Chemistry Reference
In-Depth Information
Fig. 9 (a) Conductance
spectra of an Au
7
chain and
the bare oxide film. The
insets
display dI/dV maps
taken at the peak positions of
the respective spectrum
(5
5nm
2
). (b) Calculated
LDOS for an Au
7
chain. The
blue line
denotes the seven
Au 6s-like QWS (termed
S1-S7), while the
red line
depicts the 6p state density.
The quantized nature of the
6p states is not resolved due
to the small energy
separation between adjacent
levels. The energy position
is however marked by
arrows (P1-P7). The
calculated orbital shapes for
the fourth to sixth s-like and
the first to third 6p-like QWS
are shown as
insets
[
39
]
ˈ
n
2
) is given by n maxima separated by
n
-1 nodal
planes along the chain axis, while the eigen-energies follow a parabolic dispersion
relation:
distribution in the nth QWS (
2
2
π
n
L
h
E
n
¼
E
0
þ
(2)
2
m
eff
The electronic signature of ultra-small Au aggregates discussed above has been
reproduced experimentally by STM conductance spectroscopy (Fig.
8d-f
)[
39
]. The
Au dimer displays one dI/dV maximum at +2.6 V, while two maxima at +2.3/
+3.0 V are detected for the trimer. These peaks are compatible with the lowest
p
z
-like QWS in the Au aggregates. This assignment is supported by dI/dV maps of
the trimer taken at the respective peak positions. While a homogeneous dI/dV
intensity distribution is revealed for the lower state, reflecting the constant density
probability of the ground-state p
z
orbital, a region of suppressed conductance marks
the nodal plane in the first excited p
z
-
derived state (Fig.
8f
). Note that the LUMO
of the monomer was not detected, most likely because of the overlap of this state
with the alumina conduction band.
Also conductance spectra of longer Au chains are in agreement with the elec-
tronic structure sketched above [
37
,
39
]. In positive-bias spectra, two dI/dV peaks
are observed for Au
4
and Au
5
chains, while three maxima are resolved for the gold
heptamer (Fig.
9
)[
40
]. Based on their energy position, the maxima are assigned to
