Importance of Kinetics in Surface Alloying: A Comparison of the Diffusion Pathways of Pd and Ag Atoms on Cu(111)

The microscopic details of how metals alloy have important consequences for both their material properties and their chemical reactivity. In this study, the initial stages of alloying of Pd and Ag with Cu(111) are compared. Low-temperature scanning tunneling microscopy reveals that physical vapor deposition of Pd and Ag at or above room temperature yields remarkably different Surface alloys: Pd predominantly incorporates at the nearest ascending Cu step edge, whereas Ag appears to be able to traverse step edges rather easily and alloys into terraces both above and below its initial adsorption site. Density functional theory calculations reveal that even though Pd adatoms have a lower barrier than Ag for traversing step edges, unlike Ag they bind very strongly to ascending step edges and remain there permanently. This leads to a situation in which Pd atoms have at most a very small number of attempts to leave the terrace on which they are deposited before they are incorporated into the nearest ascending step edge. Ag adatoms, however, have many opportunities to cross step edges and can alloy at positions far from their initial starting point. This direct comparison demonstrates the importance in combining theory and experiment in order to understand complicated surface alloying mechanisms and illustrates how both the kinetics and the thermodynamics of the process must be considered to fully understand experimental observations.