Structure, Stability, Edge States and Aromaticity of Graphene Ribbons

We determine the stability, the geometric, the electronic and magnetic structure of hydrogen-terminated graphene-nanoribbons edges as a function of the hydrogen content of the environment by means of density functional theory [1]. Antiferromagnetic zigzag ribbons are stable only at extremely-low ultra-vacuum pressures. Under more standard conditions, the most stable structures are the mono- and di-hydrogenated armchair edges and a zigzag edge reconstruction with one di- and two mono-hydrogenated sites. At high hydrogen-concentration ``bulk'' graphene is not stable and spontaneously breaks to form ribbons, in analogy to the spontaneous breaking of graphene into small-width nanoribbons observed experimentally in solution [2]. The stability and the existence of exotic edge electronic-states and/or magnetism is rationalized in terms of simple concepts from organic chemistry (Clar's rule). [1] T. Wassmann, et al. Phys. Rev. Lett. 101, 096402 (2008). [2] X. Li et al., Science 319, 1229 (2008); X. Wang et al., Phys. Rev. Lett. 100, 206803 (2008).