Formation of molecular order on a disordered interface layer: Pentacene/Ag(111)

In this paper, the formation of ordered structures at the pentacene/Ag(111) interface is investigated. Pentacene molecules are thermally evaporated and deposited onto a Ag(111) substrate held at room temperature. The interface is studied with a combination of x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), high resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM). To begin with, pentacene molecules form a loosely bound, disordered contact layer at the interface in which the molecules are oriented parallel to the surface. Remarkably, the molecular layer which grows on top of the contact layer exhibits order, with molecules arranged in rows and tilted around their long axis out of the surface plane. The in-plane orientation of molecular rows in this second molecular layer is templated by the steps of the substrate. A few further ordered layers with similar in-plane structure can be grown on top of the bilayer. The results presented here illustrate a mechanism by which ordered structures of molecular materials can form in spite of an initial lack of order in the contact layer at the interface.