Research

Overview

Our work focuses on the growth of complex oxide thin films, mulitlayers, superlattices and nanostructures for electronic and energy applications. These applications include next-generation memory and logic devices, quantum computation, catalysis and photovoltaics. Complex oxides such as perovskites with the chemical formula ABO₃ and spinels with the formula AB₂O₄ have excellent properties for these applications. Through a variety of doping techniques, design of interfaces, and epitaxial growth techniques, we are looking at ways to produce synthetic materials with non-equilibrium properties that can improve the functionality of these systems for future technologies.

We use hybrid molecular beam epitaxy to grow extremely high quality epitaxial films. We employ in situ x-ray photoelectron spectroscopy (XPS) to measure the film stoichiometry to ensure that we are making ideal films. Our XPS capabilities also allow us to measure valence band structure, band alignment across interfaces, built-in electric fields, charge transfer, and the oxidation state of the constituent ions in the crystal. This allows us to understand the properties of our materials as they are grown and allows us to quickly generate high impact results.

Through collaborations with researchers at Delaware, outside universities, and Department of Energy national labs we characterize the structural, chemical, and functional properties of the materials that we synthesize. Some of these techniques include scanning transmission electron microscopy (STEM), X-ray absorption spectroscopy (XAS), magnetotransport measurements, and electrochemical impedance spectroscopy (EIS). Much of our work also relies on collaborations with condensed matter theory groups who can model the electronic structure of uniform films and at interfaces. Students and postdocs in the group regularly learn some of these techniques as well to advance their own work.

Projects

In situ Studies of Charge Transfer in 3d and 5d Complex Oxides

Complex oxide thin films exhibit a wide range of unique phenomena that have been investigated for use in technologies ranging from renewable energy, data storage, and quantum information processing. Perovskite oxides with the formula ABO₃ represent a particularly interesting class of complex oxides because of the wide array of transition metal ions that can occupy the B site of the material. To date, most research has focused on 3d transition metals on the B site, such as cobalt. However, emerging work focused on 5d transition metal B site ions has shown that these materials exhibit stronger spin-orbit coupling and a wide array of correlated electronic phenomena. Many of these properties are modulated through charge transfer across epitaxial interfaces or between cations in ordered phases.

We will employ hybrid molecular beam epitaxy technique for the delivery of refractory tantalum and iridium cations and examine charge transfer to cobalt. Through systematic studies of superlattices and double perovskites that combine the three B site cations, we will validate models governing charge transfer in oxide systems. Additionally, in situ experiments at the Advanced Photon Source will measure charge transfer and cation ordering in films during the growth process using X-ray absorption spectroscopy and X-ray diffraction. Ultrafast optical spectroscopy will be employed to examine the electronic structure and induced polarization in these materials that results from interfacial charge transfer. Films and heterostructures synthesized in this work will also be examined to probe emergent electronic and magnetic topological phenomena.

In this EPSCOR-State/National Laboratory Partnership, our group is working with Prof. Wencan Jin to use in situ techniques to examine the role of charge transfer processes in epitaxial oxide superlattices and ordered double perovskite thin films. Partners include Argonne National Laboratory, Brookhaven National Laboratory, and Pacific Northwest National Laboratory. Collectively, this work will advance our synthesis capabilities and provide a fundamental understanding of interfacial charge transfer in complex oxides. Both results are key goals for the Department of Energy Basic Energy Sciences roadmap for development and control of new materials for quantum information technologies and renewable energy.

This project is funded through the Deparment of Energy Office of Science under award number DE-SC0023478.

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Topological Phenomena in 4d and 5d Complex Oxide Interfaces

Complex oxides comprised of 4d and 5d transition metals exhibit significantly higher spin-orbit coupling than those comprised of 3d elements. These materials have been predicted to exhibit high temperature superconductivity and other emergent topological phenomena when formed in epitaxial thin film heterostructures and superlattices. These properties make 4d and 5d materials promising for use as materials to enable topological quantum computation. In this project, systematic studies of synthesis of several candidate 4d and 5d complex oxides via hybrid molecular beam epitaxy are performed. Using this novel approach, a metal-organic precursor replaces the traditional elemental source for the transition metal, enabling easier synthesis and producing higher quality materials. Exploration of emergent interfacial phenomena includes high-temperature superconductivity, ferroelectricity, and topological electronic states. By synthesizing superlattice films comprised of repeating interfaces, the research focuses on the development of materials that exhibit interfacial phenomena in a bulk film. This project provides educational experiences for undergraduate students to learn about materials characterization through machine learning analysis of diffraction and spectroscopy data. Computational codes to analyze such data are shared for the research community to advance real-time analysis of film synthesis. Graduate student researchers gain experience in materials synthesis and characterization in the lab and at user facilities around the world. Such experiences help prepare students for careers in materials research in the integrated circuit and electronic device industry.

This CAREER project is funded by the National Science Foundation, Division of Materials Research, Ceramics program under award number 2045993.

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Complex Oxide Thin Films and Nancomposites as Bifunctional Catalysts for OER and ORR

Complex oxide thin films have been studied for many years for their wide array of properties, including ferromagnetism, ferroelectricity, and superconductivity. The development of metal oxide materials for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalysis is directly relevant to fuel cell technologies. In this project the investigators perform systematic synthetic studies of transition metal oxides using molecular beam epitaxy to generate epitaxial matrix-pillar nanocomposite thin films where the perovskite oxide matrix phase is a strong OER catalyst and the spinel oxide pillar phase targets ORR. The work is delineated into three tasks: synthesis of perovskite and spinel powders, epitaxial thin films, and nanocomposites; spectroscopic and electrochemical characterization of electronic structure and band alignment, including ambient-pressure x-ray photoelectron spectroscopy studies; and preliminary electrocatalytic studies of uniform thin films and nanocomposites. These studies address fundamental materials chemistry and surface science questions as they relate to improving the performance and economic viability of transition metal oxide hybrid bifunctional materials. The project also provides educational experiences for undergraduate students to learn about materials characterization and for graduate students to perform state-of-the-art experiments at national user facilities.

This work is funded by the National Science Foundation, Division of Materials Research, Solid State and Materials Chemistry program under award number 1809847 with additional funding from EPSCOR.

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Metastable Oxides for High-Mobility and Spin-Orbit 2D Electronics

High-speed wideband communications rely on devices that can operate in the GHz and THz regime for electronic switching, making the engineering and materials challenges for these systems very different from those of traditional integrated circuits for computers. Electronic switches for Air Force applications such as radar and satellite communications must operate at frequencies above 10 GHz, which has led to the development of epitaxial thin film materials that employ a two-dimensional electron gas (2DEG). THz sources with sub-millimeter wavelengths rely on generation of radiation using 2DEGs. Far greater electron mobilities and concentrations are required for these applications, opening the door for new classes of materials that outperform conventional semiconductors. Complex metal oxides are promising in this regard, as they can sustain much greater doping levels than traditional semiconductors. BaSnO₃ is an intriguing complex oxide for such 2DEGs because its s-electron conductivity produces room temperature mobility > 100 cm²/V-sec. However, integrating BaSnO₃ with other oxides is a challenge because of the inability to transfer sufficient electron concentrations across an interface into BaSnO₃. For THz oscillators, materials with strong spin-orbit coupling such as 5d transition metals are also desirable to produce spin-Hall nano-oscillators. To develop high-mobility and spin-orbit-coupled 2DEGs we employ molecular beam epitaxy to synthesize SrNbO₃ and SrTaO₃ thin films that have been proposed as strong donors to a variety of complex oxides. We study the interfaces of SrNbO₃ with BaSnO₃ to produce a 2DEG with high mobility and carrier concentration. We also synthesize SrTaO₃ 2DEGs with strong spin-orbit coupling for THz oscillators. This work opens new avenues for the use of complex oxide films in GHz and THz devices for the Air Force.