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Emergent behavior at oxide interfaces has driven research in complex oxide films for the past 20 years. Interfaces have been engineered for applications in spintronics, topological quantum computing, and high-speed electronics with properties not observed in bulk materials. Advances in synthesis have made the growth of these interfaces possible, while X-ray photoelectron spectroscopy (XPS) studies have often explained the observed interfacial phenomena. This review discusses leading recent research, focusing on key results and the XPS studies that enabled them. We describe how the in situ integration of synthesis and spectroscopy improves the growth process and accelerates scientific discovery. Specific techniques include determination of interfacial intermixing, valence band alignment, and interfacial charge transfer. A recurring theme is the role that atmospheric exposure plays on material properties, which we highlight in several material systems. We demonstrate how synchrotron studies have answered questions that are impossible in lab-based systems and how to improve such experiments in the future.

Abstract

Transition metal oxides have long been an area of interest for water electrocatalysis through the oxygen evolution and oxygen reduction reactions. Iron oxides, such as LaFeO3, are particularly promising due...

The titanomagnetites (Fe 2−x Ti x O 4 , x 1) are a family of reducible spinel-structure oxides of interest for their favorable magnetic, catalytic, and electrical transport properties. To understand the stability of the system during low temperature deposition, epitaxial thin films of Fe 2 TiO 4 were deposited by molecular beam epitaxy (MBE) on MgO(001) at 250-375 • C. The homogeneous incorporation of Ti, Fe valence state, and film morphology were all found to be strongly dependent on the oxidation conditions at the low substrate temperatures employed. More oxidizing conditions led to phase separation into epitaxial, faceted Fe 3 O 4 and rutile TiO 2 . Less oxidizing conditions resulted in polycrystalline films that exhibited Ti segregation to the film surface, as well as mixed Fe valence (Fe 3+ , Fe 2+ , Fe 0 ). A narrow window of intermediate oxygen partial pressure during deposition yielded nearly homogeneous Ti incorporation and a large fraction of Fe 2+ . However, these films were poorly crystallized, and no occupation of tetrahedral sites in the spinel lattice by Fe 2+ was detected by x-ray magnetic circular dichroism at the Fe L-edge. After vacuum annealing, a small fraction of Fe 2+ was found to occupy tetrahedral sites. Comparison of these results with previous work suggests that the low temperature deposition conditions imposed by use of MgO substrates limits the incorporation of Ti into the spinel lattice. This work suggests a path towards obtaining stoichiometric, well-crystallized Fe 2 TiO 4 by MBE by utilizing high substrate temperature and low oxygen partial pressure during deposition on thermally stable substrates.