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Fig. 2 STM image of (a) single Au and (b) single Pd adatoms on a 3 ML thick MgO film grown
on Ag(001) (30
30 nm
2
). While the negatively charged Au atoms self-assemble into a hexag-
onal pattern (see
inset
), the neutral Pd atoms are randomly distributed on the oxide surface and
show a large tendency to aggregate into clusters. A negative charge on the Au species is also
compatible with a distinct sombrero-like shape observed in low-bias STM images.
Black dots
in
both images are attributed to defects in the oxide film [
32
]
However, charge-mediated adsorption on oxide thin films is rather the exception
than the rule and requires a favorable electronic structure of the adsorbate. Coun-
terexamples are Pd atoms that, in contrast to Au atoms, do not possess a low-lying
affinity level that can be filled with extra electrons from the MgO/Ag support. In
fact, the Pd 5s, as the lowest-unoccupied atomic orbital, is located well above the
Fermi level of the thin-film system and therefore not accessible to electron transfer.
As a result, the Pd atoms remain neutral upon adsorption and do not experience any
self-ordering on the MgO surface (Fig.
2b
)[
33
]. Apart from their random distribu-
tion, the Pd atoms exhibit a high tendency to assemble into small aggregates even at
low coverage, mimicking the anticipated binding behavior of metals on bulk oxides
in the absence of charge transfer.
Charge-driven adsorption schemes were observed for many other thin-film
systems, using not only metal adatoms but also molecular species [
34
,
35
]. Partic-
ularly interesting in this context are experiments on a 5
thick alumina film grown
on NiAl(110) [
36
], as the results provide direct insight into the direction of the
charge transfer and the number of exchanged electrons (Fig.
3
)[
37
]. Although
charge transfer into the Au atoms prevails also in this case, the interaction involves
a certain bond reorganization in the oxide and therefore deviates from the more
simple MgO/Ag(001) case [
32
]. The Au atoms bind exclusively to Al
3+
ions in the
alumina surface, and no adsorption to anionic lattice sites is revealed (Fig.
3a
).
Upon bond formation, the Al ion below the gold is lifted above the surface plane.
This upward motion of the Al
3+
leads to a homolytic rupture of the bond to the
oxygen in the layer beneath. The Au atom takes up the electron donated by the Al
ion, while the electron deficiency at the oxygen is balanced by back-bonding it to an
Al atom in the NiAl(110) metal surface. The involvement of the metal substrate
thus reinforces the interaction between adatom and surface, whereby not only
Å
