Adsorbate-enhanced transport of metals on metal surfaces: Oxygen and sulfur on coinage metals

Abstract

Coarsening (i.e., ripening) of single-atom-high, metal homoepitaxial islands provides a useful window on the mechanism and kinetics of mass transport at metal surfaces. This article focuses on this type of coarsening on the surfaces of coinage metals (Cu, Ag, Au), both clean and with an adsorbed chalcogen (O, S) present. For the clean surfaces, three aspects are summarized: (1) the balance between the two major mechanisms-Ostwald ripening (the most commonly anticipated mechanism) and Smoluchowski ripening-and how that balance depends on island size; (2) the nature of the mass transport agents, which are metal adatoms in almost all known cases; and (3) the dependence of the ripening kinetics on surface crystallography. Ripening rates are in the order (110)>(111)>(100), a feature that can be rationalized in terms of the energetics of key processes. This discussion of behavior on the clean surfaces establishes a background for understanding why coarsening can be accelerated by adsorbates. Evidence that O and S accelerate mass transport on Ag, Cu, and Au surfaces is then reviewed. The most detailed information is available for two specific systems, S/Ag (111) and S/Cu(111). Here, metal-chalcogen clusters are clearly responsible for accelerated coarsening. This conclusion rests partly on deductive reasoning, partly on calculations of key energetic quantities for the clusters (compared with quantities for the clean surfaces), and partly on direct experimental observations. In these two systems, it appears that the adsorbate, S, must first decorate-and, in fact, saturate-the edges of metal islands and steps, and then build up at least slightly in coverage on the terraces before acceleration begins. Acceleration can occur at coverages as low as a few thousandths to a few hundredths of a monolayer. Despite the significant recent advances in our understanding of these systems, many open questions remain. Among them is the identification of the agents of mass transport on crystallographically different surfaces e.g., 111, 110, and 100. KeywordsAmes Laboratory, Materials Science and Engineering, Mathematics, Physics and Astronomy Disciplines Biological and Chemical Physics | Materials Science and Engineering | Mathematics | Physical Chemistry CommentsThe following article appeared in Journal of Vacuum Science and Technology A 28, no. 6 (2010) Coarsening ͑i.e., ripening͒ of single-atom-high, metal homoepitaxial islands provides a useful window on the mechanism and kinetics of mass transport at metal surfaces. This article focuses on this type of coarsening on the surfaces of coinage metals ͑Cu, Ag, Au͒, both clean and with an adsorbed chalcogen ͑O, S͒ present. For the clean surfaces, three aspects are summarized: ͑1͒ the balance between the two major mechanisms-Ostwald ripening ͑the most commonly anticipated mechanism͒ and Smoluchowski ripening-and how that balance depends on island size; ͑2͒ the nature of the mass transport agents, which are metal adatoms in almost all known cases; and ͑3͒ the dependence of the ripe...

Anisotropic coarsening: One-dimensional decay of Ag islands on Ag(110)

“…10 Thus, we also performed controlled studies in which we exposed the surface to O 2 (gas) to assess this possibility. However, we found no significant effect of oxygen on coarsening in the Ag/Ag(110) system, as discussed later.…”

Transition metals on the (0 0 0 1) surface of graphite: Fundamental aspects of adsorption, diffusion, and morphology

“…These observations are probably explained by the adsorption of hydrocarbons, which causes the surface properties of graphene and graphite to change during exposure to air (over a few tens of minutes) [28]. Presumably, the hydrocarbons are too mobile to be imaged effectively with scanning probe techniques, at least at the typical observation temperature of 300 K, so the surface may appear deceptively clean when analyzed with such techniques.Independent of the environment's effect on the graphite substrate, environment may affect the chemical state of the metal or the distribution of metal on the surface during or after deposition, especially via oxidation or via enhancement of restructuring rates [29]. Recently, for instance, it has been reported that exposure to CO(g) accelerates coarsening of Pd nanoclusters on a graphene surface [30].…”

“…Independent of the environment's effect on the graphite substrate, environment may affect the chemical state of the metal or the distribution of metal on the surface during or after deposition, especially via oxidation or via enhancement of restructuring rates [29]. Recently, for instance, it has been reported that exposure to CO(g) accelerates coarsening of Pd nanoclusters on a graphene surface [30].…”

Communication: Structure, formation, and equilibration of ensembles of Ag-S complexes on an Ag surface

“…[8][9][10][11][12][13] More specifically, these complexes can destabilize metal nanostructures and can accelerate coarsening or sintering of arrays of nanoclusters via a reactive version of Ostwald ripening. 11,14 The modern tools of surface science-particularly scanning tunneling microscopy (STM), diffraction techniques, and density functional theory (DFT)-have proven powerful enough to decipher even intricate extended structures, but direct experimental observations of isolated complexes between adsorbates and metal atoms extracted from the surface have been limited, due in part to difficulties in imaging mobile species. Also missing from the above observations is a characterization of the formation mechanism for complexes, the possibility that there exists a variety of distinct complexes interacting with each other, and a potential connection between these complexes and extended reconstructions.…”