Metal-on-Metal Bonding and Rebonding Revisited

Density-functional calculations for a wide variety of metals show that, contrary to the rebonding view of adsorbate bonding, addimers do not have notably longer surface bonds than adatoms, do not reside farther above the surface, and do not meet the rebonding arguments for augmented mobility. Rebonding concepts are found to have some utility in explaining addimer stability

Atom Dynamics and Diffusion on Surfaces

This thesis is a theoretical study of atomic diffusion processes in condensed matter, with particular emphasis on surface diffusion. Oxygen adsorption and diffusion on the (111) surface of platinum is studied using a first-principles pseudopotential plane-wave approach based on density functional theory, with local (LDA) and semi-local functionals (GGA) for the exchange-correlation energy. The study resolves an inconsistency between experimental and theoretical results in the literature. The oxygen induced surface buckling and vibrational frequencies are found to be in good agreement with experimental data. This first-principles method is also used to explore the diffusion processes at edges, kinks, and corners of islands on the Al(111) surface. The calculated diffusion barriers are converted into a set of activation temperatures through ordinary transition state theory, and used to predict the temperature evolution of the surface morphology. Recent scanning tunneling microscopy experiments confirm several calculated results in great detail, which indicates a good accuracy in the theoretical approach. The Al(111) surface is further investigated in a first-principles study of the dynamics and diffusion of Al dimers. An unexpected ground state is found and explained in terms of elastic energy. A new diffusion path is revealed and shown to be blocked by compressive strain. A smooth potential energy surface enables exotic dynamic behavior, and the possibility of a pure metal quantum rotor is addressed.<p /> The time domain is stretched considerably in a kinetic Monte Carlo study on the motion of large atom islands. A fundamental break-down of regular scaling theory is found, which is now confirmed by experiments and other simulations. This result calls for a reevaluation of experimental analysis where the atomic migration mechanism has been deduced from the size scaling of the diffusion coefficient, and bears consequences in long-time studies of the evolution of the surface morphology by coarsening and Ostwald ripening. The shortcomings of scaling theory are addressed with the development of a new, kinetic, theory.<p /> Self-diffusion in liquid gold is studied using a many-atom interaction potential within the effective-medium theory. Thermal and structural properties are in good agreement with experimental results. The molecular dynamics simulations reveal a parabolic temperature dependence of the self-diffusion coefficient, as recently measured in micro-gravity experiments on other non-simple metals

Variations in the Nature of Metal Adsorption on Ultrathin Al(2)O(3) Films

First-principles density-functional calculations are used to study metal adsorption (Li, K, Y, Nb, Ru, Pd, Pt, Cu, Ag, Au, and Al at 1/3-4 monolayer coverages) atop 5 ~ A1203 films on Al(Ill). The oxide-metal bond is ionic at Iow coverages but, with interesting exceptions, caused by polari@i ,~-cE!vED at high coverages where the overlayer is metallic. Binding trends are explained in terms of s'imp e concepts. Increasing overlayer thickness can cause the adsorbate-oxide interface structure to than . %lEc o ~ 1998 and while some metals wet, most do not

Ultrathin aluminum oxide films: Al-sublattice structure and the effect of substrate on ad-metal adhesion

First principles density-functional slab calculations are used to study 5 {angstrom} (two O-layer) Al{sub 2}O{sub 3} films on Ru(0001) and Al(111). Using larger unit cells than in a recent study, it is found that the lowest energy stable film has an even mix of tetrahedral (t) and octahedral (o) site Al ions, and thus most closely resembles the {kappa}-phase of bulk alumina. Here, alternating zig-zag rows of t and o occur within the surface plane, resulting in a greater average lateral separation of the Al-ions than with pure t or o. A second structure with an even mix of t and o has also been found, consisting of alternating stripes. These patterns mix easily, can exist in three equivalent directions on basal substrates, and can also be displaced laterally, suggesting a mechanism for a loss of long-range order in the Al-sublattice. While the latter would cause the film to appear amorphous in diffraction experiments, local coordination and film density are little affected. On a film supported by rigid Ru(0001), overlayers of Cu, Pd, and Pt bind similarly as on bulk truncated {alpha}-Al{sub 2}O{sub 3}(0001). However, when the film is supported by soft Al(111), the adhesion of Cu, Pd, and Pt metal overlayers is significantly increased: Oxide-surface Al atoms rise so only they contact the overlayer, while substrate Al metal atoms migrate into the oxide film. Thus the binding energy of metal overlayers is strongly substrate dependent, and these numbers for the above Pd-overlayer systems bracket a recent experimentally derived value for a film on NiAl(110)

Origin of Compact Triangular Islands in Metal-on-Metal Growth

The microscopic origin of compact triangular islands on close-packed surfaces is identified using kinetic Monte Carlo simulations with energy barriers obtained from density-functional calculations. In contrast to earlier accounts, corner diffusion anisotropy is found to control the shape of compact islands at intermediate temperatures. We rationalize the correlation between the orientation of dendrites grown at low temperatures and triangular islands grown at higher temperatures, and explain why in some systems dendrites grow fat before turning compact