Chemistry Reference
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s-electrons are intrinsic to the Au atoms in this case, while the other three originate
from the substrate again. For the Au
7
, being the longest chain observed experimentally,
the fourth and fifth QWS are detected at
1.3 V, respectively. In agreement
with the particle-in-the-box model, the lower state displays four density maxima and
the higher five, along the chain axis in the dI/dV maps (Fig.
10
). The Au
7
chain has
consequently five occupied QWS carrying ten s-electrons in total, which brings the
number of transfer electrons from the NiAl to three again. It should be mentioned that
no six-atom chains have been identified in the experiment, which might be explained
with an unfavorable magnetic ground state of the linear Au
6
arising from an unpaired
electron at
E
F
.
Both the experimental and theoretical behaviors of s-like QWS in different Au
chains are in good agreement with the particle-in-the-box model sketched above.
Fitting the computed level energies to a parabolic dispersion relation yields a
potential depth of
E
0
¼
1.8 and
0.85 m
e
.
It is interesting to note that Au chains assembled directly on the metallic NiAl(110)
support have a smaller electron mass of 0.5 m
e
[
51
,
52
]. This finding indicates
higher electron mobility in the metal-supported chains, despite a somewhat larger
Au-Au distance in that case (2.89
2.65 eV and an effective electron mass
m
eff
¼
on alumina). The
difference reflects the role of indirect coupling between the Au atoms, mediated
by electronic states in the NiAl substrate. Naturally, this contribution is missing on
the insulating alumina film [
53
].
on NiAl versus 2.6
Å
Å
2.4 Development of Two-Dimensional Metal Islands
on Oxide Thin Films
The charge transfer through a thin oxide spacer prevails also for larger Au aggre-
gates and keeps controlling their geometry and electronic structure. On 2 ML
MgO/Ag(001) films, for example, gold first forms flat, single-layer islands and
develops a nearly complete wetting layer with increasing exposure (Fig.
11
)
[
15
]. The formation of 2D islands is in sharp contrast to the 3D growth that is
typically observed on bulk oxides [
16
,
26
,
54
]. It reflects the tendency of gold to
increase the contact area with the oxide film, as this maximizes the charge transfer
into the gold affinity levels and therewith the strength of the metal-oxide adhesion.
Similar to Au monomers and chains, the reinforcement of the Au bonding results
from increased electrostatic and polaronic interactions upon charge transfer
[
55
]. As a rough number, the average charge transfer per adatom has been calcu-
lated to be
0.2|e| for a close-packed Au layer on 2 ML MgO/Ag(001) [
56
].
Especially in larger 2D gold islands, the excess electrons are not homogenously
distributed but show a preference to accumulate at the island perimeter [
57
]. The
reason for this particular charge distribution is similar to the one that leads to the
development of 1D chains at small atom numbers. The excess electrons repel each
other, which increases the internal Coulomb repulsion in the island and hence the
total energy. To minimize this Coulomb term, the extra charges maximize their
