Crescita di X-eni su superfici: proprietà geometriche, elettroniche e chimiche di monostrati di B e P

Starting with the discovery of graphene in 2004, which provided the first experimental evidence of an atomically thin material, and advancing in the investigation of its peculiar and intriguing properties towards the successful synthesis of high-quality graphene films for industrial applications, the scientific interest into two-dimensional materials has grown exponentially. In the recent years, the desire to miniaturise devices capable of exploiting quantum phenomena has driven significant research interest in two-dimensional mono- elemental graphene-like materials called X-enes. Within this extended materials family, this thesis focuses on two selected members, namely borophene and phosphorene, consisting of elemental monolayers of boron and phosphorus atoms, respectively. Boron is characterised by an intrinsic electron deficiency with respect to carbon, which induces, specifically in two dimensions, the tendency to multi-centric bonding and polymorphism. At surfaces, the electron deficiency could be compensated by electron doping provided by a supporting surface. In this thesis, I have characterised the growth of two-dimensional B structures at the Al(111) and Ni\ped{3}Al(111) terminations, with particular interest towards the investigation of the complex interplay between B and Al alloying and segregation, which again gives rise to the formation of a variety of B and B-Al polymorphs. In the following, we investigated how the physical phenomena characterising the two-dimensional B structures at surfaces could be influenced by the presence of ubiquitous gases as oxygen and hydrogen. Upon gases exposure, the relative chemical affinity of B and of the supporting metals with respect to the adsorbates turned out to play a crucial role in determining the behaviour of the system: while atomic H exposure resulted in the direct reduction of the B layer, yielding the synthesis of a honeycomb borophane phase on Al(111), the interaction of the system with O\ped{2} turned out to be far more complex, involving Al atoms segregation, the formation of Al and B oxide terminal structures, depending on the Al concentration in supporting surface. Phosphorus shares with Boron the tendency to form different polymorphs, specifically at surfaces. Several stable two-dimensional phosphorus allotropes have been theoretically predicted, experimentally synthesised, and eventually exploited in technological devices, stimulating an increasing scientific interest. Among the various phosphorene polymorphs, the single-layered blue phosphorus (BlueP) seemed to deliver promising premises for optical and electronic technologies, and it was experimentally synthesised in 2016, exploiting the Au(111) surface as a growth substrate. Interestingly, the effective structure model for the BlueP on this template turned out to be particularly debated, especially about the possible incorporation of Au adatoms from the underlying substrate. In the last part of this thesis, we provided solid experimental evidence in favour of the formation of a long-range ordered network of triangular BlueP islands linked together and stabilised by the incorporation of Au adatoms. The experimental characterisation of growth and properties of X-enes at surfaces has been carried out in a multi-technique experimental approach, exploiting Infrared-Visible Sum Frequency Generation spectroscopy (IR-Vis SFG), Scanning Tunnelling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS), Surface X-Ray Diffraction (SXRD), Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS), and Low-Energy Electron Diffraction (LEED). The experimental measurements were corroborated by theoretical calculations in collaboration with SISSA (group of Prof. Stefano Baroni) and the University of Trieste (group of Prof. Maria Peressi).