Chemistry Reference
In-Depth Information
successfully realized in experiments. In a first scenario, the bulk oxide support is
transformed into a thin film grown on a metal single crystal [
24
-
26
]. The metal
substrate acts as infinite charge reservoir and on readily exchange electrons with
adatoms bound to the surface of the thin film. A different electron potential in both
subsystems is a requirement for the charge transfer to occur; however, this situation
is often fulfilled as thin oxide films usually alter the work function of the pristine
metal beneath [
27
]. In the second scenario, doping with aliovalent impurity ions
may be exploited to introduce charges into the interior of thick films and even bulk
oxides. Also in this case, charge transfer into the ad-species has been revealed, the
direction of which is given by the nature of the dopant in conjunction with the
electronegativity of the adsorbate. Again, charge exchange is connected with a
considerable increase of the metal-oxide adhesion. To give an estimate for the
efficiency of charge-mediated interaction schemes, DFT calculations have revealed
an increase in binding energy from 0.9 eV for neutral Au on defect-free MgO to
2.3 eV for Au
-
species on thin MgO/Mo(001) films [
28
]. In the following, we will
substantiate these general considerations with a number of experiments performed
in our group. Whereas in the first part, the relevance of charge transfer for binding
of single atoms to oxide surfaces is discussed, consequences on the growth and
electronic properties of metal aggregates are in the focus of the later sections.
2.2 Role of Charge Transfer for Binding Single Adatoms
to Oxide Thin Films
A first hint for the formation of charged adsorbates on thin oxide films came from
low-temperature scanning tunneling microscopy (STM) experiments performed on
3 ML MgO/Ag(001) exposed to small amounts of Au [
15
]. Whereas on bulk oxides,
gold shows a strong tendency for aggregation, mainly isolated atoms are detected
on the thin film even at high gold exposure. Moreover, the ad-species self-assemble
into a hexagonal superlattice on the film surface, with the Au-Au distance
depending exclusively on the coverage and not on the MgO crystal structure
(Fig.
2a
). This unexpected ordering effect provides first evidence for the charged
nature of the Au atoms on MgO films, as equally charged species tend to maximize
their interatomic distance in order to reduce the mutual Coulomb repulsion. A
similar phenomenon was found for alkali atoms on metal and semiconductor
surfaces before and was assigned to a positive charging upon adsorption
[
29
-
31
]. Conversely, the Au atoms on the MgO films charge up negatively, as
their 6s orbital gets filled with electrons from the Ag(001) support below the oxide
spacer layer. The charge exchange is enabled by the high electronegativity of gold
in combination with a small work function of the Ag-MgO system, both effects
promoting an electron release towards ad-gold [
27
,
28
]. The experimentally
deduced charging scenario was corroborated by DFT calculations that computed
a Bader charge of
1|e| at the Au atoms in addition to the expected increase in
binding energy [
32
].
