Model Catalysts Based on Au Clusters and Nanoparticles - Gold Clusters, Colloids and Nanoparticles - page 96

Fig. 3 (a) STM image of Au monomers and small aggregates on alumina/NiAl(110) ( 1.5 V,
18 18 nm 2 ). The inset shows a ball-stick model of a monomer bound to an Al surface ion.
(b) Image series, showing a small alumina region as a function of bias voltages (15 15 nm 2 ).
The various Au species undergo large contrast changes as a function of the imaging conditions [ 39 ]

charges are shuttled through the oxide, but the film reorganizes its internal bonding
network in order to accommodate the gold. In numbers, a participation of the NiAl
support leads to an increase of the Au/alumina adsorption strength from 1.0 to
2.1 eV, emphasizing once more the importance of charge transfer [ 37 ]. It should be
added that the Au-induced bond cleavage in the alumina film can be considered as
an extreme form of a polaronic lattice distortion, being a typical response of an
ionic oxide to stabilize charged adsorbates on its surface [ 38 ].
The extra charges localized in alumina-bound Au atoms are responsible for a
distinct contrast evolution in STM images taken as a function of sample bias
(Fig. 3b )[ 39 ]. At negative voltage (probing the occupied sample states), Au mono-
mers are imaged as circular protrusions of ~6 apparent height.

This height value decreases at positive polarity and finally reverses at 3.0 V, when
the Au atoms show up as shallow depressions in the surface (Fig. 4a ). At interme-
diate voltages (between +2.0 and +3.0 V), the ad-species are surrounded by a
characteristic sombrero ring, again being the typical fingerprint of charged adsor-
bates (Figs. 2a , 3b) [ 40 , 41 ]. According to the Tersoff-Hamann theory for vacuum
tunneling, a negative contrast indicates a lower state density of the adatom as
compared to the surrounding oxide, which forces the tip to approach the surface
in order to maintain the preset tunnel current [ 42 ]. This decrease in state density is
easily explained with a local upward bending of the oxide bands, in response to the
negatively charged Au atom. Whereas tunneling is efficient in regions away from
the Au, as the oxide conduction band provides suitable final states, it remains
blocked at the adatom site due to upshift of the band onset at this location
(Fig. 4b , inset). Such upward bending is compatible with the accumulation of
negative surface charges, exerting a repulsive interaction on the alumina electronic
states [ 43 ], while positive charges would result in a downward bending of the bands

diameter and ~1

Å Å