Atomic view of cluster diffusion on metal surfaces

Field ion microscope measurements show a strong correlation between the mobility and shape of small clusters on fcc(100) metal surfaces. For self-diffusion on Rh(100) this correlation leads to an oscillatory behavior in the activation energy of surface diffusion as a function of cluster size. Comparison of the measured activation energies to previous theoretical calculations indicate that the mechanism of cluster diffusion involves individual displacements of edge atoms (i.e., perimeter diffusion). The rate-determining step in the migration of clusters is the partial detachment of one of the perimeter atoms. The relative ease of adatom motion along straight edges of stationary clusters also permits measurements of diffusion barriers at steps — measurements useful in the interpretation of fractal vs. compact island growth on fcc metal surfaces.