Chemistry Reference
In-Depth Information
Fig. 23 STM topographic images of (a) a bare (see also Fig.
12
) and (b) a CO-saturated Au island
on 2 ML MgO/Ag(001) (7.0
5.5 nm
2
,
V ¼
150 mV). Whereas the bare island is surrounded by a
bright rim, indicating charge accumulation at the perimeter, the CO-covered island exhibits
charge-density waves in the interior. The standing electron waves are due to electron displacement
from the island boundary upon CO adsorption [
66
]
The preferential CO binding to the island perimeter might be ascribed to two
effects. On the one hand, the boundary atoms have a lower coordination than the
ones enclosed in the Au plane. The same mechanism controls CO adsorption on
stepped metal surfaces [
98
,
99
]. On the other hand, electrons transferred from the
MgO/Ag support are localized at the island perimeter, as discussed above, and may
alter the CO binding characteristic. According to DFT calculations, the coordina-
tion effect clearly dominates the CO binding position. Whereas CO adsorbs with
0.72 eV to 3-fold coordinated edge atoms of a negatively charged Au island on
MgO/Ag(001), it is nearly unbound to the 5-fold coordinated atoms in the interior
of island. A similar binding enhancement is revealed for edge versus inner atoms of
a neutral Au
12
island on bulk MgO [
100
]. The extra electrons localized on the
perimeter of charged islands actually counteract this trend, as they impede charge
donation from the CO 5
orbital into the metal and hence increase the CO-Au
repulsion [
101
]. Clearly, CO attaches to the island perimeter not because but in
spite of the extra charge, and the binding preference with respect to the top facet is
governed by the reduced coordination of the edge atoms.
This conclusion is supported by the experimental observation that CO adsorption
modifies the charge distribution within the Au islands. The specific contrast at the
rim of the islands (Fig.
23a
) disappears after CO dosage, when standing electron
waves emerge in the island center (Fig.
23b
)[
102
]. Apparently, the CO removes the
localized charges from the perimeter sites by pushing them into the interior of the
island. Also this finding is in line with DFT results for charged Au
12
clusters on
MgO/Ag(001) [
100
].
While on ultrathin films we resorted to IETS to probe the vibrations on adsorbed
CO, on thick insulating films, the CO binding properties have been studied with
IRAS, whereas STM still provides information on the nucleation density and size of
the Au aggregates. Figure
24a
displays an STM topographic image of a 14 ML thick
MgO/Mo film, being exposed to electrons to produce F
0
and F
+
centers. Those point
defects that mainly form along step edges and misfit induced dislocation lines are
˃
