Physics & Astronomy Colloquium

Speaker: Ryan Comes  (Auburn University)

Title: Atom-by-Atom Engineering of Oxide Thin Films and Nanocomposites via Molecular Beam Epitaxy

Abstract: Complex oxides comprised of multiple positively charged metal cations exhibit a host of intriguing and useful properties for new technologies. Perovskite oxides with the chemical formula ABO3 and spinel oxides with the formula AB2O4 have some of the richest behavior. These materials may be metallic, semiconducting, or insulating, and exhibit ferroelectricity, with a built-in electric polarization, ferromagnetism, or superconductivity. This combination of properties in a single class of materials offers rich opportunities for engineering of unusual combinations of behavior through the design of multi-layer thin film materials. Through the use of molecular beam epitaxy (MBE), we are able to engineer these materials down to the atomic level so that interfaces between two different perovskites can be controlled to produce desirable properties. In this talk I will present two examples of this type of interfacial engineering, showing how we can design, model, and characterize these properties through a wide variety of techniques. I will discuss our work using interfacial termination in polar/non-polar heterojunctions and superlattices to engineer electric fields in these materials. Using in situ x-ray photoelectron spectroscopy (XPS) characterization of the LaFeO3/n-SrTiO3 junction with differing interfacial termination, we extract the valence and conduction band alignment between the materials and show that we can tune the electronic structure by interfacial engineering. In LaFeO3-NiFe2O4 nanocomposites, we show for the first time that MBE can be used to grow these vertically-aligned nanocomposites that are of interest for magnetic and catalytic applications. Using a combination of atom probe tomography and scanning transmission electron microscopy, we visualize the lateral interfaces down to the atomic level with sensitivity to the elemental composition in each phase. These results open up a wide range of new opportunities to design multilayer and nanostructured materials to achieve specific properties that cannot be found in the bulk.