Scanning Tunneling Microscopy Studies of Cluster Diffusion in a Highly Mismatched System: Copper on Silver(111)

I have used low-temperature scanning tunneling microscopy to investigate cluster diffusion phenomena in both the early stages of nucleation and growth and the late stages for a highly mismatched system, Cu on Ag(111). In particular, activation energies and prefactors for cluster diffusion are measured directly by tracking cluster motion with atomic precision at multiple temperatures to determine the temperature dependence of hop rates. In contrast with larger homoepitaxial islands on Ag(111) and Cu(111), small-to-medium sized clusters (3-30 atoms) of Cu on Ag(111) display a non-monotonic size dependence of the diffusion barrier, with surprisingly low diffusion barriers for clusters containing up to ~26 atoms. Molecular dynamics simulations reveal a novel dislocation-mediated island diffusion mechanism and predict barriers that are in very good agreement with experiment. In this mechanism, the barrier to nucleate a dislocation, and hence diffuse, is sensitive to island size and shape. Experimental studies of the early stages of nucleation and growth of Cu on Ag(111) reveal that trimers, once formed, have significantly higher mobilities than either atoms or dimers. While transient, this mobility makes trimers the dominant contributor to mass transport at temperatures allowing trimer formation (T ??? 24 K). Using the STM tip, we constructed linear and compact trimers at 5 K and investigated their stabilities and diffusion parameters as a function of temperature. Analysis shows that the diffusion barrier for linear trimers is very low, 13.6 meV, compared to 65 meV for atoms, while the compact trimer is stable and immobile. The details of trimer diffusion and rotation provide insights into the intermediate diffusion steps and indicate that the large lattice mismatch plays an important role. The properties of Cu trimers on Ag(111) contrast with those reported for homoepitaxial trimers on fcc(111) surfaces. Because the diffusion phenomena for Cu trimers and other clusters on Ag(111) are largely a result of lattice mismatch, similar phenomena may exist in the early stages of nucleation and growth of other heteroepitaxial systems