Magic silver cluster on a MgO(100) terrace with defects

The structure and energetics of ${\mathrm{Ag}}_N$ clusters $(N=2–10)$ adsorbed on a double vacancy (DV) neutral defect of a MgO(100) terrace are investigated via a density-functional basin-hopping approach. It is found that ${\mathrm{Ag}}_8$ is a surface magic cluster from both electronic and structural points of view, i.e., it exhibits enhanced stability with respect to neighboring sizes and a large highest occupied molecular orbital–lowest unoccupied molecular orbital gap. The possibility of synthesizing monodisperse ${\mathrm{Ag}}_8∕\mathrm{DV}∕\mathrm{Mg}\mathrm{O}\left(100\right)$ clusters is discussed. The first ionization potential, the electron affinity, and the interaction energies with atomic and molecular oxygen are evaluated for the adsorbed Ag clusters, showing that ${\mathrm{Ag}}_8$ exhibits appreciable differences in its electronic and chemical properties with respect to neighboring sizes.

Figure 1

(Color online) Schematic pictures of the putative global minima of ${\mathrm{Ag}}_N$ clusters adsorbed on the DV-defected MgO(100) surface. Silver atoms appear as gray (light) spheres, magnesium atoms as orange (small, dark) spheres, and oxygen atoms as light blue (big, dark) spheres. The silver atom occupying the empty Mg site is labeled with $A$, while the one above the empty O site is labeled with $B$.

Figure 2

Incremental formation energy of gas-phase and adsorbed ${\mathrm{Ag}}_N$ clusters, ionization potential, and electron affinity of adsorbed ${\mathrm{Ag}}_N$ clusters. All quantities in eV.

Figure 3

(Color online) Schematic pictures of the lowest-lying isomers of ${\mathrm{Ag}}_N$ clusters adsorbed on the DV-defected MgO(100) surface with the corresponding energy differences with respect to global minima. Silver atoms appear as gray (light) spheres, magnesium atoms as orange (small, dark) spheres, and oxygen atoms as light blue (big, dark) spheres. The silver atom occupying the empty Mg site is labeled with $A$, while the one above the empty O site is labeled with $B$.

Figure 4

(Color online) Interaction geometries of O and ${\mathrm{O}}_2$ on ${\mathrm{Ag}}_N$ clusters. For ${\mathrm{O}}_2$, two interaction modes are shown, mode (I) and mode (II), in which the ${\mathrm{O}}_2$ molecule does or does not interact with the MgO surface, respectively. Silver atoms appear as gray (light) spheres, magnesium atoms as orange (small, dark) spheres, oxygen atoms of MgO as light blue (big, dark) spheres, and adsorbed oxygen atoms as dark blue (darkest) spheres. The silver atom occupying the empty Mg site is labeled with $A$, while the one above the empty O site is labeled with $B$. In interaction mode (II), a dotted line highlights the interaction of oxygens from the ${\mathrm{O}}_2$ molecule with magnesium atoms at the surface.