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.

Giant thermal expansion of a two-dimensional supramolecular network triggered by alkyl chain motion

“…From the lattice measurements, we extract of a thermal expansion of this system of about α = 520 ± 60 × 10 −6 K −1 . While the same hexagonal ordering has been reported by Meissner et al 34 from pristine HBC molecule (without alkyl chains), no lattice variation has been observed between 1.2 K and 300 K STM measurements, resulting in a thermal expansion of only α = 14 × 10 −6 K −134, 35 .…”

“…The cores of these precursors are quite different accounting for the slight difference in observed thermal expansion since the HBC lies flat on the surface 24 while the SW core tends to protrude upon adsorption. In contrast to these interdigitated systems, supramolecular networks formed by pristine HBC molecules do only show a negligible expansion 34,35 . This observation again points towards the central role of the alkyl chains in the thermal expansion mechanism of interdigitated systems.…”

First-principles calculations of hybrid inorganic–organic interfaces: from state-of-the-art to best practice

“…However, also the opposite effect may occur, if, e.g., the stretching of the substrate introduces a lattice mismatch between the docking groups of the adsorbate and the docking sites of the substrate. The magnitude of either effect can be appreciable 116 and can easily be larger than the energetic differences between different adsorption sites or polymorphs. 117 Consequently, for such interfaces authoritative first-principles structure search or the reliable simulation of surface phase diagrams becomes an unsurmountable challenge.…”

Searching for New Polymorphs by Epitaxial Growth

“…The discovery of unknown polymorphs through epitaxy is reported for several cases. 22 − 28 A number of experimental methods can be used to prove the presence of a new phase; spectroscopic techniques and diffraction methods are used. 29 − 31 However, the presence of several different phases is a frequently observed case within epitaxially oriented crystallites at surfaces.…”