Exploring the driving forces behind the structural assembly of biphenylthiolates on Au(111)

In this contribution, we use dispersion-corrected density functional theory to study inter- and intramolecular interactions in a prototypical self-assembled monolayer (SAM) consisting of biphenylthiolates bonded to Au(111) via thiolate groups. The goal is to identify the nature of the interactions that drive the monolayer into a specific conformation. Particular focus is laid on sampling realistic structures rather than high symmetry model configurations. This is achieved by studying conceptually different local minimum structures of the SAM that are obtained via exploring the potential energy surface from systematically varied starting geometries. The six obtained packing motifs differ in the relative arrangement of the two molecules in the unit cell (co-planar versus herringbone) and in the intramolecular configuration (twisted versus planar rings). We find that van der Waals interactions within the organic adsorbate and between the adsorbate and substrate are the main reason that these molecular assemblies can form stable structures at all. The van der Waals interactions are, however, very similar for all observed motifs; by analyzing various types of interactions in the course of three notional SAM-formation steps, we find that the main driving force stabilizing the actual global minimum structure originates from electrostatic interactions between the molecules.