Chemistry Reference
In-Depth Information
Fig. 1 Different binding mechanisms of metal adatoms on thin films and bulk oxides
oxide surface. Especially the overlap between the d-states of transition metal atoms
and the 2p orbitals of the surface oxygen plays an important role and enables an
increase in the metal-oxide bond strength to more than 1.0 eV [
19
-
21
]. Naturally,
this channel is dominant for metals with partly filled d-shells (Cr, Mn, Fe) and loses
influence for semi-noble and noble metals, e.g., Pt, Cu, Ag, and Au.
Oxide materials are never perfect and therefore surface defects need to be
considered as potential binding sites for metal atoms [
22
]. Oxide defects often
given rise to considerable variations in the electrostatic potential, which originate
from unbalanced charges and cannot be screened due to the low density of free
carries in the insulating material. In covalently bound oxides, dangling bond states
may emerge at the defect site, reflecting the rigid lattice structure of the system that
does not support bond reorganization. Whereas dangling bond states are highly
susceptible to form covalent bonds to metal adatoms, electrostatic forces and
charge transfer processes become relevant in the presence of charged defects in
ionic oxides. Oxygen vacancies in MgO, for example, are able to exchange elec-
trons with metallic adsorbates, which enable strong Coulomb attraction between
both partners. A model case for this scenario is the interaction between a doubly
occupied O-defect (F
0
center) and an Au atom, in which the gold turns anionic and
binds with more than 3.0 eV to the surface [
23
]. In the opposite scenario, electron
flow into an electron trap in the oxide lattice (e.g., an F
2+
center) is observed for
electropositive ad-species and governs for instance the adsorption of alkali metals
to different surfaces. Defect-mediated interaction schemes exceed the binding
potential of the regular surface by up to a factor of three, underlining the signifi-
cance of such lattice irregularities for the nucleation and growth of metals on oxide
materials.
A particularly interesting approach to modify the metal-oxide adhesion without
generating surface defects is the insertion of charge sources directly into the oxide
lattice (Fig.
1
). Two approaches have been proposed in the literature and were
