Microstructural Analysis of Perfluoropentacene Films on Graphene and Graphite: Interface-Mediated Alignment and Island Formation

The epitaxial growth of the n-type organic semiconductor perfluoropentacene (C₂₂F₁₄, PFP) on graphite and graphene substrates is analyzed by combining X-ray diffraction, scanning tunneling microscopy (STM), atomic force microscopy, and transmission electron microscopy. On both graphite and graphene substrates, PFP forms islands where molecules adopt the π-stacked polymorph with lying molecular orientation relative to the substrate. By performing azimuthal analyses, we identify the epitaxial relation between the PFP islands and the graphite substrate and gain insights into the crystal habitus of the PFP islands. Combining low-temperature STM measurements with molecular mechanics calculations, we provide a consistent mechanism for the azimuthal alignment at the PFP/graphene interface and the lateral positioning of the organic adsorbate on the graphene substrate.