Equilibrium structures and shapes of clusters on metal fcc(111) surfaces

Using embedded-atom-method potentials, the lower-energy structures (LESs) of adatom clusters are obtained directly on a series of metal fcc(111) surfaces by the method based on the genetic algorithm. The structural features, energy distributions, number of LESs and their differences on different surfaces are discussed and explained in terms of the nearest-neighbor and next-nearest-neighbor (NN, NNN) adatom-adatom interactions, and the edge-type difference. When the energetic preference for one edge type over another is slight, e.g., on Ag(111), only one type of structure is included, and it does not change with the increment of cluster size. However, when there is a strong energetic preference for one of the edge types, e.g., on Pt(111), an interesting phenomenon of structure replacement is revealed, by which the structures in the LES group deviate more and more from the configuration with the maximum number of NN bonds as the cluster size increases. The structure replacement also finally leads to the shape of the two-dimensional island on Pt(111) being quite distinct from that on Ag(111). Based on these results, the general trend of the variation of lower-energy structures with cluster size is discussed further for other metal fcc(111) surfaces.