Paper SS1-FrM8
Misfit-Dislocation-Mediated Migration of Cu Nanostructures on Ag(111)

Friday, October 19, 2007, 10:20 am, Room 608

Experimental studies of island migration, an important process in crystal growth and nanostructure synthesis, have mostly been limited to homoepitaxial systems. In these systems, either diffusion or evaporation and condensation of adatoms at the island edge gives rise to motion of the entire structure, leading to size-independent barriers, and diffusivities that scale with radius according to D ~ R^-B, with B =1, 2, or 3 for the simplest cases. The present work with Cu-Ag(111), a lattice-mismatched system, provides compelling evidence for a collective mechanism involving glide of misfit dislocations. Here, the entire structure is moved by one Burger's vector b= 1/6 <211> as the dislocation nucleates and glides through the island. Scanning tunneling microscopy (STM) images show that islands with magic sizes have a propensity for defect nucleation and glide leading to enhanced mobility. Quantitative analysis of island trajectories from STM movies collected at 130-200 K yield energy barriers as low as 0.35 eV for this process, even for multilayer islands containing up to ~90 atoms. Significantly, the barriers are very sensitive to island size, and hence island diffusivities do not scale with size in the ways predicted for traditional atomistic mechanisms, greatly affecting the overall coarsening kinetics.