Accelerated coarsening of Ag adatom islands on Ag(111) due to trace amounts of S: Mass-transport mediated by Ag–S complexes

Scanning tunneling microscopy studies reveal that trace amounts of adsorbed S below a critical coverage on the order of $10mML$ have little effect on the coarsening and decay of monolayer Ag adatom islands on Ag(111) at $300K$⁠. In contrast, above this critical coverage, decay is greatly accelerated. This critical value appears to be determined by whether all S can be accommodated at step edges. Accelerated coarsening derives from the feature that the excess S (above that incorporated at steps) produces significant populations on the terraces of metal-sulfur complexes, which are stabilized by strong Ag–S bonding. These include $AgS2$⁠, $Ag2S2$⁠, $Ag2S3$⁠, and $Ag3S3$⁠. Such complexes are sufficiently populous and mobile that they can potentially lead to greatly enhanced metal mass transport across the surface. This picture is supported by density functional theory analysis of the relevant energetics, as well as by reaction-diffusion equation modeling to assess the mechanism and degree of enhanced coarsening.