Self-Assembly and Electronic Structure of Tribenzotriquinacenes on Ag(111)

Abstract

The structure of two-dimensional layers of organic compounds on metal surfaces is of great interest for the fabrication of devices in the field of organic electronics. To progress in this area, a deeper understanding of intermolecular interactions and charge transfer processes between organic molecules and a metal substrate is required. Here, the two-dimensional self-assembly of tribenzotriquinacenes (TBTQs) on Ag(111) was studied by scanning tunneling microscopy under ultrahigh vacuum conditions. Highly ordered monolayers of tribenzotriquinacene (TBTQ) and centro-methyl tribenzotriquinacene (Me-TBTQ) were observed for the first time. At low to moderate coverage, both of these bowl-shaped molecules adsorb with the bowl opening faced downward, appearing in two orientations that are rotated by 14° from the high-symmetry axis of the Ag(111) surface. At high TBTQ coverage, extended islands with a self-assembled highly ordered windmill nanostructure dominate the surface, interspaced with nanometer-sized openings. This windmill structure is absent in Me-TBTQ, which immediately forms double-layered regions at high coverage. Scanning tunneling spectroscopy measurements, which probe the local density of states, identify a shift of the Ag(111) surface state in the presence of the TBTQ molecules. Investigations such as these into the self-assembly of curved aromatics provide a foundation for further work on the construction of multilayer nanostructures and the production of electronic devices.