Equilibrium shapes and energetics of iridium clusters on Ir(111)

In an effort to characterize the energetics of surface clusters, the shape and stability of iridium clusters containing from 18 to 39 atoms have been examined on the closepacked Ir(lll) plane using a low-temperature field ion microscope. The hexagonal clusters Ir-19 and Ir-37 are found to be far more stable than less compact ones. At equilibrium, the latter occur in many different configurations conserving the total number of nearest neighbors. In these clusters, edges made up of {111} microfacets (type B) are strongly favored over type A edges, composed of {100} facets; the free energy difference amounts to similar to 30 meV per atom. For nonhexagonal clusters, those with kinked edges occur most frequently. (C) 1997 Elsevier Science B.V.