Proprietà elettroniche e vibrazionali di strati sottili adsorbiti su substrati metallici

Abstract

Understanding the adsorption, catalytic, and electronic properties of metal/metal interfaces is one of the foremost aims of surface science. In fact, through the tailoring of the characteristics of the substrate it should be possible to obtain more selective catalysts and more effective electronic devices. This thesis is focused on the vibrational and electronic properties of various metal/metal systems, studied by high-resolution electron energy loss spectroscopy. Firstly, alkali adsorption and their coadsorption with CO, O, and OH on transition-metal and noble-metal surfaces have been investigated. Vibrational measurements demonstrated that alkalis adsorb as neutral and polarized adatoms on metal surfaces. The adsorption of CO induces alkali ionization. Short-range effects dominate in alkali coadsorption systems. Alkali oxidation is more readily achieved upon CO adsorption or dissociation than by direct exposures to oxygen. The stabilization of subsurface O was obtained on alkali-doped Cu(111), while on alkali-modified transition-metal catalysts O adatoms remain in on-surface sites. In the latter case a softening of the O-substrate bond occurs. Moreover, we studied chemical reactions at clean and alkali-doped bimetallic surfaces obtained by depositing monolayers of silver on Ni(111) and Cu(111). Important information on the metal surfaces and interfaces could be provided also by the analysis of collective electronic excitations. Surface plasmon dispersion was investigated in metal/metal systems exhibiting electron quantum confinement. Screening effects are enhanced by the presence of quantum well states and a direct correlation between the free-electron density of states of the system, the chemical reactivity, and dynamical screening processes exists. Measurements have been performed on both flat thin films and films nanostructured in islands in order to shed light on the influence of the growth mode on the electronic response of the system. The knowledge gained herein has far-reaching implications to the general understanding of processes at metal/metal interfaces. Keywords: chemisorption, alkali metals, carbon monoxide, oxygen, metallic thin films, silver, bimetallic surfaces, electronic excitations, electron energy loss spectroscopy, surface chemical reactivity, charge transfers, electron confinement.