Coronene on Ag(111) investigated by LEED and STM in UHV

The adsorption of the aromatic molecule hexabenzobenzene (coronene) on a Ag(111) single crystal surface was investigated under UHV conditions by means of LEED and variable temperature STM. For a very low coverage randomly distributed individual molecules adsorbed on terraces were found. All molecules are surrounded by concentric rings caused by scattering of surface electrons, leading to the formation of standing waves. These surface electrons mediate an additional intermolecular interaction with an oscillatory potential in the distance between molecules. The balance of all forces leads to a typical spacing of 2.4 nm for isolated molecular dimers. For monolayer coverage, a long-range ordered close packed structure was found. Calibration of the coronene LEED pattern with the substrate spots provided the superstructure unit cell parameters. These were used for distortion correction and a subsequent correlation analysis, which improves the quality of the submolecular resolved images. The zoom shows a tilt of the molecules against the substrate plane.