STM/STS Observations of Graphene Edges

Abstract

The electronic structure of nanographene having open edges around its circumference crucially depends on its edge shape. The circumference of an arbitrary shaped nanographene sheet is described in terms of a combination of zigzag and armchair edges. According to theoretical suggestions, nanographene has a nonbonding π-electron state (edge state) localized in zigzag edges. This is reminiscent of the nonbonding π-electron state appearing in non-Kekule-type aromatic molecules. The localized spins of the edge states can give rises to unconventional magnetism in nanographene, such as carbon-only ferromagnetism, magnetic switching phenomenon, spin glass state, etc. STM/STS investigations of well defined graphene edges which are hydrogen terminated in ultra-high vacuum condition confirm the presence of edge states around zigzag edges, in good agreement with theoretical works. The feature of the edge state depends on the detailed edge shape. The edge state in a short zigzag edge embedded between armchair edges becomes less localized due to the state mixing with the adjacent armchair edges. The electrons in the edge state in a finite-length zigzag edge are subjected to electron confinement effect.