Microscopic view of epitaxial metal growth: nucleation and aggregation

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Surface chemistry of catalysis by gold

Physical and chemical properties of bimetallic surfaces

The formation of a surface metal-metal bond can produce large perturbations in the electronic, chemical, and catalytic properties of a metal. Recent studies indicate that charge transfer is an important component in surface metal-metal bonds that involve dissimilar elements. The larger the charge transfer, the stronger the cohesive energy of the bimetallic bond. On a surface, the formation of a heteronuclear metal-metal bond induces a flow of electron density toward the element with the larger fraction of empty states in its valence band. This behavior is completely contrary to that observed in bulk alloys, indicating that the nature of a heteronuclear metal-metal bond depends strongly on the structural geometry of the bimetallic system.

http://science.sciencemag.org/content/sci/257/5072/897.full.pdf

The Nature of the Metal-Metal Bond in Bimetallic Surfaces

The interaction of oxygen with Al(111) was studied by scanning tunneling microscopy (STM). Chemisorbed oxygen and surface oxides can be distinguished in STM images, where for moderate tunnel currents and independent of the bias voltage the former are imaged as depressions, while the latter appear as protrusions. An absolute coverage scale was established by counting O adatoms. The initial sticking coefficient is determined to so=0.005. Upon chemisorption at 300 K the O adlayer is characterized by randomly distributed, immobile, individual O adatoms and, for higher coverages, by small (1×1) O islands which consist of few adatoms only. From the random distribution of the thermalized O adatoms at low coverages a mobile atomic precursor species is concluded to exist, which results from an internal energy transfer during dissociative adsorption. These ‘‘hot adatoms’’ ‘‘fly apart’’ by at least 80 A, before their excess energy is dissipated. A model is derived which explains the unusual island nucleation scheme by trapping of the hot adatoms at already thermalized oxygen atoms. Oxidation starts long before saturation of the (1×1) O adlayer, at coverages around ΘO≂0.2. For a wide coverage range bare and Oad covered surfaces coexist with the surface oxide phase. Upon further oxygen uptake both chemisorbed and oxide phase grow in coverage. Oxide nucleation takes place at the interface of Oad islands and bare surface, with a slight preference for nucleation at upper terrace step edges.Further oxide formation progresses by nucleation of additional oxide grains rather than by growth of existing ones, until the surface is filled up with a layer of small oxide particles of about 20 A in diameter. At very large exposures up to 5×105 L they cover the entire surface as a relatively smooth, amorphous layer of aluminum oxide. The difference in Al atom density between Al metal and surface oxide is accommodated by short range processes, with no indication for any long range Al mass transport. Based on our data we discuss a simpler two step model for the interaction of oxygen with Al(111), without making use of an additional subsurface oxygen species. The complex spectroscopic data for the O/Al(111) system are rationalized by the wide coexistence range of bare and Oad covered surface with surface oxide and by differences in the electronic and vibronic properties of the surface atoms depending on the number of neighboring O adatoms in the small Oad islands.

/pdf/interaction-of-oxygen-with-al-111-studied-by-scanning-2p2cgmvxlo.pdf

Interaction of oxygen with Al(111) studied by scanning tunneling microscopy

Direct observation of a nucleation and growth process on an atomic scale

We show, from scanning tunneling microscopy measurements, that the strain at the interface between Cu films and a Ru(0001) substrate is reduced by a structural transformation from a more tightly bound, strained pseudomorphic first Cu layer to a unidirectionally contracted second Cu layer with periodic partial misfit dislocations. These results for a two-dimensional structure confirm the mechanism of stress accommodation in strained layers predicted in the one-dimensional dislocation model of Frank and ven der Merwe.

G. Pötschke

Papers

Direct observation of a nucleation and growth process on an atomic scale

Interface structure and misfit dislocations in thin Cu films on Ru(0001)

A STM investigation of the nucleation and growth of thin Cu and Au films on Ru(0001)