Self-Organization of Gold Atoms on a Polar FeO(111) Surface

The spatial distribution of single Au atoms on a thin FeO film has been investigated by low-temperature scanning tunneling microscopy and spectroscopy. The adatoms preferentially adsorb on distinct sites of the Moiré cell formed by the oxide layer and the Pt(111) support and arrange into a well-ordered hexagonal superlattice with 25 Å lattice constant. The self-organization is the consequence of an inhomogeneous surface potential within the FeO Moiré cell and substantial electrostatic repulsion between the adatoms.

Figure 1

(a) Structural model of the Moiré cell formed by FeO film and Pt(111). (b) STM topographic images of $\mathrm{FeO}/\mathrm{Pt}(111)$ taken at 0.5 V and (c) 4.5 V sample bias. The current was set to 0.1 nA. (d) Apparent-barrier-height image of $\mathrm{FeO}/\mathrm{Pt}(111)$ taken at 3.5 V sample bias. The current was set to 0.1 nA, the $z$ modulation to 1.0 Å. All images are $100\times 100{\AA }^2$ in size.

Figure 2

(a) STM topographic image of Au adatoms on $\mathrm{FeO}/\mathrm{Pt}(111)$ taken at 0.55 V and 0.1 nA. (b) Conductance spectra of bare FeO and Au adatoms bound to different domains of the Moiré cell. The tip was stabilized at 1.0 V and 0.1 nA for spectroscopy. (c) Conductance images of the same area as in (a) taken at 0.55 V and (d) 0.35 V. All images are $130\times 130{\AA }^2$ in size.

Figure 3

(a) STM topographic image of Au adatoms on $\mathrm{FeO}/\mathrm{Pt}(111)$ taken at 0.55 V and 0.1 nA. (b) Histogram of the occupation probability of different domains in the FeO Moiré cell determined from roughly 700 adatom positions. The inset shows a power spectrum of (a). (c) Conductance images of the same area as in (a) taken at 0.55 V and (d) 0.75 V. The current was set to 0.1 nA. All images are $430\times 430{\AA }^2$ in size.

Figure 4

Pair distribution function of Au adatoms on $\mathrm{FeO}/\mathrm{Pt}(111)$ extracted from STM images at 0.01 ML Au coverage (lower part). Most events occur within hcp domains; the substructure in the distribution maxima reflects the atomic binding sites within these domains. The corresponding potential energy landscape for Au adsorption on the FeO film is calculated from the relative occupations using a Boltzmann distribution, as shown in the upper part.