Chemistry Reference
In-Depth Information
higher density of states as compared to the island interior. The contrast enhance-
ment at the edge is compatible with specific edge states that enable accommodation
of the extra electrons that have been transferred through the oxide film into the gold
sheets. This mechanism has been corroborated by tight-binding DFT calculations,
which allow for an explicit treatment of the atomic structure and the edge config-
uration even of extended metal nanostructures [
57
]. Also, the computed charge-
density plots unambiguously demonstrate the localization of electronic states along
the island perimeter (Fig.
12
). These states are able to store one extra electron per
low-coordinated edge atom and get filled up with transfer electrons although the
island interior remains neutral. This charge localization in the low-coordinated edge
atoms renders the 2D Au islands supported on thin oxide films particularly inter-
esting for adsorption and chemical reactions involving electron-accepting mole-
cules (Lewis acids) [
58
].
Also for flat Au islands on MgO/Ag(001), STM conductance spectroscopy can
be exploited to determine their charge state [
56
]. As discussed before, the devel-
opment of well-defined QWS is the necessary precondition for this analysis,
whereby symmetry and electron filling of the QWS have to be compared to the
findings in neutral aggregates. As electron confinement in a 2D potential is more
sensitive to structural irregularities, Au islands with suitable QWS need to be highly
symmetric and free of defects.
A particularly instructive example shows an ultrasmall Au cluster with ~10
Å
diameter and 0.8-0.9
apparent height grown on a 2 ML MgO/Ag(001) film
(Fig.
13
)[
56
]. In low-bias STM images, mainly the cluster morphology is revealed,
as no eigenstates of the aggregate are available in the probed energy window.
At slightly higher bias, the apparent cluster height doubles and flowerlike pro-
trusions emerge in the image. This bias-dependent contrast change provides evi-
dence that tunneling is now governed by the electronic and not the topographic
properties of the nanostructures. More precisely, a distinct eigenstate, the LUMO of
the Au aggregate, becomes accessible to the tunneling electrons and dominates the
image contrast at positive bias. A similar observation is made at negative bias, when
the Au HOMO moves into the bias window and a comparable “nano-flower”
becomes visible in the STM. The two observed QWS closely resemble the
eigenstates of a free-electron gas confined in a 2D parabolic potential (Fig.
14
).
They are derived from the Au 6s states of the participating atoms and preserve their
characteristic symmetry, as they neither mix with the states of the wide-gap oxide
material nor with the Au 5d and 6p states positioned at much lower or higher
energy, respectively [
43
,
59
]. The symmetry of the states is defined by the angular
momentum quantum number
m
, being a measure of the number of nodes in the
2D electron-density probability [
60
]. The two lowest QWS in Fig.
13
feature four
nodal planes, which corresponds to an
m
of 4 or, equivalently, to a state with
G-symmetry.
Again, the STM reveals not only the symmetry of the eigenstates, but via the
spectroscopic channel also their energy position. For the Au cluster shown in
Fig.
13
, the HOMO and LUMO are clearly identified as dI/dV peaks at
Å
0.4 and
+0.8 V, respectively, separated by a region of zero conductance of 1.0 V width.
