Growth study of para-sexiphenyl on Ag(001): From single molecule to crystalline film

In this work we investigate the formation of crystalline organic thin films for p-sexiphenyl (6P) on Ag(001) with a combination of STM, LEED, TPD, SFM, UPS, XRD and DFT. For weakly interacting van-der-Waals-dominated systems of small organic molecules on single crystalline metals we demonstrate the complex epitaxial evolution from sub-monolayer to bulk structure. In the delicate interplay between molecule-substrate and intermolecular interactions we identify the oligo-phenyl's flexibility of central importance for the film growth. We observe a change from a planar to a twisted molecular conformation and, accompanying, a molecular realignment during the formation of a dense monolayer as the result of enhanced 2-dimensional intermolecular interaction. A second molecular realignment occurs collectively for whole domains with the formation of 3-dimensional bilayer structures. Finally the bilayer acts as a template for the epitaxial growth of the molecular thin film, determining the azimuthal orientation and a single contact plane for the organic crystallites.