Chemistry Reference
In-Depth Information
Fig. 16 STM images of 0.7 ML Au dosed onto (a) pristine and (b) doped CaO films (4.5 V,
50
50 nm
2
). The
insets
display close-ups of two characteristic particles (
5.0 V, 10
10 nm
2
)
material (Fig.
16
). Evidently, the donor character of the Mo dopants is responsible
for the 2D growth morphology, as the bare CaO(001) surface interacts with gold
only weakly. The Mo impurity ions mainly occupy Ca substitutional sites and, in
the absence of gold, adopt the typical 2+ charge state of the rock salt lattice in order
to maintain charge neutrality. In the 2+ configuration, four Mo 4d electrons are
localized in the dopant, three of them occupying (t
2g
-
ʱ
) crystal field states and one
ʲ
sitting in a (t
2g
-
) level close to the upper end of the CaO band gap (Fig.
17
)
[
80
]. Especially the latter one is in an energetically unfavorable position and
therefore susceptible to be transferred into an acceptor state with lower energy.
Such states are indeed provided by the Au atoms that exhibit half-filled Au 6s levels
at lower energy. DFT calculations therefore reveal a spontaneous transfer of the
topmost Mo 4d-electron into the Au 6s affinity level, resulting in the formation of
an Au
anion (Fig.
17
). As discussed for the thin films, the charged gold experi-
ences reinforced bonding to the CaO surface, reflected by the increase of the
binding energy from ~1.5 eV without to ~3.5 eV with Mo dopants in the film.
We emphasize that the charge transfer does not require the presence of a Mo ion in
the surface and remains active over relatively large Mo-Au distance of up to ten
atomic planes.
The increase of the metal-oxide adhesion due to dopant-induced charge transfer
fully explains the 2D growth regime of gold observed in the experiment. Gold tends
to wet the CaO surface in an attempt to maximize the number of exchanged
electrons, hence the interfacial interaction. Further DFT calculations suggested
that also a Mo
3+
species that has already lost one electron remains a potential
donor, as two of the residual
d
-electrons are still higher in energy than the Au 6s
affinity level [
81
]. Consequently, even a second and a third electron may be
transferred into the ad-metal, leaving behind thermodynamically stable Mo
4+
and
Mo
5+
ions in the CaO lattice. It is this behavior of the Mo ions that is responsible for
the robust donor behavior of Mo-doped CaO [
80
].
