Orientation of C60 molecules in the (3r3 x3r3 )R30° and (r13 xr13 )R14° phases of C60/Ge(111) single-layer

The structure of the (33 x 33 )R30° and (13 x 13 )R14° phases of C60/Ge(111) single-layer has been studied by Scanning Tunneling Microscopy (STM). Sub-molecular resolution allows to distinguish among differently orientated molecules. In the (33 x 33 )R30° phase the molecules result to be arranged in rhomboidal groups of 4 molecules, named tetramers. The (2x2) periodicity in the domains of homogeneously oriented tetramers is due to the alternating orientation of the molecules within the tetramer, accounting for the observed (33 x 33 )R30° Low Energy Electron Diffraction (LEED) pattern. The symmetry of the molecular lattice suggests that the molecules interact only with the first layer of substrate atoms. The orientation of each molecule is mainly determined by the configuration of the substrate atoms in the adsorption site, even though a contribution from the intermolecular interaction is likely present. In the (13 x 13 )R14° phase the observed sub-molecular features indicate that all the molecules have the same adsorption configuration, with a hexagon facing the substrate. The threefold symmetry of the molecular lattice suggests that the C60 - Ge interaction involves also the atoms of the second layer of the substrate.