title: Flexible 2D Crystals of Polycyclic Aromatics Stabilized by Static Distortion Waves.
abstract: The epitaxy of many organic films on inorganic substrates can be classified within the framework of rigid lattices which helps to understand the origin of energy gain driving the epitaxy of the films. Yet, there are adsorbate-substrate combinations with distinct mutual orientations for which this classification fails and epitaxy cannot be explained within a rigid lattice concept. It has been proposed that tiny shifts in atomic positions away from ideal lattice points, so-called static distortion waves (SDWs), are responsible for the observed orientational epitaxy in such cases. Using low-energy electron diffraction and scanning tunneling microscopy, we provide direct experimental evidence for SDWs in organic adsorbate films, namely hexa-peri-hexabenzocoronene on graphite. They manifest as wave-like sub-Ångström molecular displacements away from an ideal adsorbate lattice which is incommensurate with graphite. By means of a density-functional-theory based model, we show that, due to the flexibility in the adsorbate layer, molecule-substrate energy is gained by straining the intermolecular bonds and that the resulting total energy is minimal for the observed domain orientation, constituting the orientational epitaxy. While structural relaxation at an interface is a common assumption, the combination of the precise determination of the incommensurate epitaxial relation, the direct observation of SDWs in real space, and their identification as the sole source of epitaxial energy gain constitutes a comprehensive proof of this effect.
authors.affiliation: Institute of Solid State Physics, Friedrich Schiller University , Helmholtzweg 5, 07743 Jena, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Meissner
authors.affiliation: Institute of Solid State Physics, Friedrich Schiller University , Helmholtzweg 5, 07743 Jena, Germany.
authors.forename: Falko
authors.initials: F
authors.lastname: Sojka
authors.affiliation: Institute of Condensed Matter Theory and Solid State Optics, Friedrich Schiller University , Fröbelstieg 1, 07743 Jena, Germany.
authors.forename: Lars
authors.initials: L
authors.lastname: Matthes
authors.affiliation: Institute of Condensed Matter Theory and Solid State Optics, Friedrich Schiller University , Fröbelstieg 1, 07743 Jena, Germany.
authors.forename: Friedhelm
authors.initials: F
authors.lastname: Bechstedt
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Xinliang
authors.initials: X
authors.lastname: Feng
authors.affiliation: Max Planck Institute for Polymer Research , Ackermannweg 10, 55128 Mainz, Germany.
authors.forename: Klaus
authors.initials: K
authors.lastname: Müllen
authors.affiliation: Center for Advancing Electronics, University of Technology Dresden , Würzburger Strasse 46, 01187 Dresden, Germany.
authors.forename: Stefan C B
authors.initials: SC
authors.lastname: Mannsfeld
authors.affiliation: Institute of Solid State Physics, Friedrich Schiller University , Helmholtzweg 5, 07743 Jena, Germany.
authors.forename: Roman
authors.initials: R
authors.lastname: Forker
authors.affiliation: Institute of Solid State Physics, Friedrich Schiller University , Helmholtzweg 5, 07743 Jena, Germany.|Graduate School of Science and Institute for Academic Initiatives, Department of Chemistry, Osaka University , 1-1 Machikaneyama, Toyonaka 560-0043, Osaka, Japan.
authors.forename: Torsten
authors.initials: T
authors.lastname: Fritz
keywords: density functional theory (DFT); epitaxial graphene; low-energy electron diffraction (LEED); natural graphite; orientational epitaxy; polycyclic aromatic hydrocarbon (PAH); scanning tunneling microscopy (STM)