The work for project ClusterDynamics was performed at the Technical University of Munich. We investigated the dynamics of model catalysts with a very high degree of definition, combining size-selected (i.e. defined to the exact number of atoms) metal clusters with single crystalline supports. Fast scanning tunneling microscopy (FastSTM) was used to monitor cluster diffusion, sintering into larger particles and internal restructuring on the atomic scale with a time resolution down to some tens of msec.
In a first step, Pd clusters, which are a very versatile model system thanks to their rich redox chemistry, were deposited on the moiré films of graphene and boron nitride grown on Rh(111). These well-defined models were used to test and further develop the so-called FAST module, an add-on electronics module which we connected to our standard STM instrument, to allow us to measure STM at up to 20 fps. We found that Pd clusters containing only a few atoms are highly mobile on boron nitride and identified motion ranging from long-range diffusion leading to sintering, over confined diffusion inside a boron nitride pore or the moiré structure, to the internal restructuring of clusters. To our surprise, the cluster dynamics did not appear to be influenced by adsorbate molecules.
In another part of the project, we investigated the stability of clusters when exposed to air for transportation between different vacuum instruments. We found that Pt clusters are highly stable on the native oxide film of a silicon wafer, SiO2. Scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS) showed that the clusters can be transported and stored in ambient conditions without loss of the size selection. Furthermore, this system proved highly sinter-resistant, even at elevated temperatures and in reactive environments.
After initial measurements on boron nitride and graphene films which established the FastSTM technique as a routine analysis tool in our lab, we investigated Pt clusters on a more relevant support material, Fe3O4(001). We found that even the support itself already exhibits rich surface dynamics, ranging from adsorbate diffusion to the dynamics the Fe atoms themselves depending on temperature. In contrast, Pt clusters adsorbed on the support are stabilized and do not sinter at temperatures up to 600 K. The temperature-dependence of the apparent cluster height and adsorption site and the influence of adsorbates on the clusters were investigated. In order to expose the clusters to a higher pressure environment (up to 10-3 mbar), a sniffer-TPR setup was installed and characterized.
