Alloying, Dealloying, and Reentrant Alloying in (Sub)monolayer Growth of Ag on Pt(111)

An
in situ nanoscopic investigation of the prototypical surface
alloying system Ag/Pt(111) is reported. The morphology and the structure
of the ultrathin Ag–Pt film is studied using low energy electron
microscopy during growth at about 800 K. A very rich dynamic behavior
is uncovered in which stress relief plays a governing role. Initial
growth leads to surface alloying with prolonged and retarded nucleation
of adislands. Beyond 50% coverage dealloying proceeds, joined by partial
segregation of Pt toward the center of large islands in violent processes.
Subsequently, the growing islands coalesce and consequently irregularly
shaped vacancy clusters evolve. These clusters are progressively filled
by segregating Pt and assume a compact shape (black spots). As a result,
at around 85% coverage, the strain of the initially pseudomorphological
film is almost completely relieved and Pt segregation is at its maximum.
Further deposition of Ag leads to transient reentrant alloying and
recovery of the pseudomorphological layer. The black spots persist
even in/on several-layer-thick films. Ex situ atomic force microscopy
data confirm that these are constituted by Pt­(-rich) structures. The
(sub)­monolayer films are strongly heterogeneous.