Realization of an atomic sieve: Silica on Mo(112)

Abstract

The adsorption of Pd, Ag and Au atoms on a porous silica film on Mo(112) is investigated by scanning tunneling microscopy and density functional theory. While Pd atoms are able to penetrate the holes in the silica top-layer with virtually no barrier, Ag atoms experience an intermediate barrier value and Au atoms are completely unable to pass the oxide surface. The penetration probability does not correlate with the effective atom size, but depends on their electronic structure. Whereas Pd with an unoccupied valence s-orbital has a low penetration barrier, Ag and Au atoms with occupied s-states experience a substantial repulsion with the filled oxide states, leading to a higher barrier for penetration. In the case of Ag, the barrier height can be temporally lowered by promoting the Ag 5s-electron into the support. The Mo-supported silica film can thus be considered as a primitive form of an atomic sieve whose selectivity is controlled by the electronic structure of the adatoms.

Lithium incorporation into a silica thin film: Scanning tunneling microscopy and density functional theory

“…3͒. This finding is not surprising, as the eightmembered rings along the boundaries are generally associated with reduced penetration barriers compared to the regular hexagons 22,23 and should be easily filled with Li. For a nominal Li coverage exceeding one atom per pore, nucleation of Li adclusters is observed on the silica surface ͓Fig.…”

“…A similar behavior was earlier observed for Pd and Ag atoms and explained by the fact that the atoms actually occupy binding sites below the surface at the metal-oxide interface. 22,23 The distinct contrast is hereby triggered by a hybridization of the valence s state of the adatom and the O 2p orbitals of the surface oxygen atoms, which locally increases the unoccupied state density of the silica ring above the adsorbate. Similar adsorption patterns in the present experiment suggest that also Li penetrates the topmost oxide layer and binds to interfacial sites.…”

Properties of oxide thin films and their adsorption behavior studied by scanning tunneling microscopy and conductance spectroscopy

“…A similar adsorption behavior is revealed upon silver deposition onto the silica surface at cryogenic temperatures (not shown). 423 Also there, distinct hexagonal stars emerge in the topographic images at low positive bias, which develop into circular maxima above +1.25 V.…”

“…The binding behavior of silver is comparable, because also the Ag atoms are able to penetrate the silica top-layer and attach to the metal-oxide interface with 1.6 eV. 423 The penetration barrier is with 0.3 eV larger than for Pd, for which a value below 0.1 eV has been computed. The star-like contrast of the inserted Ag atoms at low bias is caused again by hybridization between the Ag 5s and the O 2p orbitals.…”

“…425 The barrier height is in fact determined by an electronic repulsion, which the 2p states of the surface oxygen ions exert on the filled orbitals of the incoming adatom. 423 Na, whereas the highly electronegative Pt will be unable to pass the oxide surface-holes in analogy to the Au species. .…”