Kyoko Nakada

Lattice Distortion in Nanographite Ribbons

We study the lattice distortion in graphite ribbons of a nanometer width by taking account of the electron-phonon interaction in the tight binding model. In the ribbons with armchair edges, the typical Kekule structure appears near the edges depending on the distribution of the bond orders. On the other hand, the zigzag ribbons do not undergo bond alternations along the ribbon axis, implying less Peierls instability. Special emphasis is put on the survival of the edge state which forms almost flat bands and a sharp peak in the density of states in consideration of the electron-phonon interaction.

Electronic Structure of Graphene with a Topological Line Defect

Using the first-principle total-energy procedure within the framework of density functional theory, we study the electronic structure of graphene with a topological line defect that is comprised of a pair of fused pentagons and an octagon. We find that introduction of the topological line defect effectively terminates the sp 2 network of graphene leading the flat dispersion band around the Γ point. A detailed investigation of the wave function of this flat-band state reveals its edge-state nature, which is particular to graphene nanoribbons with zigzag edges. Our tight-binding molecular dynamics simulation also reveals that the topological line defect is spontaneously formed from defects in the graphene sheet when held at a temperature of 1000 K.

Electron-Electron Interaction in Nanographite Ribbons

We study the π electronic structure of graphite ribbons of a nanometer width by taking account of the long-range electron-electron interaction. The primary features like energy dispersion and bond order distribution are essentially determined by the nearest-neighbor transfer alone, meaning their strong dependency on the connectivity relation in the π electron network. The Coulomb interaction, on the other hand, is induced by the sites of longer distances as well, which means that the Coulomb repulsion reflects the site geometry in the atomic configuration. We focus on the geometry-dependent Coulomb interaction and discuss how it influences the electronic state near the Fermi level which is governed by the network topology. The Coulomb repulsion under the geometry of armchair ribbons works against the bond order distribution determined by the network topology. This may lead to a frustrated system especially for a quite narrow ribbon. The Coulomb interaction in zigzag ribbons tends to open a gap with/withou...

Electronic and geometric structures of fluorine adsorbed graphene

We study electronic and geometric structures of the fluorine adsorbed graphene sheet using an ab initio pseudopotential method. We find that the fluorine atom forms covalent bond with the carbon atom and locally terminates the π-states on the graphene sheet. The charge distribution exhibits a typical √3 x √3 structure near the Fermi energy. This means that the doped fluorine atom acts as a local defect on the π-electron network of a graphene sheet.

Polymorphism of Extended Fullerene Networks: Geometrical Parameters and Electronic Structures

Abstract The morphology of fullerene networks can be widely extended by introducing heptagonal or octagonal rings, which produce a Gaussian negative curvature. Their presence makes it possible to form donut-, coil- and sponge-shaped networks of carbon atoms. We discuss the geometry of the polymorphous forms based on the net diagram method relative to a honeycomb lattice, and further study the electronic structures constructed by the network of electrons system. Special emphasis is put on how the geometrical paramateres, which specify the relative arrangement of polygonal ring, control the electronic structures in the various extended-fullerene networks. In addition, we mention that the presence of a certain type of edge in fullerene network derives critical localized edge stages at the Fermi level.

On Construction of Clar Structures for Large Benzenoids

Abstract We present an algorithm for construction of Clar valence structures for large benzenoids. The algorithm starts with a single Clar structure and systematically generates the remaining structures by considering all possible “moves” for the π-sextets on the “board” of the fused hexagons representing carbon skeleton of a benzenoid. We illustrate the approach on hexabenzocoronene, its higher homologue and few other cases. We end the presentation by discussing some limitations of the Clar model.

Electronic band structures of penta-heptagonal carbon networks

Abstract The electronic band structures of planar carbon networks consisting solely of pentagons and heptagons were calculated. Two delocalized structures analogous to graphite were proposed as the candidates of the most possible penta-heptagonal network. The electronic structures of these networks were found to show rather metallic properties marking quite a contrast to graphite.

Possible three-dimensional all-sp2 phases of carbon with edge states

We discuss a possibility of new three-dimensional all-sp 2 phases of carbon. The most interesting property of them is appearance of special surface states, i.e. the edge states. One of characters of edge states is large degeneracy at the Fermi level. Thus, these structures are expected to be metallic carbon-materials. In this paper, we show π-band structure and band structure based on the first principle calculation within local density approximation(LDA).

First-principles study on the π electronic structure of nanographite

Abstract We studied electronic structure of one-dimensional nanographite ribbons with a pair of zigzag edges by performing first-principles calculations within the local density approximation. The edge state of zigzag ribbons, which was predicted by the previous tight-binding calculations, was well reproduced by the present first-principles scheme. Nearly flat bands of the edge state were seen in zigzag ribbons which are condensed in a manner of AB-stacking as in bulk graphite. We took account of dangling bonds at edge sites and confirmed that the dangling-bond state near the Fermi level does not affect the nearly flat dispersion of the edge state. We also examined electronic structure of zigzag ribbons made up of an odd number of C-C chains and discussed the possibility of lattice distortion.