Stability of Pd Islands on Ag(111) and Au(111)

Scanning tunneling microscopy was used to compare the evolution with time and annealing temperature of the structure of Pd islands formed after room-temperature (RT) deposition of Pd on Ag(111) and Au(111). Although silver and gold share many properties, their clean (111) surfaces have distinctly different structures, and this leads to distinctly different behavior following Pd deposition. On Ag(111), large Pd islands form and become capped by Ag atoms with the simultaneous formation of vacancy pits on the Ag(111) surface. On Au(111), smaller Pd islands form for comparable Pd coverages. In contrast to Ag(111), much less capping of Pd by Au atoms occurs on the Au(111) surface at RT and no vacancy pits were observed. Upon annealing to 340 K, 98% of the Pd islands were capped by Ag atoms on Ag(111), whereas less than 4% of Pd islands on Au(111) were covered by surface Au atoms, but again without forming vacancy pits. Upon further annealing to 470 K, Pd diffuses into the subsurface of both Ag(111) and Au(111). The contrasting behavior is attributed to the herringbone reconstruction on Au(111), which provides sites with higher binding energies for Pd atoms, whereas such sites are not present on Ag(111).