Oxidation of Transition and Noble Metal Size-Selected Clusters Supported on Epitaxial Graphene

Size-selected clusters have emerged in the past decades for their unique features, which are often very different from their bulk analogues. In a cluster, the electronic and chemical features depends on the exact number of atoms, offering an unmatched playground to look for new properties of matter. However, clusters are hard to handle, due to their high reactivity and mobility when they are deposited on solid surfaces. In particular, understanding the interaction of supported metallic size-selected clusters with simple molecules such as oxygen and their oxidation process in conditions where the mass selection is not compromised is experimentally challenging. During my PhD, I was able to focus on this aspect using ENAC (Exact Number of Atoms in each Cluster), the cluster source designed and built at the Nanoscale Materials Laboratory of the Elettra synchrotron in Trieste. The unique feature of ENAC is that it can be connected it directly to the SuperESCA beamline of the Elettra synchrotron to deposit and study the cluster in situ by means of experimental techniques based on synchrotron radiation, such as X-ray photoelectron spectroscopy (XPS). This offers the opportunity to characterize the clusters in controlled conditions, avoiding critical issues such as the presence of contaminants. In the first part of my research activity, I have been involved in the commissioning and optimization of the cluster source, and in its installation at the SuperESCA beamline. When this crucial goal was accomplished, I participated to the first experiment ever performed with the cluster source at the beamline. We studied the adsorption of size selected Ag on Ru(0001) by means of XPS and with the support of density functional theory calculations. Our results indicate that using different clusters as building blocks, it is possible to form two-dimensional Ag islands on Ru(0001) with different degrees of order that depend on the size of the cluster. This experiment acted as a benchmark to test the capabilities of ENAC, and was instrumental to address our investigation towards more complex studies, i.e., the the oxidation of graphene-supported size-selected clusters. Graphene, which was epitaxially grown on Ru(0001) and Ir(111), is a versatile template, whose corrugated morphology allows to reduce the mobility of the clusters while affecting their properties only in a minor extent. To further reduce the cluster mobility and preserve the mass selection, all the experiments were performed at the temperature of 20 K. The cluster oxidation was achieved by exploiting a method based on the photo-induced dissociation of physisorbed O2, which leads to an efficient production of atomic O in an extremely clean environment. We studied the oxidation of supported Ag, Pt and Fe size-selected clusters by means of XPS and DFT calculations, highlighting similarities and differences with their counterparts at larger scale and discussed the potential technological applications for the oxidized clusters in the light of our new results.