Orientation of C-60 molecules in the, (3 root 3x3 root 3)R30 degrees and, (root 13x root 13)R14 degrees phases of C-60/Ge(111) single layers

The structure of the (3 root 3x3 root 3)R30 degrees and (root 13x root 13)R14 degrees phases of C-60/Ge(111) single layer has been studied by scanning tunneling microscopy. Submolecular resolution allows to distinguish differently oriented molecules. In the (3 root 3x3 root 3)R30 degrees phase, the molecules are arranged in rhomboidal groups of four 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 (3 root 3x3 root 3)R30 degrees low energy electron diffraction 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 (root 13x root 13)R14 degrees phase, the observed submolecular 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 C-60-Ge interaction involves also the atoms of the second layer of the substrate.