Simulating the Deposition of Atomic Clusters of Palladium on a Magnesium Oxide Surface

Deposition of atomic clusters onto solid surfaces is a versatile surface-processing tool, with applications ranging from micromachining and surface smoothing to thin-film growth and fabrication of model nanocatalysts. Theoretical investigations, most often employing molecular dynamics (MD) simulations with semiempirical interatomic potentials, provide valuable insights into the microscopic mechanisms of the deposition process. However, when cluster-surface interaction involves surface chemistry (that is, the creation or breaking of chemical bonds), spin-dependent (magnetic) processes, or surface defects of electronic origin (e.g., F center on an ionic surface), a full quantum description of the cluster deposition process is necessary.

	Figure 4   Structural evolution of PdN supported clusters (N = 2 , 3, 4, 6, 7, and 13). Pd atoms are depicted as blue, Mg as green, and O as red spheres, except for Pd13, where a subset of the Pd atoms is colored in yellow in order to highlight the Pd7 subunit (blue).

Moseler et al. have performed the first ab initio molecular dynamics simulation of low-energy deposition of metal clusters on a solid surface (Figure 4). The simulation (based on the density functional formalism) provided insights into the electronic mechanisms that govern the deposition process. It was found that palladium clusters larger then a trimer remain magnetic when deposited in the vicinity of surface F-center defects of MgO(001). This suggests that the soft-landing of magnetic clusters on purposefully patterned surfaces may allow preparation of ordered arrays of magnetic quantum dots. These results are important for understanding the activity of Pd/MgO nanocatalysts as well as for future investigations of supported magnetic nanoclusters.

The simulations show that an F center creates an attractive funnel for the approaching metal cluster, resulting in preferred binding configurations with one Pd atom atop the F center. For adsorbed Pd₂–Pd₆ the gas-phase geometry is retained, while Pd₇ and Pd₁₃ adapt to the underlying MgO structure. Although the surface tends to reduce the spin of the adsorbed cluster, clusters larger than Pd₃ remain magnetic at the surface, exhibiting several low-lying structural and spin isomers. These results provide the impetus for further investigations regarding the interplay of structural and magnetic states of supported metal clusters and their catalytic properties.

INVESTIGATORS
U. Landman, R. N. Barnett, C. L. Cleveland, H. Häkkinen, and W. D. Luedtke, Georgia Institute of Technology; M. Moseler, University of Freiburg, Germany.

PUBLICATION
M. Moseler, H. Häkkinen, and U. Landman, “Supported magnetic nanoclusters: Soft landing of Pd clusters on a MgO surface,” Phys. Rev. Lett. 89, 176103 (2002).