Carbon spiral helix: a nanoarchitecture derived from monovacancy defects in graphene.

Graphene has diverse applications in molecular electronics due to its unique electronic, thermal, and mechanical properties. Lattice imperfections are introduced into graphene unavoidably during graphene growth or when a graphene sheet is irradiated with high-energy particles. These structural defects are known to significantly affect electronic and chemical properties. A comprehensive understanding of graphene defects is thus of critical importance. In particular, the monovacancy defect has attracted great attention due to its fundamental nature. Recent studies have established that monovacancies exist in graphene as a planar 5/9 isomer with lowered symmetry. The 5/9 isomer is generated by formation of an elongated carbon–carbon bridge across the hole that results from removal of a single carbon atom, which leads to fusion of a fiveand a nine-membered ring (Figure 1b). Each 5/9 isomer contributes an intrinsic magnetic moment of about 1 mB. [4] Here we report that the stability and therefore electronic structure of the monovacancy defect is determined by the distance from the graphene periphery: The magnetic, planar 5/9 isomer is most stable in the center of graphene flakes, whereas the nonmagnetic spiro isomer (Figure 1b) is more stable when the shortest center-to-periphery distance d of the vacancy is less than 7 (for definition of d, see Figure 1a). Creation of spiro isomers in the planes of graphene nanoribbons (GNRs) paves the way to achieving novel nanoarchitectures such as spiral helices. To investigate the low-lying structural and electronic conformation of graphene monovacancies, monovacant graphene flakes 1–6 with peripheries terminated by hydrogen atoms were used as model systems (Figure 1a). The largest flake, 1, contains 351 carbon and 46 hydrogen atoms (Table 1). With decreasing molecular size from 1 to 6, the d value decreases from 15.6 to 4.2 (Table 1). Thus, 1–6 provide reasonable models for monovacancy defects in graphenes with shrinking size and decreasing d. Geometry optimizations and energy calculations were performed by four methods: B3LYP/6-31G(d), GGA-PBE/DZP, and selfconsistent-charge density functional tight binding with finite Figure 1. a) 1–6 : Hydrogen-terminated graphene flakes with monovacancies; the empty circle designates the vacant carbon position; d is the shortest center-to-periphery distance. b) 6F and 6S : Structures derived from 6 with 5/9 and spiro defects. Hydrogen atoms of all structures are omitted for clarity.