EQUILIBRIUM SHAPES AND ENERGETICS OF IRIDIUM CLUSTERS ON IR(111)

In an effort to characterize the energetics of surface clusters, the s hape and stability of iridium clusters containing from 18 to 39 atoms have been examined on the closepacked Ir(lll) plane using a low-temper ature field ion microscope. The hexagonal clusters Ir-19 and Ir-37 are found to be far more stable than less compact ones. At equilibrium, t he 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.