Chemistry Reference
In-Depth Information
Fig. 19 STM images of 0.5 ML Au deposited onto (a) pristine CaO, (b) doped with 4% Mo,
(c) doped with 4% Mo + 2% Li, and (d) doped with 4% Mo + 8 % Li (50
50 nm
2
, 6.0 V). Note
that monolayer Au islands in (b) and (c) appear as depressions at high sample bias. Histograms of
the particle shapes (height-to-diameter ratio) are plotted below. Note the transitions from 2D to 3D
particles when the overvalent doping with Mo in (b) is balanced with increasing amounts of
monovalent Li impurities
more deposits turn 3D, as the ad-particles are unable to accumulate enough excess
charges from the Mo donors (Fig.
19c,d
). The extra electrons are trapped by lattice
defects that arise from the presence of Li in the lattice. Every monovalent Li
+
ion
sitting in Ca
2+
substitutional sites produces a deep hole in the O2p states of an
adjacent oxygen forming an effective trap for the Mo 4d electrons. As a result,
charge transfer to the surface ceases at a critical Li doping level and the Au deposits
adopt the typical 3D geometry of pristine CaO films.
We note that pure hole doping could not be realized in the experiments so far.
Neither MgO nor CaO films, doped exclusively with Li, were found to alter the
morphology and electronic structure of gold, and no sign for the generation of
positively charge ad-species was obtained. This finding is in agreement with the
occurrence of intrinsic compensation mechanisms in the oxides that remove the
energetically unfavorable holes produced by the Li
+
species. Even in well-prepared
oxide films and at perfect vacuum conditions, competing electron sources are
present, such as electron-rich oxygen vacancies (F
o
centers) and donor-type adsor-
bates from the rest gas (water, hydrogen) [
87
,
88
,
90
,
91
]. Hole doping of oxides as
a means to tailor the properties of metal ad-particles is therefore more difficult to
realize than electron doping with donor-type impurities.
Interestingly, the competition for excess electrons occurs not only in Mo and Li
co-doped films but is observed also in the presence of electron-accepting species on
the oxide surface. Gold forms monolayer islands if deposited onto a Mo-doped CaO
at vacuum conditions, but 3D particles if oxygen is present during growth. The
reason is that O
2
molecules bound to the surface act as electron acceptors as well
and may trap charges in their O 2
* antibonding orbitals. These electrons are lost
for the Au islands, resulting in a gradual transition from a 2D to a 3D growth regime
π
