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In my PhD activity I was involved in the study of graphene-supported metal oxide nanoparticles and the effect of the graphene doping on their electronic and chemical properties. Nanostructured materials are nowadays at the centre of the scientific investigation in the condensed matter field. The fundamental concept that drives this research topic is that the microscopic features of a nano-designed material can affect its macroscopic properties. This is the reason why, often, the experimental results lead to applications in many different contexts as for example in chemistry, quantum optics, in the field of energy storage, biosensing or quantum optics, rapidly paving the way towards the developments of new technologies. Nano-architectured materials are strictly related to low dimensionality materials. The term nano, in fact, refers to a length scale at which quantum confinements begins to be non-negligible and it gives rise to microscopic modifications and phenomena that have consequences on the macroscopic behaviour of the system. The building blocks of these structures are typically 2D, 1D and 0D objects i.e., namely, layered materials, nanowires and nanoclusters. In this PhD work I investigated the interaction between nanoparticles and their solid substrate with the aim to tune the properties of the first ones by controlling the structure of the latter. To do so, we combined 2D and 0D materials to fabricate novel nanostructured interfaces. Metal oxides nanoparticles have been studied, with an attention on their possible application as heterogeneous photocatalysts. The thesis starts describing the methods used to create differently nanostructured supports for the metal oxide nanoparticles. Graphene has been used as the key building block in this context because of its several remarkable properties. From the electronic point of view, its outstanding transport properties promise to be an efficient way to increase the charge separation in photocatalytic reactions. On the other hand, its mechanical strength and its thermal stability are two features that a substrate must have to be reliably exploited. In order to modify the electronic structure of graphene, intercalation procedures have been performed to grow a metal oxide thin layer between graphene and its original substrate, whose effect can be described as an electronic doping of graphene. The focus of the experimental activity is the investigation of the correlation between the doping state of graphene and the electronic and chemical properties of the supported particles. Three different metal oxides have been used as both particle constituents and intercalant agents to collect information about different possible graphene doping levels and particles modifications: iron, cobalt and titanium oxide. Synchrotron radiation spectroscopy techniques were used as the principal measurement methods for the characterization of the electronic structure of these interfaces. For titanium oxide, photocatalytic measurements were performed in collaboration with the Department of Chemistry at the University of Trieste, demonstrating that the graphene-based substrate can be designed to enhance the activity of the supported particle-photocatalyst by more than one order of magnitude respect to the same material supported by a metal surface. Theoretical calculations have been also performed to better understand the mechanisms behind this enhancement and possibly predict the behaviour of further nanostructures. In parallel to this research activity I worked on the development and commissioning of a mass-selected nanocluster source, designed to produce clusters with a precise number of atoms in order to exploit space-averaging experimental techniques to investigate their properties. During my PhD period the machine was completed and the first functional tests were performed.

The photocatalytic activity of titania nanoparticles deposited on epitaxial graphene is proven to be significantly affected by the substrate on which graphene is supported. In particular, it has been revealed that the addition of a two-dimensional TiO1.5 layer sandwiched between graphene and the supporting metal induces a p-doping of graphene itself and a consistent shift in the Ti d states. These modifications in the electronic structure are compatible with the reduction of the probability of charge carrier recombination and enhance the photocatalytic activity of the heterostructure. This is indicative of the capital role played by the interfacial thin oxide films in fine-tuning the properties of heterostructures based on graphene and pave the way to new combinations of graphene/oxides for photocatalysis-oriented applications.

We present a study on the growth and characterization of high-quality single-layer MoS$_2$ with a single orientation, i.e. without the presence of mirror domains. This single orientation of the MoS$_2$ layer is established by means of x-ray photoelectron diffraction. The high quality is evidenced by combining scanning tunneling microscopy with x-ray photoelectron spectroscopy measurements. Spin- and angle-resolved photoemission experiments performed on the sample revealed complete spin-polarization of the valence band states near the K and -K points of the Brillouin zone. These findings open up the possibility to exploit the spin and valley degrees of freedom for encoding and processing information in devices that are based on epitaxially grown materials., Comment: 25 pages, 10 figures