphite where Li diffusion is confined in the interlayer space (in-plane diffusion), the unique atomic arrangement and electronic structures enable both inplane and out-plane diffusion of Li ions in graphyne with moderate barriers, 0.53À0.57 eV. The highest Li intercalation density in graphyne can be LiC 4 , exceeding the up limit of LiC 6 in the commonly used graphite. The high lithium mobility and high storage capacity make graphyne a promising candidate for the anode material in battery applications.

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.

Tin-chalcogenides SnX (X = Te, Se and S) have been arousing research interest due to their thermoelectric physical properties. The two-dimensional (2D) counterparts, which are expected to enhance the property, nevertheless, have not been fully explored because of many possible structures. Generating variable composition of 2D Sn 1−x X x systems (X = Te, Se and S) has been performed using global searching method based on evolutionary algorithm combining with density functional calculations. A new hexagonal phase named by β -SnX is found by Universal Structure Predictor Evolutionary Xtallography (USPEX), and the structural stability has been further checked by phonon dispersion calculation and the elasticity criteria. The β -SnTe is the most stable among all possible 2D phases of SnTe including those experimentally available phases. Further, β phases of SnSe and SnS are also found energetically close to the most stable phases. High thermoelectronic (TE) performance has been achieved in the β -SnX phases, which have dimensionless figure of merit (ZT) as high as ∼0.96 to 3.81 for SnTe, ∼0.93 to 2.51 for SnSe and ∼1.19 to 3.18 for SnS at temperature ranging from 300 K to 900 K with practically attainable carrier concentration of 5×10 12 cm −2 . The high TE performance is resulted from a high power factor which is attributed to the quantum confinement of 2D materials and the band convergence near Fermi level, as well as low thermal conductivity mainly from both low elastic constants due to weak inter-Sn bonding strength and strong lattice anharmonicity.

Thermoelectric (TE) properties of monolayered α-In2Se3 are investigated using the first-principles calculations based on the density functional theory and Boltzmann transport theory. The results show that monolayered α-In2Se3 is a great candidate for high-performance thermoelectric materials with the power factor PF and the figure of merit ZT as high as 0.02 W/mK2 and 2.18 at room temperature, respectively. We attribute such great TE performance to the large electrical conductivity and low lattice thermal conductivity, which originate from unique band structures of group III chalcogenides and anharmonic scattering. Furthermore, we prove that the quantum confinement effect can realize up to an order of magnitude enhancement in the PF. Our findings may open up new possibilities for two-dimensional thermoelectric materials in practical applications.

New boron-rich sulfide B6S and selenide B6Se have been discovered by combination of high pressure – high temperature synthesis and ab initio evolutionary crystal structure prediction, and studied by synchrotron X-ray diffraction and Raman spectroscopy at ambient conditions. As it follows from Rietveld refinement of powder X-ray diffraction data, both chalcogenides have orthorhombic symmetry and belong to Pmna space group. All experimentally observed Raman bands have been attributed to the theoretically calculated phonon modes, and the mode assignment has been performed. Prediction of mechanical properties (hardness and elastic moduli) of new boron-rich chalcogenides has been made using ab initio calculations, and both compounds were found to be members of a family of hard phases.

The energetic stability and electronic properties of hydrogenated silicon carbide nanowires (SiCNWs) with zinc blende (3C) and wurtzite (2H) structures are investigated using first-principles calculations within density functional theory and generalized gradient approximation. The [111]-orientated 3C-SiCNWs are energetically more stable than other kinds of NWs with similar size. All the NWs have direct band gaps except the 3C-SiCNWs orientating along [112] direction. The band gaps of these NWs decrease with the increase of wire size, due to the quantum-confinement effects. The direct-band-gap features can be kept for the 3C-SiCNWs orientating along [111] direction with diameters up to 2.8 nm. The superior stability and electronic structures of the [111]-orientated 3C-SiCNWs are in good agreement with the experimental results.