Energetics and Electronic Structure of Triangular Hexagonal Boron Nitride Nanoflakes

We studied the energetics and electronic structures of hexagonal boron nitrogen (h-BN) nanoflakes with hydrogenated edges and triangular shapes with respect to the edge atom species. Our calculations clarified that the hydrogenated h-BN nanoflakes with a triangular shape prefer the N edges rather than B edges irrespective of the flake size. The electronic structure of hydrogenated h-BN nanoflakes depends on the edge atom species and their flake size. The energy gap between the lowest unoccupied (LU) and the highest occupied (HO) states of the nanoflakes with N edges is narrower than that of the nanoflakes with B edges and the band gap of h-BN. The nanoflakes possess peculiar non-bonding states around their HO and LU states for the N and B edges, respectively, which cause spin polarization under hole or electron doping, depending on the edge atom species

Strain-induced charge transfer and polarity control of a heterosheet comprising C60 and graphene

Using density functional theory combined with the effective screening medium method, the energetics and electronic structure of a C60 molecular sheet adsorbed on graphene were studied in terms of biaxial strains. The optimum spacing and interlayer interaction monotonically decreases and increases, respectively, with an increasing biaxial tensile strain. The biaxial compressive strain induces electron transfer from the graphene to C60 at a 2% lateral compression, leading to an all-carbon charge transfer complex. The heterosheet possesses an intrinsic dipole moment along the graphene-to-C60 molecular layer direction

Unconventional Gapless Semiconductor in Extended Martini Network in Honeycomb Covalent Materials

We study characteristic electronic structures in the extended martini lattice model and propose its materialization in $\pi$-electron networks constructed by designated chemisorption on graphene and silicene. By investigating the minimal tight-binding model, we reveal rich electronic structures tuned by the ratio of hopping parameters, ranging from the band insulator to the unconventional gapless semiconductor. Remarkably, the unconventional gapless semiconductor is characterized by the flat band at the Fermi level. Further, the density functional theory calculations for candidate materials reveal that the characteristic electronic structures can be realized by the designated chemisorption or the chemical substitution on graphene and silicene, and that the electronic structure near the Fermi level is tunable by the choice of the atomic species of adsorbed atoms. Our results open the way to search exotic electronic structures and their functionalities induced by the extended martini lattice.Comment: 5 pages, 4 figures for the main text, 5 pages, 5 figures for Supplemental Materia

欠陥や境界を有する二次元sp2炭素ネットワークの電子物性

筑波大学 (University of Tsukuba)201

Three-dimensional covalent networks of sp2 and sp3 C atoms: energetics and electronic properties of polymerized diphenylmethane and tetraphenylmethane

Based on the density functional theory with the generalized gradient approximation, we investigate geometric and electronic structures of three-dimensional covalent organic frameworks of polymerized diphenylmethane and tetraphenylmethane in which phenyl and biphenyl are arranged in a tetrahedral manner connected via methane vertexes. These three-dimensional covalent networks are energetically stable with the total energy of 90 and 65 meV atom−1 for diphenylmethane and tetraphenylmethane polymers, respectively, with respect to that of diamond. Polymerization reactions of diphenylmethane and tetraphenylmethane are endothermic with the reaction energy of 1.62 and 0.68 eV, respectively. These polymers have the peculiar electronic band structures in their valence and conduction states, which consist of the combination of the doubly degenerate flat band and two dispersive states forming a Dirac cone at the W point. The wavefunction analyzes and the simple tight-binding calculations reveal that the peculiar electron states are ascribed to the delicate balance between inter- and intra-phenyl/biphenyl π electron transfers, indicating that these polymers could be regarded as the three-dimensional kagome or pyrocroa networks with the internal degree of freedom

Geometric and electronic structures of two-dimensionally polymerized triptycene: covalent honeycomb networks comprising triptycene and polyphenyl

On the basis of the density functional theory with generalized gradient approximation, we investigated the geometric and electronic structures of two-dimensional covalent networks consisting of triptycene and phenyl groups, which are alternately arranged hexagonally. Calculated total energies of the networks are 48–63 meV per atom higher than that of an isolated benzene, indicating that the networks are energetically stable. All networks were semiconductors with a moderate band gap at the Γ point, the value of which is inversely proportional to the length of polyphenyl connecting triptycene. According to a kagome topology of π electrons distributed on sp2 hydrocarbons, the characteristic kagome energy bands consisting of a flat dispersion band and a Dirac cone emerge in valence and conduction states whose structure is sensitive to the mutual orientation of phenyl groups with respect to the polymer chain

Energetics and electronic structures of MoS2 nanoribbons

We study the energetics and electronic structures of MoS2 nanoribbons with clean armchair, chiral, and zigzag edges by conducting the first-principle total energy calculations based on the density functional theory. Our calculations showed that the nanoribbon with zigzag edges is the most stable among the ribbons studied here. The ribbons with armchair or near armchair edges are semiconductors with a direct band gap at the Γ point, owing to the large edge relaxation reducing the unsaturated nature of edge atoms, while the ribbons with zigzag and near zigzag edges are metals with the finite density of state at the Fermi level. According to the asymmetric atomic arrangement in ribbons with the chiral and zigzag edges, they have polarity across the ribbon, which monotonically increase with increasing edge angle

Two-dimensional atomic-scale ultrathin lateral heterostructures

Ultrathin lateral heterostructures of monolayer MoS2 and WS2 have successfully been realized with the metal-organic chemical vapor deposition method. Atomic-resolution HAADF-STEM observations have revealed that the junction widths of lateral heterostructures range from several nanometers to single-atom thickness, the thinnest heterojunction in theory. The interfaces are atomically flat with minimal mixing between MoS2 and WS2, originating from rapid and abrupt switching of the source supply. Due to one-dimensional interfaces and broken rotational symmetry, the resulting ultrathin lateral heterostructures, 1~2 mixed-dimensional structures, can show emergent optical/electronic properties. The MOCVD growth developed in this work allows us to access various ultrathin lateral heterostructures, leading to future exploration of their emergent properties absent in each component alone