Dr. Tao Xu, Physical Chemistry II, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany

Structure Dependent Anchoring and Thermal Stability of Organic Molecules on Atomically-Defined Oxide Surfaces

Organic thin films on oxide surfaces are essential in many applications like photovoltaics and molecular electronics. For such applications, the organic entities are often bound to the surface using anchor groups such as carboxylic acids, phosphonates or hydroxyl groups. The associated binding mechanisms, kinetics and energetics are critical for the growth and the structure formation processes at organic-oxide interfaces. They are the key to control the growth and structure of the film and, therefore, their electronic and chemical properties.

We have performed surface science model studies to explore the influence of (1) the carboxylic acid functional group (2) the surface structure on the anchoring and thermal stability of organic molecules on oxide surfaces. Specifically, we investigated the adsorption and desorption of phthalic acid (PA) on atomically defined (i) MgO(100) grown on Ag(100), (ii) Co₃O₄(111), (iii) CoO(111) and (iv) CoO(100) grown on Ir(100). Formation of the PA films and interfacial reactions were monitored in-situ during growth by isothermal time-resolved infrared reflection absorption spectroscopy (TR-IRAS) under ultrahigh vacuum (UHV) conditions. It is found that PA molecules bond to MgO(100) through the carboxyl group in a tilted bidentate configuration. We observe pronounced structure dependencies on the three cobalt oxide surfaces with different binding geometries and characteristic differences as a function of temperature and coverage. Desorption of PA from the oxides was monitored by recording temperature-programmed IRAS between 130 and 600 K. Pronounced differences in stability are observed. The most stable species is found on CoO(100) where a bridging bis-carboxylate is formed involving easily accessible Co surface ions. The chelating bis-carboxylate on Co₃O₄(111) is found to be less stable and the distorted bis-carboxylate on CoO(111) shows the lowest stability due to the limited accessibility of the surface Co ions.

Host: Associate professor Jeppe Vang Lauritsen, Interdisciplinary Nanoscience Center, Aarhus University