Nanographene; Edge Geometry and Electronic Structure
榎 敏明 名誉教授
When a graphene sheet, which consists of 2D honeycomb lattice, is cut into fragments and the created edge carbon atoms are terminated with hydrogen atoms, we can create a variety of fragments having different sizes and shapes; semi-infinite graphene, nanographene, and polycyclic aromatic hydrocarbon molecules, to benzene. Here we have two independent cutting directions, zigzag and armchair directions. The electronic structures of these fragments seriously depend on the geometry of edges created by cutting. In zigzag edged nanographenes, nonbonding localized π-state (edge state) having a large local density of states and strong spin polarization is created in the vicinity of edges, while electron wave interference appears in the armchair edged nanographenes, resulting in the creation of a standing wave with energy gap opening. This geometry dependence can be understood both from physics and chemistry aspects; massless Dirac fermion moving in 2D bipartite honeycomb lattice in physics and aromaticity expressed by the Clar’s aromatic sextet rule with aromatic sextet as fundamental unit in chemistry. Namely, the creation of edge state is a consequence of broken symmetry of pseudo-spin in the Dirac fermion or degradation of aromaticity, whereas the electron wave interference is explained in terms of inter-valley electronic transition in the bipartite lattice or fully benzenoid structure expressed by only one unique Clar’s representation. Consequently, as an arbitrary shaped nanographene is described in terms of a combination of armchair and zigzag edges, the edge geometry gives rise to a variety of electronic structures in nanographenes. This is important in giving electronic, chemical and magnetic activities to graphene nanosructures. The issue of this geometry dependence is reviewed on the basis of experimsntal results of scanning tunneling microscopy/spectroscopy, non-contact atmoic force microscopy, Raman scattering, X-ray absorption, magnetic susceptibility, and ESR measurements.
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- S. Fujii and T. Enoki, "Nanographene and Graphene Edges: Electronic Structure and Nanofabrication" Acc. Chem. Res. (2013)abstract