The present work is divided in two part. The first is dedicated to the investigation of the gasmetal interactions, an interesting area in the basic surface science but also in applied one, since it could provide a more efficient way to design corrosionresistant structural metals. In particular, we concentrate our attention on the study H2S on Fe surface. Experimental studies, of adsorption of H2S on Fe, and firstprinciple calculations were carried out for these systems, clarifying some important questions, such as adsorption geometry and dissociation pathways for H2S, on the above closepacked metal surfaces. However, real samples will also include a number of defects, in particular step edges where bonding of adsorbates is usually stronger than at facets. It is therefore interesting to investigate adsorption of H2S on a stepped Fe surface, a task which has not been considered yet to the best of our knowledge. In the present work we study the H2S interaction with Fe(310) surfaces by DFT calculations in order to understand the role of step defects in the adsorption properties. We recall that the (310) surface is relatively stable, and its surface energy predicted to be even smaller than that of Fe(110). We do not only obtain the binding sites and adsorption energies of H2S and its components, but we also relate bonding to the detailed features of the localdensity of states (LDOS). The second part of the present thesis is devoted to the dynamics of scattering. Scattering underlies various physical processes in different field of physics, mainly in solid state, as for example in thermoelectricity, about the filtering of hot electrons by defects, or adsorption and desorption by a surface, or in charge injection and field emission trough interface, usually associated with tunneling mechanisms. The recent developments of nanotechnology and the advent of modern highspeed highdensity MOS devices, have revived the technological and theoretical interest of the scientific community on the scattering problem and in particular on quantun tunneling mechanism usually associated. Ultrascaled nanometric CMOS compatible single electron transistors (SETs) and single atom trasistors has lead the emergence of density of states graining and fluctuations in the contacts which may determine discretization of energy levels, charge localization at intradopant length scale and selection rules on quantum states in tunnelling. Consequently, the understanding of dependence of charge dynamics, across a barrier, from the initial position constitutes a relevant aspect in such systems. In this work we study the scattering process in the non stationary framework using Gaussian wave packet (GWP) to describe the particle wave function of the system so as to consider the dependence of scattering dynamics from the initial conditions. Through a numerical solution of the Schr¨odinger equation we analyse the evolution of the system calculating the transmission of the scattering GWP as a function of the initial spread and position x(0), and comparing simulated data with theoretical results. By our analysis a new important issue emerges: the time spent by the particle to reach its asymptotic probability to be observed beyond the barrier ( that we call formation time), strongly depends on initial conditions, and in particular on x(0). Finally, to analytically express such a dependence, we propose a semiclassical approximated model in which tf is described as the time spent by a finite support (accounting for the 0.99 of the probability) of the incident wave packet to cross the barrier, namely the time required to locate, in coordinate space, the greatest amount of the GWP’s probability distribution beyond the barrier interface.
Adsorbption and scattering phenomena in materials science
CARONE FABIANI, FILIPPO
2014
Abstract
The present work is divided in two part. The first is dedicated to the investigation of the gasmetal interactions, an interesting area in the basic surface science but also in applied one, since it could provide a more efficient way to design corrosionresistant structural metals. In particular, we concentrate our attention on the study H2S on Fe surface. Experimental studies, of adsorption of H2S on Fe, and firstprinciple calculations were carried out for these systems, clarifying some important questions, such as adsorption geometry and dissociation pathways for H2S, on the above closepacked metal surfaces. However, real samples will also include a number of defects, in particular step edges where bonding of adsorbates is usually stronger than at facets. It is therefore interesting to investigate adsorption of H2S on a stepped Fe surface, a task which has not been considered yet to the best of our knowledge. In the present work we study the H2S interaction with Fe(310) surfaces by DFT calculations in order to understand the role of step defects in the adsorption properties. We recall that the (310) surface is relatively stable, and its surface energy predicted to be even smaller than that of Fe(110). We do not only obtain the binding sites and adsorption energies of H2S and its components, but we also relate bonding to the detailed features of the localdensity of states (LDOS). The second part of the present thesis is devoted to the dynamics of scattering. Scattering underlies various physical processes in different field of physics, mainly in solid state, as for example in thermoelectricity, about the filtering of hot electrons by defects, or adsorption and desorption by a surface, or in charge injection and field emission trough interface, usually associated with tunneling mechanisms. The recent developments of nanotechnology and the advent of modern highspeed highdensity MOS devices, have revived the technological and theoretical interest of the scientific community on the scattering problem and in particular on quantun tunneling mechanism usually associated. Ultrascaled nanometric CMOS compatible single electron transistors (SETs) and single atom trasistors has lead the emergence of density of states graining and fluctuations in the contacts which may determine discretization of energy levels, charge localization at intradopant length scale and selection rules on quantum states in tunnelling. Consequently, the understanding of dependence of charge dynamics, across a barrier, from the initial position constitutes a relevant aspect in such systems. In this work we study the scattering process in the non stationary framework using Gaussian wave packet (GWP) to describe the particle wave function of the system so as to consider the dependence of scattering dynamics from the initial conditions. Through a numerical solution of the Schr¨odinger equation we analyse the evolution of the system calculating the transmission of the scattering GWP as a function of the initial spread and position x(0), and comparing simulated data with theoretical results. By our analysis a new important issue emerges: the time spent by the particle to reach its asymptotic probability to be observed beyond the barrier ( that we call formation time), strongly depends on initial conditions, and in particular on x(0). Finally, to analytically express such a dependence, we propose a semiclassical approximated model in which tf is described as the time spent by a finite support (accounting for the 0.99 of the probability) of the incident wave packet to cross the barrier, namely the time required to locate, in coordinate space, the greatest amount of the GWP’s probability distribution beyond the barrier interface.File  Dimensione  Formato  

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https://hdl.handle.net/20.500.14242/72326
URN:NBN:IT:UNIMIB72326