POINT DEFECTS AND ADATOMS AT SURFACES:CHALLENGES AND OPPORTUNITIES

In many occasions defects have been proved to be an opportunity more than a limit, as they may be used to tailor the properties of a given material. To this end, a practical route to introduce a controlled amount of defects as well as a deep knowledge of the defect nature is always desirable. As well, recombinative and dissociative processes involving gas molecules are well known to likely occur on metal surfaces, that may then be used in a number of industrial applications. In this thesis I report on both the isolated carbon atom vacancy, that is a common lattice defect in graphene, and the Eley-Rideal formation of H2 molecules in the limit of an single adsorbed atom on the Ag(111) surface. In the first part of this thesis I consider the details of the electronic structure in the neighbourhoods of a carbon atom vacancy in graphene by employing magnetisation-constrained density-functional theory on periodic slabs, and spin-exact, multi-reference, second-order perturbation theory on a finite cluster. The picture that emerges is that of two local magnetic moments (one sigma-like and one pi-like) decoupled from the pi-band and coupled to each other. The ground state is identified as a triplet with a planar equilibrium geometry resulting upon a Jahn-Teller distortion, in which an apical C atom opposes a pentagonal ring. This state lies 0.2 eV lower in energy than the open-shell singlet with one spin flipped, which is a bistable system with two equivalent equilibrium lattice configurations (for the apical C atom above or below the lattice plane) and a barrier 0.1 eV high separating them. Accordingly, a bare carbon-atom vacancy is predicted to be a spin-one paramagnetic species, but spin-half paramagnetism can be accommodated if binding to foreign species, ripples, coupling to a substrate, or doping are taken into account. In the second part, I study by DFT means the process of hydrogenation of the carbon vacancy, starting from the bare defect atom up to the case of six hydrogen atoms chemisorbed onto its nearest neighbours. I initially consider the formation of a mono-hydrogenated vacancy, finding a binding energy of 4.2 eV and no activation barrier to the adsorption. As well, I study a variety of possible mutual arrangements of the adsorbates at higher coverages discussing their reactivity and local magnetic moments. In this way the overall hydrogenation process turns out to be thermodynamically favoured and exothermic with respect to both atomic and molecular hydrogen gas sources at least up to four H atoms. This follows from the fact that the driving force in this process is the saturation of the (3sigma + 1pi) unpaired electrons at the vacancy. Moreover, these DFT energies are used to build a phase diagram in a broad range of temperatures and H2 partial pressures, thus finding that at room T and p conditions, the magnetic (M = 1muB) 3H-anti structure is the most stable in agreement with recent magnetic measurements. In addition, by considering the stable phase at TEM conditions, it seems reasonable to identify the recently detected three-fold and distorted vacancy with the 3H-anti and the 1H vacancy, respectively. In the end, in these calculations the 2H-geminal phase detected in muSR experiments, is found to be unfavoured both from a thermodynamic and a kinetic point of view with respect to other di-hydrogenated structures. In the third part I consider the formation of hydrogen molecules on the Ag(111) surface by abstraction of the adsorbed H atom according to the Eley-Rideal reaction mechanism. To follow the time evolution of the system, I rely on ab initio molecular dynamics and on the quasiclassical trajectory method based onto an external potential energy surface, originally built for quantum calculations on the same system within the flat and rigid surface approximation. In general the reaction is not activated, in fact it has a sizeable cross section even at collision energies in the order of few meV. In terms of cross sections, the differences between ab initio and quasiclassical results at collision energies below 0.5 eV are proved to depend on the surface corrugation and the energy exchange between hydrogen atoms and surface atoms, which are ignored in the quasiclassical study, following from the reference PES used. In this energy interval, the target vibration may be safely neglected but this is not the case for higher collision energies where it strongly affects the final outcome. Moreover, by considering the product molecules the reaction mechanism is identified as mainly based on a non-collinear scheme with the reactive encounter occurring upon a bounce of the incident atom on the surface. By means of all these dynamics calculations a large cross-section (compared to the typical value on transition metals) is found in quite good agreement with a recent experimental estimate at very low coverage. Anyway, in future in order to get closer to the experimental result, it seems to be necessary to account for the initial surface temperature, the surface precoverage and the incident angle of the incoming atoms.