Invited Paper SS1+NC-WeM5
Additive-Enhanced Mass Transport on Metal Surfaces: Hunting Elusive Agents of Change

Wednesday, October 22, 2008, 9:20 am, Room 208

Sulfur is well known to enhance dynamic rearrangements of single-crystal metal surfaces for the coinage metals (Ag, Cu, and Au). It has been proposed that the associated enhanced surface mass transport of metal is due to the formation and diffusion of stable metal-sulfur clusters or complexes. From scanning tunneling microscopy (STM) images of a Ag(111) surface with adsorbed sulfur below room temperature, we have been able to “see” clusters that probably contribute to enhanced transport. We propose that the imaged clusters are Ag₃S₃.¹ Our ability to see these trimeric clusters derives from the feature that they self-organize into a distinctive dot-row structure. The dot-rows are very robust, in the sense that they exist over a coverage range that spans an order of magnitude (0.03 to 0.5 monolayers). The dots are assigned as Ag₃S₃ clusters, based on DFT calculations of their energetic stability, on the calculated and measured dimensions of the dots in STM, and on experimental evidence that they incorporate Ag. We have also explored the effect of S on Ag surface dynamics by measuring the stability of Ag adatom islands produced by vapor deposition and subsequently exposed to S. By varying both surface temperature and S-coverage, we identify three regimes: At high S-coverage where a dense row-dot structure forms, the Ag islands are stable or “frozen” over long periods of time. At intermediate coverage, S serves to destabilize the Ag islands. At very low coverage (0.01 monolayer), S has no effect because it passively decorates step and island edges. Thus, S is not universally effective in accelerating mass transport, but rather exhibits complex dependencies upon temperature and coverage.

¹ M. Shen, D.-J. Liu, C.J. Jenks, and P.A. Thiel, J. Phys. Chem. C, 112, in press (2008). .