Ultrathin oxide films: Epitaxy at the two-dimensional limit
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he epitaxial growth of two-dimensional oxide layers on metal surfaces is examined in view of the 1949 van der Merwe proposition that epitaxy requires a pseudomorphic monolayer. It is argued that this limitation is relaxed in the 2-D case and that ordered oxide phases can grow out of a variety of interface scenarios, ranging from pseudomorphic to incommensurate. Prototypical examples of binary and ternary oxides supported on noble metal surfaces are presented, and the structural peculiarities of 2-D oxide phases are emphasized. The usually strong coupling at the oxide–metal interface leads to the stabilization of novel structure concepts that are not encountered in the, respective, bulk phases. The structural flexibility of 2-D lattices is discussed, and their ability to accommodate strain in generating novel 2-D oxide phases is emphasized. In the case of weakly coupled systems, it is reported that more subtle interactions at the interface can create periodic nanoscale morphologies and particular growth patterns in subsequent layers.
I. INTRODUCTION
For many decades, studies of the growth of ordered thin films have been a prolific endeavor in the scientific community of materials science, and this continued interest has been and is motivated by the ubiquitous application of thin films in almost all fields of modern advanced technologies. With the advent of the emerging nanotechnologies during the last decade, ultrathin films have become the focus of attention, with films approaching the two-dimensional (2-D) limit with only one or a few atom layers or, in the case of compounds, a few structural polyhedra thick. In conventional thin film terminology, (hetero)epitaxy denotes the formation of a crystalline overlayer on the surface of a single crystal of a different material. If the lattice constants of the two materials are similar, the atomic lattice of the substrate surface can act as a template and can promote the growth of an ordered crystalline film. In 1949, Charles Frank and van der Merwe have postulated that the pseudomorphic growth of the first monolayer is a necessary condition for the growth of an epitaxial film.1 This implies that the interface between the substrate and the overlayer is coherent, in which the corresponding atom planes and lines are continuous and there is an atom-by-atom matching across the interface. In this article, we will examine the epitaxial growth of ultrathin oxide films on metal surfaces, in which a variety of different interface scenarios can give rise to epitaxially ordered overlayers.
Contributing Editor: Johan Brand Malherbe a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.391
Phenomenologically, thin film growth modes have been categorized by Ernst Bauer2,3 using a thermodynamic equilibrium approach based on surface energies. The categories were named after their original investigators as: (i) Frank–van der Merwe (F–M) or 2-D growth, i.e., the substrate is wetted by a pseudomorphic monolayer, one monolayer is completed after another; (ii
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