Step edge diffusion and the structure of nanometer-size Ir islands on the Ir(111) surface

We report an FIM study of the structure of nanometer-size Ir islands on the Ir(111) surface. In this experiment, the number of atoms in an island is carefully controlled by field evaporation and vapor deposition. When this number can be fitted to a hexagonal atomic arrangement, the stable structure is found to be a perfect hexagon. In other cases, an addition of one ledge atom can reverse the symmetry of a small island or change its shape. We also compare diffusion of adatoms on the Ir(311) and (331) surfaces to that of ledge-atoms along the A- and B-type steps of the (111) layer, and the relative binding energies of a ledge atom at these steps.