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 (${\theta }_{\mathrm{S}}$) is consistent with the proposal that ${\mathrm{C}\mathrm{u}}_3{\mathrm{S}}_3$ clusters are responsible for the enhancement. Unexpectedly, the decay and ripening are diffusion limited with very low and very high ${\theta }_{\mathrm{S}}$ but not for intermediate ${\theta }_{\mathrm{S}}$. To explain this result we propose that surface mass transport in the intermediate region is limited by the rate of reaction to form ${\mathrm{C}\mathrm{u}}_3{\mathrm{S}}_3$ clusters on the terraces.

Figure 1

Trace S drastically speeds up self-diffusion on Cu(111). (a) LEEM images showing the decay of a Cu island (dark innermost circle) at 215 °C after exposure to 14 mML S. The cartoon illustrates the configuration of the decaying Cu island (light gray), which is monolayer high sitting on top of a thick Cu stack (dark gray). (b) STM results showing similar speedup in the decay of a Cu island at $-60°\mathrm{C}$ after exposure to $\sim 5\mathrm{m}\mathrm{M}\mathrm{L}$ S. Both LEEM and STM observe a change in the Cu island shape from hexagonal to triangular during the decay (more prominent in the STM images because of the lower temperature) due to the preferential decoration of S at the $\{100\}$ microfacet step edges [9].

Figure 2

S-mediated decay rate as a function of S coverage ${\theta }_{\mathrm{S}}$ at 215 °C. Averaged decay rates $r_{\mathrm{S}}$ of Cu islands with area $0.2\mu {\mathrm{m}}^2$ for different ${\theta }_{\mathrm{S}}$ are compared to the clean Cu island decay rate $r_0=1/94\mathrm{m}\mathrm{i}\mathrm{n}$.

Figure 3

The three regimes of island area evolution as a function of time $t$ during Cu island decay at 215 °C. Surface is exposed to S at $t=0\mathrm{m}\mathrm{i}\mathrm{n}$. (a) Clean Cu limit. (b) Intermediate ${\theta }_{\mathrm{S}}$ with linear $t$ dependence. (c) High ${\theta }_{\mathrm{S}}$.

Figure 4

Three coarsening regimes observed by LEEM. (a) Clean Cu at 270 °C. Ostwald ripening is observed with one big Cu island left after 19 min. (b) No Ostwald ripening with 11.8 mML S at 160 °C. Mass from islands is added to the step, which advances to the right on the lower terrace (arrow). (c) With 12.3 mML S at 215 °C, Ostwald ripening occurs again but at accelerated rate.