Chemistry Reference
In-Depth Information
Fig. 18 STM images of (a) bare and (b) Mo-doped CaO(001) films of 60 ML thickness after
dosing 0.5 ML of Au (60
50 nm
2
, +6.0 V). The 2D Au islands appear as depressions on the
insulating oxide, because electron transport into the gold is inhibited at the high positive bias
needed for scanning. The
insets
show typical 3D and 2D islands taken at lower bias (3.5 V,
10
10 nm
2
). Similar measurements on (c) bare and (d) Cr-doped MgO(001) films of 20 ML
thickness. The particle shape is not affected by the dopants in this case. (e) Ball models explaining
possible charge-transfer processes between the doped oxide and the ad-metal and their influence
on the particle shape
center (
V
-centers) decreases from more than 8 eV in bare CaO and MgO to
1.0-1.5 eV in the presence of high-valence dopants [
81
]. As a result, internal
compensation effects become favorable, in which every dopant in the lattice pro-
duces a balancing vacancy. While this internal compensation remains insufficient to
cancel the donor potential of Mo ions, it annihilates the effect in Cr-doped MgO.
Consequently, no charge-mediated changes in the Au growth morphology are
observed for Cr-MgO, although the effect is huge for the Mo-doped CaO
(Fig.
18
). One conclusion of these results is that the donor behavior of a transi-
tion-metal-doped oxide cannot be predicted by simple valence arguments but needs
to be checked in every single case.
The influence of overvalent dopants on the equilibrium morphology of metal
particles can, to a certain extent, be annihilated by undervalent dopants in the oxide
lattice (Fig.
19
)[
87
,
88
]. Dopants with lower charge state generate holes in the
oxide electronic structure that are able to trap the extra electrons provided by the
charge donors. This mutual compensation between donors and acceptors has been
explored for CaO films co-doped with Li and Mo ions [
89
]. At low Li concentration
with respect to Mo, most of the Au particles still adopt 2D shapes, indicating a
prevailing charge transfer into the gold. With increasing Li doping level, however,
