Study of physical properties of mass-selected atomic nanoclusters deposited on solid surfaces

The research activity I have performed during my PhD was focused on the production and study of the properties of size-selected sub-nanometer atomic clusters. The motivations of the scientific community behind the study of these object is twofold: 1. To understand the behaviour of the matter for aggregates of few atoms, a regime which is intermediate between the single atom and the bulk material. 2. To individuate clusters with specific properties to be exploited for technological applications. The unexpected properties of the clusters make them interesting for their possible applications in a wide variety of fields, such as: single-molecule optoelectronics, as bio-label and molecular sensors, for medical purposes, as catalysts. In order to study these kind of systems, whose properties depend strongly on their exact size, I worked on the development of the cluster source ENAC (Exact Number of Atoms in each Cluster), This source is able to produce mass-selected clusters with atomic precision and has been designed and developed at the Nanoscale Materials Laboratory, in collaboration with the group led by Prof. Ulrich Heiz from the Technical University of Munich, Germany. ENAC is able to produce clusters from homogeneous or heterogeneous solid metallic targets. In the framework of this thesis, the size-selected clusters were deposited and characterized in situ, with photoemission techniques, at the SuperESCA beamline of Elettra. The first part of my PhD was dedicated to the development of the source. I worked on clusters production, taking care of both increasing the clusters current and expanding the mass range of the generated clusters. I also participated in the operations to connect ENAC with the SuperESCA beamline of the synchrotron-radiation facility Elettra. In order to achieve this, I cooperated to build and install an additional stage on the cluster source and contributed to the development and optimization of the electronic equipment needed to the functioning of the RF-octupole installed inside the new stage. The second part of my PhD wad focused on: to the upgrade of the cluster source, to the production of clusters of different elements and to the characterization of the produced clusters. The activities that i carried out focused on mass-selected clusters of different elements: Ag, Pt, Fe, Ta, Mo, deposited on different substrates. The experiments with Ag were the first performed connecting ENAC and SuperESCA. Specifically, I have studied the formation of Ag islands on Ru(0001) using Ag clusters with different sizes as building blocks to individuate effects on the morphology of the islands depending on the size of the clusters used during the deposition. Moreover, I have studied the oxidation of Ag11 clusters on graphene on Ru(0001) at low temperature, in order shed some light on the mechanism of oxidation for these very small objects.