Dresden 2014 – scientific programme

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O: Fachverband Oberflächenphysik

O 86: Organic Electronics and Photovoltaics VI (DS jointly with CPP, HL, O)

O 86.6: Talk

Thursday, April 3, 2014, 19:00–19:15, CHE 91

Commensurability as the determining factor for molecular tilt and multilayer growth: In-situ and real-time growth study of the nanographene HBC on SiO₂ and HOPG — •Paul Beyer¹, Tobias Breuer², Saliou Ndiaye², Anton Zykov¹, Andreas Viertel¹, Manuel Gensler¹, Jürgen P. Rabe¹, Stefan Hecht¹, Gregor Witte², and Stefan Kowarik¹ — ¹HU Berlin, 12489 Berlin — ²Philipps-Universität Marburg, 35032 Marburg

We investigate the influence of symmetries and the substrate-molecule commensurability on the structure and growth kinetics of the nanographene hexa-peri-hexabenzocoronene (HBC). We study organic molecular beam deposited ultrathin HBC films on SiO₂, pristine and sputtered HOPG because of the similar surface energies, singling out the influence of the lattice-matching between HBC and graphite. Using real-time and in situ x-ray growth oscillations we find the first ad-layer to grow lying down on both substrates. On pristine HOPG we find a superstructure of hexagonally arranged, recumbent molecules in a new polymorph using GIXD, XRR and NEXAFS. The loss of commensurability in the sputtered HOPG leads to mainly upright molecules. Equally, we find a transition to upright molecules on amorphous SiO₂ surfaces. On SiO₂ micro-crystallites in the bulk structure that completely cover the substrate are found, while the strong ordering on HOPG leads to island growth of the new polymorph as observed by AFM. Our results demonstrate that the (lack of) symmetry- and lattice-matching critically determines the molecular orientation and occurrence of surface induced polymorphs.