We introduce an interfactant for zirconium oxide heteroepitaxy and propose that dispersed, epitaxial films of materials which are hard to crystallize are accessible by growing them on top of an ultrathin, polar FeO(111) film.

It was not possible to achieve atomic resolution on these films which may be due to their insulating character which requires high bias voltages.

There, the well-known O/Ru(0001)-(2 × 2) structure may have formed,⁴⁷ which we always observe on FeO-uncovered Ru terraces after growth of submonolayer FeO(111) films.²⁹

Thus, in the holes no FeO(111) buffer layer is present, and this indicates that the presence of the FeO(111) film is beneficial for the adhesion of the zirconia film while the O/Ru(0001)-(2 × 2) structure is not.

We can only speculate about the origin of the hole formation: It may be due to diffusion of FeO into the substrate leaving a submonolayer FeO(111) film with uncovered Ru regions.

Alternatively, a restructuring of the FeO(111) monolayer film may occur, since previous studies on Pt and Ru showed that the lattice constant of these films varies strongly as it adjusts to the electrostatic, thermodynamic and interfacial energy gain associated with the interaction at the specific interface.²⁹

However, even after 2 h annealing at 1200 K, STM shows that the morphology of these films has not changed at all (compare Fig. 3b with Fig. 3d) indicating a stable film structure and that decomposition mainly occurs via desorption of Zr and O. Although a direct chemical evidence for the presence of FeO at the interface is not available with our methods (Fe Auger signal intensities are too weak below the zirconia film), the observation of a lattice constant which matches a FeO(111) film even after annealing for long periods suggests that this layer separates the Ru substrate from the zirconia film.

The FeO(111) buffer layer seems to “float” between the zirconia and Ru.

However, we cannot exclude that decomposition and diffusion of the iron oxide may have occurred.

The new, ordered LEED structure may then be attributed to an ordered Ru/ZrO_x interface structure as has been proposed to occur for zirconia films on Pt(111) after annealing beyond 1100 K.¹⁸

In this study, we have introduced a new interfactant for heteroepitaxial metal oxide growth by precovering a metal substrate with a monolayer FeO(111) film.

In this way, we have obtained dispersed, cubic ZrO₂(111) films with improved long-range order compared to previous studies.

An effective interfactant requires (I) that the rigidity of the substrate is reduced, so that strain relieve can lead to a better epitaxy, (II) that it acts as a diffusion barrier, while on the other hand (III) the interaction with the substrate and also with the growing film has to be strong in order to achieve wetting of all layers.

In other words: An interfactant has to behave like a ‘glue’ sticking together the substrate and the film, a condition which is perfectly fulfilled for wetting, ‘floating’ FeO(111) films with their variable lattice constant adjustment and their strong adhesiveness which is due to a high tendency to reduce the polarity by an interfacial electron redistribution.

The development of such multi-layer oxide systems is highly relevant in all fields, where thin closed oxide layers are required.

We are aware that the presented study is preliminary and quantitative structural information by surface crystallography or electron microscopy are appreciated to verify the presented hypotheses, especially for the presence of the FeO(111) interface layer after deposition of zirconia and annealing.

However, even if Fe diffusion occurs in the zirconia film, this is no drawback as promotion of sulfated zirconia with Fe and Mn enhances the catalytic activity,⁵¹ and in this respect, the described substrate system may be an ideal candidate for model catalytic studies.

Such dissolution behaviour can be regarded as being due to a very low or even negative interface free energy,⁵² and thus from the point of view of the total energy balance, eqn. (1), would enhance the dispersion of the zirconia layer.

The novel concept of interfactant behavior is extended in this paper to oxide-on-oxide heteroepitaxy.

It presents an explanation for the unexpected wetting behavior on top of the polar FeO(111) interface layer.