Influence of the Metal Substrate on the Adsorption Properties of Thin Oxide Layers: Au Atoms on a Thin Alumina Film on NiAl(110)

Thin oxide films grown on metal substrates are widely used in surface science to model bulk oxides, assuming their chemical and electronic properties to be similar. In some cases, however, this might not be justified as the present scanning tunneling microscopy studies demonstrate for Au atoms on a thin alumina film on NiAl(110). Au atoms were evaporated onto the oxide film at a sample temperature of $\sim 10\mathrm{K}$. At low coverage, this leads to the formation of one-dimensional clusters with unusually large Au-Au distances of 5.6–6.0 Å. A direct interaction between the Au atoms can be excluded, and a substrate-mediated mechanism is supposed instead. This assumption is strengthened by the finding that the Au chains exhibit a preferential orientation: They are almost aligned with the $[001]$ direction of the NiAl(110) substrate, clearly indicating that the metal substrate participates in the binding of the Au atoms.

Figure 1

STM image showing Au adatoms and clusters on a thin alumina film on NiAl(110) ($430\AA \times 430\AA$, $V_S=3.0\mathrm{V}$). At the displayed coverage, mostly monomers (M) and dimers (D) are found. The bright stripes are domain boundaries, regularly appearing line defects of the oxide film [12, 15]. The marked orientation of the NiAl substrate is the same in all STM images of this work.

Figure 2

STM images ($43\AA \times 43\AA$) and according height profiles of a Au monomer (a) and self-assembled Au chains containing two to five atoms [(b)–(e)] on an alumina film on NiAl(110). The STM images are recorded at negative sample bias $V_S$, whereby $V_S$ is chosen either well above (left side) or close to (middle) the highest occupied state of the Au chain, as determined by STS.

Figure 3

(a)–(b) STM images ($47\AA \times 47\AA$, $V_S=-1.8\mathrm{V}$) of Au pentamers on the alumina film on NiAl(110), which are adsorbed in the $A$ and $B$ domains, respectively. The angles between the chain axis (dotted lines) and the NiAl $[001]$ direction (straight lines) are indicated. (c)–(d) Schemes showing the orientation of the (idealized) hexagonal ${\mathrm{Al}}_S$ lattice (gray circles) with respect to the NiAl substrate. The dotted lines reflect the extension and orientation of Au pentamers. The unit cell parameters are ($a_1=4.08\AA$, $a_2=2.89\AA$) for NiAl and ($b_1=17.88\AA$, $b_2=10.55\AA$, $\alpha =89°$) for the alumina film. (e) STM image showing a Au trimer ($V_S=-0.7\mathrm{V}$) and the ${\mathrm{Al}}_S$ layer with atomic resolution due to supercorrugation. The atom positions are marked, whereby the lattice is extrapolated to the area underneath the Au trimer. (f) STM image of the same region with a metallic tip at $V_S=-1.2\mathrm{V}$. The lattice from (e) is superposed.

Figure 4

Conductance spectra, constant current, and conductance ($dI/dV$) images of a Au pentamer on the alumina film on NiAl(110). For better visualization, the intensity of the $dI/dV$ images was scaled. (a) At negative sample bias $V_S$, two states are detected, which are localized at each of the five lobes visible in the inset (position I). In between the lobes, nodal planes with strongly reduced conductance are observed (position II). The symmetry of these states is also reflected in the $dI/dV$ images of (b) [14]. (c) Conductance spectra taken at positive $V_S$ differ along the axis of the Au chain. (d) According conductance images.