Growth and redox of borophene on Al(111) substrate

This thesis presents a comprehensive investigation of borophene growth on an Al(111) substrate, combining our theoretical analysis with the experimental findings obtained by our collaborators at the Elettra synchrotron radiation facility, in the framework of the FERMAT project, funded by the Italian Ministry of University and Research (MUR). The theoretical aspects of this study focus on understanding the role of oxygen and hydrogen adsorbates in the formation of various configurations, as well as their impact on the structural and electronic properties of the system. To this end, we have paid special attention to several quantum-mechanical effects, including the charge transfer mechanisms, the thermodynamic stability of different phases, and the characterization of the bond strength through the computation of the relevant vibrational frequencies. Our theoretical results greatly contribute to the understanding of the experimental findings, which are also rather extensively reported in the thesis for a comprehensive analysis of borophene growth on Al(111). Density functional theory (DFT) combined with a genetic algorithm have been employed to predict the electronic and structural properties of borophene, oxidized borophene, and hydrogenated borophene (borophane) on Al(111). Ab initio thermodynamics has been implemented to find the most stable structures of the above systems in realistic conditions. Vibrational frequencies have been computed from density functional perturbation theory (DFPT). As this methodology is very compute-intensive, we have explored new methods and algorithms to reduce the compute time while enhancing its applicability. To this end, we have started a new project aimed at extending the scope of Neural Network Potentials (NNP) for calculating the phonon dispersion of materials explicitly accounting for the effects of long-range interactions. The calculated total energy, phonon frequency, band structure, and density of states of borophene, oxidized borophene, and borophane on Al(111) provide the theoretical counterpart to the experimental observations performed with Low-Energy Electron Diffraction (LEED), Infrared-Visible Sum Frequency Generation (IR-Vis SFG), and X-ray Photoelectron Spectroscopy (XPS), and other techniques. Our computational study reveals that: (i) The coupling between Al(111) and borophene leads to the formation of strong Al–B bonds, which can be modulated by oxygen doping due to the higher tendency of oxygen to combine with aluminum. Experimental results confirm this finding. (ii) The hydrogenation of borophene on Al(111) induces the formation of a well-ordered honeycomb borophane phase with interesting electronic conductivity properties. In summary, the study contributes to the understanding of the mechanism of borophene growth on metal substrates. It provides insights into the control of the electronic properties of boron-based materials. The results have implications for developing novel 2D materials with tailored properties for technological applications such as nanoelectronics and energy storage. In the course of this research, several studies were conducted and significant findings were obtained.