Structure of Ag Clusters Grown on F_s-Defect Sites of an MgO(100) Surface

摘要

The structure of Ag_N clusters (N=1-4,6,8,10),both in the gas phase and grown on the MgO(100) surface containing F_s-defects,has been investigated by a density functional basin-hopping (DF-BH) approach.In analogy with what observed in the case of gold clusters,it is found that the presence of the defect implies a double frustration and a cylindrical invariance of the metal-surface interaction,causing small Ag clusters growing around the F_s defect to be highly fluxional.Nevertheless,two different structural crossovers are found to be induced by the metal-defect interaction for the adsorbed clusters such that;1) planar structures prevail for N<=4 (as in the gas phase);2) noncrystalline (fivefold symmetric) structures,which are the lowest energy ones in the gas phase for medium sized Ag_N clusters (N>=7),prevail for N=6 and N=8;3) distorted face-centered cubic (fcc) structures grown pseudomorphically on the defected surface prevail for N=10.The transition from fivefold to fcc motifs is rationalized in terms of the double-frustration effect,which increases the bond strain of the noncrystalline structures.Detrapping energies from the defect were also calculated;the lowest energy pathway corresponds to the detachment of a dimer.