Adsorption and Growth of Bis-pyrene Molecular Layers on Au(111) Studied by STM

The adsorption and growth of 1,4-di-n-octyloxy-2,5-bis(pyren-1-yletheny1)-benzene (bis-pyrene) thermo-evaporated onto a Au(111) single crystal surface has been investigated using low temperature scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. At low coverage, bis-pyrene molecules arrange into molecular chains, which exclusively grow on the fcc domains of Au(111) herringbone reconstruction. In these molecular chains, the individual molecules are connected by a van der Waals interaction between the alkyl chains of the molecules. As the coverage is increased the hcp domains of Au(111) herringbone reconstruction are also decorated by molecular chains. Finally, for coverages above 0.5 ML, four distinct ordered commensurate phases are observed, which suggests a delicate balance between the intermolecular and the molecule-surface interactions. A detailed analysis of the STM images suggests that distinct molecular configurations of the bis-pyrene compound in these ordered phases are at the origin of their formation.