Alkali impurities on quantum thin films : adsorption, electron scattering, and impurity-induced nano-structure formation in the quantum regime
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For thin epitaxial metal films, when the thickness is on the order of the Fermi wavelength, [lambda subscript F], quantum confinement can dramatically alter the physical properties of the film. These so-called Quantum Size Effects (QSE) can dramatically alter the morphology of thin films by an intricate interplay between kinetics and surface energy driven thermodynamics. These effects lead to rich growth-related phenomena in Pb(111) films grown on semiconductor substrates such as Si(111). For example, QSE can drive flat film formation when growth is dominated by surface energy oscillations. This is rather surprising for Pb/Si systems because of a rather high lattice mismatch. However, these films are not defect free, but rather show common occurrences of three defect types. Low Temperature Scanning Tunneling Microscopy (LT-STM) was utilized to characterize these defects on the atomic scale. Furthermore, these defects create modulations in the electron density resulting in fluctuations in QWS near defect sites. Another topic of recent interest is how QSE affect adsorption of as well as how adsorbates modify QSE for these Pb films. In this thesis, LT-STM and first principles calculations were utilized to study Cs adsorbates on Pb film surfaces, defects, and step edges. Cs adsorption is intricately related to the electronic structure of the surface, especially the defect sites which can act as surface traps. These Cs adsorbates, which are assumed to be ionized, enhance elastic surface scattering of empty-state electrons. This results in observable wave interference patterns near Cs impurities. Furthermore, Cs adsorbates, by an overall step energy reduction, can promote QSE-related nanostructures, which are otherwise too weak when kinetic effects cannot be ignored. This enhancement of "quantum stability" is driven by favorable Cs step binding and can be explained within the contexts of Density Functional Theory (DFT).