Layer-by-Layer Growth of Incommensurate, Polycrystalline, Lying-Down Pentacene Thin Films on Au(111)

Pentacene, as well as other polyacenes, is known to adsorb on metal surfaces in a lying-down geometry, that is, with the molecular plane parallel to the surface. Here we show using scanning tunneling microscopy that the lying-down pentacene monolayer seeds the growth of a multilayer polycrystalline film in a layer-by-layer fashion, with the molecular planes parallel to the substrate surface. Growth in the submonolayer region is characterized by a large number of ordered molecular structures as coverage increases. The saturated monolayer structure remains approximately the same throughout multilayer coverages studied. Statistical analysis of molecular domain orientation reveals that the ordered pentacene structures are incommensurate with the Au(111) substrate and growth beyond the monolayer region is a mixture of epitaxy and nonepitaxy. We conclude that the relatively large footprint of a lying-down pentacene molecule smoothes out lateral corrugation in the adsorption energy landscape on the Au(111) surface, leading to the growth of an incommensurate, polycrystalline, and partially nonepitaxial thin film.