Vertical Bonding Distances Impact Organic-Metal Interface Energetics

The X-ray standing wave (XSW) technique is ideally suited to measure element-specific bonding distances of organic (sub)monolayers on metal single crystalline surfaces. These bonding distances are crucial for the energy-level alignment and thus for the charge injection properties at these interfaces. We review experimentally determined bonding distances of several prototypical interfaces. Even for weakly interacting systems site-specific interactions can lead to adsorption induced molecular distortions and consequently to additional intramolecular dipoles, which impact the energy-level alignment. Guidelines to tune adsorption heights by rational molecular design are discussed. For strongly interacting system (charge transfer complex formation) distortions of the molecules on the substrate play a pivotal role in the process of surface-induced aromatic stabilization, which results in metallic organic monolayers.