Chemistry Reference
In-Depth Information
work function of non-alkali metals. A counterexample is the FeO thin film that can
be grown on Pt(111). Already the bare Pt(111) surface features an exceptionally
high work function and this situation does not change upon FeO deposition.
The reason is that the FeO film is of polar nature and features an intrinsic surface
dipole with the negative side pointing towards the surface (oxygen termination)
[
45
]. In this particular case, the Au atoms lose electrons to the film and become
positively charged upon adsorption [
46
]. Although the direction of the charge flow
is opposite to the one on magnesia and alumina, the resulting binding principles are
similar and arise from a combination of electrostatic interactions and polaronic
lattice distortion of the ionic oxide in presence of the charged ad-species.
2.3 Charge-Mediated Growth of Metallic Chains on Oxide
Thin Films
The charge transfer that governs the binding of Au monomers to oxide thin films
affects also the aggregation behavior of the gold at higher exposure. At cryogenic
temperature, aggregation of equally charged adatoms is inhibited due to the
Coulomb repulsion, giving rise to the formation of ordered adatom patterns as
shown in Fig.
2a
. The repulsion is overcome, however, when dosing the gold at
elevated temperature, e.g., 100 K. In this case, the Au atoms assemble into 1D atom
chains at low coverage, as observed on both magnesia [
47
,
48
] and alumina films
[
40
]. The smallest aggregate is a flat-lying dimer with 9
Å
apparent lengths, while
longer chains contain between three and seven atoms and are 12-22
long (Fig.
5
).
The development of 1D chains on thin films is in contrast to the common behavior
on bulk oxides, where 3D clusters are energetically favorable at any coverage due to
the weak metal-oxide adhesion [
26
,
49
].
The development of Au atom chains seems unexpected at first glance as the
number of stabilizing Au-Au bonds is small with respect to 2D and 3D aggregates.
However, the linear atom arrangement is in agreement with the charged nature of
the aggregates. Similar to the monomer case, electron transfer through the oxide
film into the Au cluster is active and increases the electrostatic coupling to the oxide
lattice. Conversely, the charge transfer leads to a Coulomb repulsion in the aggre-
gate that may be minimized when the extra electrons are separated over large
distances. Minimization of Coulomb repulsion in electron-rich aggregates is there-
fore the fundamental reason for the development of 1D cluster shapes, and no 1D
chains are to be expected in the absence of charge transfer through the oxide films
[
47
,
48
].
Also for Au aggregates bound to MgO/Ag(001) [
47
] and MgO/Mo(001) films
[
50
], the concept of charge transfer has been verified by DFT calculations. Two
configurations have been identified for the Au dimer, a neutral and upright one that
binds to an O
2
ion in the MgO surface and a flat-lying, negatively charged species
(Bader charge
Å
0.8|e|) that bridges two Mg
2+
or two hollow sites. The flat-lying
