Enhanced Self-Diffusion on Cu(111) by Trace Amounts of S: Chemical-Reaction-Limited Kinetics

We find that less than 0.01 monolayer of S can enhance surface self-diffusion on Cu(111) by several orders of magnitude. The measured dependence of two-dimensional island decay rates on S coverage (θS) is consistent with the proposal that Cu₃S₃ clusters are responsible for the enhancement. Unexpectedly, the decay and ripening are diffusion limited with very low and very high θS but not for intermediate θS. To explain this result we propose that surface mass transport in the intermediate region is limited by the rate of reaction to form Cu₃S₃ clusters on the terraces.