Sebastian Stepanow
This thesis is concerned with supramolecular architectures assembled at metal surfaces. The investigations pursued two objectives. On the one hand, the focus was placed on the fabrication of low-dimensional surface-supported network structures by applying the concepts of conventional three-dimensional supramolecular chemistry at surfaces. Three main driving forces were explored for the construction of low-dimensional assemblies: hydrogen bonding, metal coordination and ionic interactions. The realized structures were characterized by scanning tunneling microscopy (STM) under ultra-high vacuum conditions. In particular, the interplay between the adsorbate-substrate coupling and lateral adsorbate interactions was addressed in the experiments. Further, the electronic and magnetic properties of the fabricated coordination structures were investigated. This characterization was done by x-ray absorption experiments at the synchrotron facility in Grenoble. Moreover, the response of the metal units and open cavity structures to the adsorption of large organic and small gas molecules was studied by temperature controlled STM experiments. The first part of the thesis deals with hydrogen bonded supramolecular assemblies. Simple aromatic carboxylic acids were employed to study the assembly principles and in particular effects arising from the substrate and molecular structure. The investigation of TPA (1,4-benzenedicarboxylic acid) adlayers on Cu(100) at different substrate temperatures revealed how the protonation status of the carboxyl moieties affects the topology of the assemblies. Further, the influence of the molecular backbone functionality and symmetry is addressed in comparative investigations on TPA and PDA (2,5-pyrazinedicarboxylic acid) as well as BDA (4,4'-biphenyldicarboxylic acid) and SDA (4',4" trans-ethene-l,2-diyl-bisbenzoic acid) adlayers on Cu(100). In the second part of the thesis, metal-ligand interactions were employed for the construction of supramolecular architectures at a Cu(100) surface. The concepts of conventional coordination chemistry were successfully applied at the surface. Iron atoms in conjunction with rod-like aromatic molecular linkers comprising carboxyl, pyridyl and hydroxyl functional groups assemble into a rich variety of open network structures with distinctly arranged mono- and diiron coordination centers and well-defined cavities. The results show that the adsorbate-substrate coupling considerably affects the local coordination environment of the iron units when the molecular backbone length is varied. The occurrence of structural isomerism in open network structures is addressed in a comparative study of heterofunctional and homofunctional linker molecules. It is shown that the former ligand allows only one topologically pure structure while the other linker forms two phases with different topology. The hydroxyl ligand has been used to construct distorted hexagonal coordination networks containing threefold coordinated single iron atoms. The comparison between the networks on Cu(100) and Ag(111) surfaces shows marked differences, which are attributed to adsorbate-substrate interactions. Finally, a novel design strategy for the fabrication of two-dimensional coordination structures at surfaces is introduced by using alkali metal ions in conjunction with carboxylate ligands. The interaction between the components is dominated by electrostatic forces and is of intermediate bond strength as in the case of hydrogen bonding and transition metal coordination. It is expected, that substrate symmetry and registry plays a larger role in the network formation, because the ionic interaction is not directional. The chemical as well as magnetic properties of the fabricated coordination networks are examined in the last part of the thesis. The adsorption of C60 molecules on open network structures reveals how cavity size and chemical functionality affect the bond strength to the guest molecules as well as the number of accommodated C60 molecules. Since transition metal clusters, and in particular diiron units, are the catalytic active centers in various proteins, the reactivity of various mono- and diiron coordination structures to molecular oxygen was investigated in temperature controlled STM experiments. In particular, the focus was placed on the structural relaxation upon gas adsorption and chemical reaction. At last, the magnetic properties of the metal coordination centers are addressed in XMCD experiments. The iron units are found to be paramagnetic at temperatures down to 5 K and show distinct magnetic anisotropy and magnetization loops. These experiments demonstrate that surface-supported metal coordination structures are a promising class of materials for applications in the field of heterogenous catalysis and surface magnetism.
EPFL2005
