Abstract
This work presents a detailed study of ultrathin films of FeO, iron silicate, and iron germanate on Ru(0001). These two-dimensional structures are suitable model systems for catalytically active structures, such as zeolites, which are known for their good catalytic properties, for instance as molecular sieves or selective catalysts. This work applies the methods of LEEM, μLEED, μXPS and XPEEM for a detailed and comprehensive investigation of chemical and physical properties of these films. The temperature dependent film formation is studied in real-time and in situ. Ultrathin FeO films were prepared by direct deposition of iron onto a Ru(0001) single crystal support at elevated temperatures. Dependent on the oxygen pressure monolayer-thick or bilayer-thick FeO films are found to grow in a Stranski-Krastanov growth, i.e. a complete wetting layer followed by three-dimensional islands, whereas the latter grow as Fe3O4 crystallites. Different metastable sub-phases are structurally and chemically characterized and their transformation into other more stable phases is addressed. Ultrathin iron silicate films consist of a monolayer of silica on top of a monolayer of iron oxide. The work, presented in this thesis, brought new insights into the structure of these layers. In particular, the measurements suggest that the number of iron atoms per silica unit cell in the iron oxide layer is two and that an additional oxygen layer at the iron/ruthenium interface is present. The Fe-Fe distance is found to be adapted to the Si-O-Si distance in unstrained silicates, rather than being influenced by the ruthenium substrate. Four different preparation methods have been developed in order to study the film formation in dependence of temperature and oxygen pressure. The films were characterized with regard to their thermal stability and chemical and physical properties. Moreover, the iron silicate structure was varied and prepared with two layers of iron oxide or two layers of silica, respectively. Both additional layers are found to be stable if grown as complete layers, and adapted to the monolayer iron silicate film structure. The study of incomplete layers brought new insights into the dynamic processes and thermal stability. In particular, iron migration was found to start at iron silicate domains, when silicon was deposited on top of FeO islands. The migrating iron binds to silica in initially iron-free domains, where again iron silicate is formed. For monolayer-thick FeO films this takes in form of about 50 nm small isolated islands. In bilayer-thick FeO films a two-step process is found: First a rim is formed consisting of monolayer-thick iron silicate. In a second step small isolated islands are formed additionally. Ultrathin iron germanate films are found to have almost the same structure as iron silicate films, i.e. a monolayer of germania is bound on top of a monolayer of iron oxide. However, the Fe-Fe distance is adapted to the length of Ge-O-Ge bonds in unstrained germanates. Different preparation methods show that these films are energetically stable in the investigated temperature (between 300 K and 840 K) and pressure (up to 10-6 mbar) range.
