Chemistry Reference
In-Depth Information
Fig. 4 (a) Line profiles and (b) apparent heights of an Au monomer and dimer on alumina/NiAl
(110) taken as a function of the bias voltages. The monomer turns into a depression above 3.0 V, as
its negative charge triggers an upward bending of the alumina bands (see
inset
). The dimer appears
bright due to tunneling through its LUMO located at 2.5 eV [
37
]
and does not explain the experimental observations [
44
]. Assuming a screened
Coulomb interaction:
q
V
¼
(1)
4
πε
0
ε
r
r
with an alumina dielectric constant of
ε
r
¼
10 and an Au excess charge of
q
¼
1|e|,
the upward bending calculates to +1.8 eV in distance
r
from the anion
[
43
]. This value matches the bias window of 3.0-4.5 V, in which the Au atom appears
with negative contrast in the STM images. The contrast reversal observed for Au
atoms on alumina thin films therefore provides additional evidence for the charge
transfer that accompanies the interaction of gold with oxide thin films [
28
]. We note in
passing that no contrast reversal is observed for Au dimers, which appear bright over
the entire bias range (Fig.
4b
). The reason is the Au
2
electronic structure, which
exhibits an intrinsic electronic state at 2.5 eV that compensates for the LDOS
reduction due to band bending [
39
].
Charge transfer across oxide thin films does not necessarily lead to the formation
of anionic gold species. In fact, the direction of charge transfer depends on the
position of the Au affinity level (the Au 6s orbital) with respect to the Fermi level of
the metal-oxide system. Because the level position of individual Au adatoms is
largely governed by the vacuum energy, which is, in turn, given by the work
function
¼
1
Å
of the metal-oxide system, it is the latter quantity that determines the
direction of the charge flow [
38
]. In general, low work function systems promote an
electron flow into the gold, while the opposite trend is revealed for systems with
high
ϕ
values. The two oxide films discussed so far, MgO/Ag and Alumina/NiAl,
are characterized by a low work function, because the oxide layer prevents electron
spill out from the metal surface and therewith removes a main reason for the high
ϕ
