Ripening of monolayer vacancy pits on metal surfaces: Pathways, energetics, and size-scaling for Ag(111) versus Ag(100)

Abstract

Scanning tunneling microscopy studies have revealed that monolayer-deep vacancy pits typically coarsen at 300 K via Smoluchowski ripening (SR) on Ag(111) surfaces and via Ostwald ripening (OR) on Ag(100) surfaces. We elucidate the underlying atomistic processes, the relevant energetics with some input from density functional theory analysis, and also the scaling of the ripening rate with mean pit size. Size scaling for SR reflects the size dependence of the pit diffusion coefficient, so we also discuss observed deviations from classical theories. SR dominates OR for pits on Ag(111) primarily due to its significantly lower effective energy barrier. However, the effective barrier for OR is not lower than that for SR for pits on Ag(100), and one must also account for the distinct size scaling of these pathways to explain the dominance of OR. We also briefly discuss the dependence on temperature of the dominant ripening pathway and the ripening behavior for adatom islands. Scanning tunneling microscopy studies have revealed that monolayer-deep vacancy pits typically coarsen at 300 K via Smoluchowski ripening ͑SR͒ on Ag͑111͒ surfaces and via Ostwald ripening ͑OR͒ on Ag͑100͒ surfaces. We elucidate the underlying atomistic processes, the relevant energetics with some input from density functional theory analysis, and also the scaling of the ripening rate with mean pit size. Size scaling for SR reflects the size dependence of the pit diffusion coefficient, so we also discuss observed deviations from classical theories. SR dominates OR for pits on Ag͑111͒ primarily due to its significantly lower effective energy barrier. However, the effective barrier for OR is not lower than that for SR for pits on Ag͑100͒, and one must also account for the distinct size scaling of these pathways to explain the dominance of OR. We also briefly discuss the dependence on temperature of the dominant ripening pathway and the ripening behavior for adatom islands.

The effect of chalcogens (O, S) on coarsening of nanoislands on metal surfaces

“…3, i.e. at lower temperature, we have determined that the mechanism is Ostwald ripening (as it would be also on the clean Ag(1 1 1) surface [9,[46][47][48]). This was done by monitoring sequential STM images, which provided unequivocal evidence that islands shrink or grow, rather than diffusing significantly.…”

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

“…This approach can be applied either for OR, i.e., simultaneous dissolution of smaller islands and growth of larger islands mediated by mass transport across terraces between them, or for Smoluchowski ripening ͑SR͒, i.e., diffusion and coalescence of islands. [3][4][5][6]43 However, such an analysis of the coarsening kinetics is sensitive to the presence of narrow terraces. Thus, here we utilize a more robust alternative for OR.…”

“…͑iii͒ Metal adatom interactions are described reasonably by effective NN M-M attractions of strength M Ϸ 0.2 eV. 43,46 These interactions imply an equilibrium state for Ag adlayers on Ag͑111͒ corresponding to 2D condensed island phase coexisting with a dilute 2D gas phase on clean and S-exposed Ag͑111͒ surfaces. They also drive island formation during deposition of Ag.…”

Dynamics of the Early Stages in Metal-on-Insulator Thin Film Deposition

“…Depending on growth conditions and properties of the film material, island mobility may cause coalescence to occur earlier as well, [2,[80][81][82][83] a process which has been termed "Smoluchowski ripening" to separate it from conventional growth, or static coalescence. For very small islands (up to a few atoms in size), coalescence likely resembles more a nucleation event, occurring in essence instantaneously be rearrangement of periphery atoms.…”