Anion/Cation Layers at Electrified Interfaces: A Comprehensive STM, XRD and XPS Case Study

Charged organic adsorbates play an important role in a number of electrochemical reactions, e.g. as additives for metal plating relevant for device fabrication in the semiconductor industry. Fundamental investigations are mandatory in order to acquire profound knowledge of the structural and electronic properties of these layers parallel and perpendicular to the surface, and to finally achieve a deeper mechanistic understanding of the kinetics of involved charge transfer reactions taking place at these complex metal/organic/electrolyte interfaces. A key structural motif of these interfaces consists in 'paired' (inorganic)anion/(organic)cation layers that can have an enormous stability even during an ongoing charge transfer reaction. In this contribution we present and discuss a selected case study on the co-adsorption of halide anions and cationic organic molecules that exhibit a pronounced redox activity. It will be demonstrated that their phase behavior at the interface crucially depends on both their particular redox-state and the surface concentration of the halide counter ions. The subtle balance between adsorbate–adsorbate and adsorbate–substrate interaction of the poly-cationic organic layer can be carefully controlled by potential dependent anion adsorption and desorption processes through the organic layer. This process can be followed by in situ high-resolution scanning tunnelling microscopy, while additional information about the structural and chemical state of the respective phase is obtained from in situ X-ray diffraction and ex situ photoelectron spectroscopy.