Multitude of PTCDA Superstructures on Ag(111) and Vicinal Surfaces

Organic molecules that covalently bond to metal surfaces, like perylene tetracarboxylic acid dianhydride (PTCDA) on Ag, may cause large-scale reconstruction of the interface, in particular if the adsorption takes place on a “stepped” (i.e., vicinal) surface. Under certain conditions, such surfaces develop a large variety of high-index facets that are stabilized by superstructures of the PTCDA adsorbate. These superstructures can be commensurate, point-on-line, or incommensurate with respect to the substrate facets depending on facet orientation and step direction. The present scanning tunneling microscopy and low-energy electron diffraction data were taken from two different vicinal surfaces, Ag(775) and Ag(10 8 7), both inclined by 8.5° with respect to the (111) direction but in different azimuthal directions. Altogether, we identify and evaluate 18 new superstructures and correlate their data with the properties of the corresponding facets. The observed richness of superstructures and the occurrence of large-scale reconstruction may be common for many other combinations of similar organic molecules and metals.