Self-Assembly of Pd-Cyclometalates and Melem on surfaces: a photo-electron spectroscopy and scanning probe microscopy investigation

The controlled assembly of organometallic complexes at solid surfaces is a key step towards the rational design of nanostructured materials for heterogeneous catalysis. Although the interest in surface-confined molecular systems is increasing, the relationship between molecular structure, deposition technique, and thermally induced morphological changes is still not completely understood. This thesis addresses these open questions for the two specific cases of three palladium cyclometalated (CyPd) complexes deposited on highly oriented pyrolytic graphite (HOPG) investigated by X-Ray photoemission spectroscopy (XPS) and Atomic Force Microscopy (AFM) at UNIGE and of the organic molecule Melem on Au(111) studied by Scanning Tunnelling Microscopy (STM) during a six month stay at the Institute of Applied Physics (IAP) at Vienna University of Technology (TU Wien). The deposition of CyPd complexes, performed either by drop-casting from a solution of dimethylformamide (DMF) or by sublimation under ultra-high vacuum (UHV) conditions, leads to non-dissociative adsorption as inferred by XPS. The direct comparison of solution-based and vacuum-based deposition routes shows that the same chemical state is obtained. Particular emphasis was placed on the Pd 3d core-level analysis in order to probe the chemical state and stability of the palladium catalytic centre. Structures with different morphologies were evidenced by AFM at the mesoscale, corresponding to dendritic and fibrous networks, as well as dense layered structures which undergo significant changes upon annealing. Melem molecules sublimated on Au(111) in UHV form Nitrogen-rich carbon-based layers on which metallic atoms can be supported, thus preventing their sintering. Then Fe atoms were deposited on the Melem honeycomb overlayer. This system was investigated by XPS and STM. XPS inspection indicated that Fe atoms end up in a 4-fold coordinated configuration. STM images show that Fe adsorption induces the formation of a more compact Melem structure. Clustering of Fe atoms was negligible up to 2 Fe atoms per Melem overlayer unit cell. Both investigated systems show that self-assembled C-based nanostructures with metallic centers can be generated by depositing organic/organometallic precursor molecules on surfaces. These nanostructures hosting single isolated metal atoms are stable well above room temperature (RT) and may therefore act as single atom catalysts.