While conjugated systems with alternant topologies and their interaction with metal surfaces have been subject to intensive investigation, much less is known about their non-alternant isomers. A conjugated system with non-alternant topology occurs in the 5-7 defects of graphene. In these defects a pair of a 5-membered ring and a 7-membered ring are formed in the pristine 6-membered graphene lattice. Such defects occur for instance at grain boundaries between graphene domains and greatly influence the mechanic and electronic properties of the graphene layer. Because of the difficulties of yielding informations about a low concentration of defects in presence of a largely undisturbed structure, little is known about the interaction of 5-7 defect with an underlying substrate. In this study, we have obtained insight into this interaction by using a molecular model system. The smallest possible model system for the 5-7 defects is azulene, which is a bicyclic aromatic compound with a 5 and a 7-membered ring. We compared this molecule and its interaction with a metal surface to naphthalene as a model for the regular 6-6 motif. Both molecules were studied on Cu(111) and Ag(111) surfaces with PES, NEXAFS, TPD, nc-AFM, STM and NIXSW experiments. To deepen the understanding of the resulting interactions, periodic DFT calculations were performed. The non-alternant topology of azulene causes non-uniform charge distribution and more localized frontier orbitals, resulting in a much stronger and more localized interaction especially with the Cu(111) surface. Manifestations of the stronger interaction of the non-alternant molecule with the surface include interfacial charge transfer and substantial in-plane and out-of-plane deformations, as well as a much smaller adsorption height. The strong interaction results from the donation of electron density into the lower lying LUMO of the non-alternant molecule.

Introducing a molecular model system for 5-7 defects in graphene

255th National Meeting (2018)

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