Ultrathin Films of Diindenoperylene on Graphite and SiO2

In situ low-temperature scanning tunneling microscopy (LT-STM), synchrotron-based high-resolution photoemission spectroscopy, and near-edge X-ray absorption fine structure measurements were used to study the supramolecular packing, molecular orientation, and electronic structures of ultrathin films of diindenoperylene (DIP) on graphite and SiO2. LT-STM measurements reveal that monolayer DIP on highly ordered pyrolytic graphite (HOPG) adopts a long-range ordered brick-wall arrangement with their molecular pi-planes orientated parallel to the graphite surface, arising from the DIP-graphite interfacial pi-pi interaction. In contrast, DIP molecules stand upright on inert SiO2 due to the weak interfacial interaction. The ionization potential (IP) of DIP films largely depends on their molecular orientation, i.e., the IP for the standing-Up DIP film on SiO2 is 0.40 +/- 0.05 eV lower than that of the lying-down film on graphite. This is attributed to the intrinsic surface dipoles that originate from the intramolecular dipolar C-H bonds exposed at the surfaces of the standing-up thin film.