Nanosized CoO Films on the α‑Al 2 O 3 (0001) Surface: A Density Functional Study Ala’ Zayed, Antonio M. Ma ́ rquez,* and Javier Fdez. Sanz Departamento de Química Física, Universidad de Sevilla, Facultad de Química, 41012 Sevilla, Spain ABSTRACT: The structural and magnetic properties of ultrathin CoO films supported on α-Al 2 O 3 (0001) surface are investigated on the basis of density functional theory calculations. By examining both stoichiometric and nonstoichiometric submonolayer structures at an atomic level we establish that the preferred surface arrangements are stabilized by the presence of a sublayer of O adatoms, even at the expense of creating O vacancies at the subsurface layer. Further addition of oxygen to this thin CoO layer fills the vacancies resulting in the formation of nondefective, atomically smooth oxygen-terminated surfaces as the preferred structural arrangement. Later, we examine the possible coexistence of rock-salt, wurtzite, or blende-type structures and different arrangements of the Co atoms magnetic moments at higher coverages. The observed structures and magnetic arrangements are rationalized on the basis of the surface dipole moment reduction induced by the O-termination of the overlayers and on the observed intra- and interlayer distances that favor, at these coverages, an intralayer antiferromagnetic arrangement of the Co magnetic moments. ■ INTRODUCTION Oxide ultrathin films grown on different supports (metals, semiconductors, or other oxides) constitute a novel class of nanostructured materials with special size, shape, composition, and properties. Recent advances in the growth of these two- dimensional materials under controlled conditions allow for the tuning of the material properties on demand by controlling the film thickness. These materials have applications in fields as diverse as microelectronics, corrosion protection, catalysis, solid fuel cells, gas sensors, and solar energy materials. 1 Some of these applications are well-known and have been used for decades. Indeed, during the last two decades there has been intense activity oriented to the preparation and characterization of these systems. 1-7 In particular, a great deal of research work is being conducted in the field of ferromagnetic semiconductors because of their potential application in spin electronics. 8 Among the 3d metal oxides, CoO is one of the most studied systems at the ultrathin layer level. There are reports in the literature of thin layer CoO deposited on diverse metal substrates and oxides. These reports are concentrated on the structure, morphology, and magnetic ordering of the deposited nanolayers, and have found that the thin films have a diversity of structures linked to both the preparation method and to the support substrate. 9-17 In particular, the formation of CoO overlayers on metal surfaces has revealed different structural patterns, related to the different lattice mismatch between the CoO and the support and to different mechanisms to reduce the surface dipolar energy. In the case of CoO bilayers grown on Ir(100), LEED studies 12 combined with XPS measurements and theoretical density functional theory (DFT) calculations 11 have shown that below two monolayers the CoO adopts a pseudohexagonal arrangement that corresponds to the (111) face of cubic CoO with the Co atoms next to the substrate and covered by an oxygen layer. However, the structure is strongly distorted by the substrate and forms a c(10 × 2) superstructure. A close relationship was found between the structural relaxation and the local magnetic order in the CoO overlayer. Later studies 15 at higher CoO coverages (4 ML) have shown that while CoO initially grew on Ir(100) epitaxially in the polar (111) cubic orientation, the film is terminated by a thin slab of wurtzite-type CoO. Some evidence is presented of metallic character for these nanosized CoO films, at least at their surface. Thus, the wurtzite-type CoO top layer structure was connected with this metallic character of the system and as a mechanism to compensate the polarity of the (111) cubic CoO surface termination. Cobalt oxide (CoO x ) supported on metal oxides is an attractive catalyst in air pollution control, 18 eliminating pollutants from water streams, 19 photoelectrochemical water splitting, 20 and gas desulfurization 21 and is a potential candidate for spin electronics. 8 However, deposition of CoO overlayers on metal oxides has not received the same detailed attention given to deposition on metals. Only some experimental papers can be found in the literature, and to the best of our knowledge no theoretical work has dealt with the formation and structure of CoO thin layers over a support oxide. For example, one recent report 16 examined the formation of hexagonal CoO thin films on Al 2 O 3 and ZnO and found that the structural and Received: June 18, 2013 Revised: September 26, 2013 Published: October 16, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 22714 dx.doi.org/10.1021/jp406016z | J. Phys. Chem. C 2013, 117, 22714-22722