Vacancy-Mediated and Exchange Diffusion in a $\mathrm{P}\mathrm{b}/\mathrm{C}\mathrm{u}(111)$ Surface Alloy: Concurrent Diffusion on Two Length Scales

Pb diffuses in a $\mathrm{P}\mathrm{b}/\mathrm{C}\mathrm{u}(111)$ surface alloy predominantly by exchange with surface vacancies and, much less frequently, by exchange with thermal Cu adatoms. Because the infrequent adatom exchanges transport Pb atoms much farther, both processes affect observations of Pb transport in the $\mathrm{P}\mathrm{b}/\mathrm{C}\mathrm{u}(111)$ surface alloy.

Figure 1

Derivative-mode STM images showing the evolution of the embedded Pb concentration profile as a function of time. The upper terrace is on the left. Scale: $550\times 366{\AA }^2$. Histograms show the atom concentration versus distance from step. The light gray line shows the initial concentration profile used in the predictive models.

Figure 2

Width of the concentration profile versus time, from which the diffusivity is extracted.

Figure 3

(a) Two consecutive frames (separated by 14 s) from a $100\times 100{\AA }^2$ STM movie acquired at 400 K. The embedded Pb atoms are seen as white protrusions. (b) Measured diffusion length distribution (solid circles). The solid line is the expected distribution for vacancy-mediated diffusion, and the dotted line is the expected Gaussian distribution from a simple random walk process.

Figure 4

Predicted evolution of the concentration profile using measured vacancy-mediated diffusivity and rate of exchange processes.