Epitaxial Growth of π-Stacked Perfluoropentacene on Graphene-Coated Quartz

Chemical-vapor-deposited large-area graphene is employed as the coating of transparent substrates for the growth of the prototypical organic n-type semiconductor perfluoropentacene (PFP). The graphene coating is found to cause face-on growth of PFP in a yet unknown substrate-mediated polymorph, which is solved by combining grazing-incidence X-ray diffraction with theoretical structure modeling. In contrast to the otherwise common herringbone arrangement of PFP in single crystals and standing films, we report a pi-stacked arrangement of coplanar molecules in flat-lying films, which exhibit an exceedingly low pi-stacking distance of only 3.07 angstrom, giving rise to significant electronic band dispersion along the pi-stacking direction, as evidenced by ultraviolet photoelectron spectroscopy. Our study underlines the high potential of graphene for use as a transparent electrode in (opto-)electronic applications, where optimized vertical transport through flat-lying conjugated organic molecules is desired.