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Perovskite oxides such as LaFeO3 are a well-studied family of materials that possess a wide range of useful and novel properties. Successfully synthesizing perovskite oxide samples usually requires a significant number of growth attempts and a detailed film characterization on each sample to find the optimal growth window of a material. The most common real-time in situ diagnostic technique available during molecular beam epitaxy (MBE) synthesis is reflection high-energy electron diffraction (RHEED). Conventional use of RHEED allows a highly experienced operator to determine growth rate by monitoring intensity oscillations and make some qualitative observations during growth, such as recognizing the sample has become amorphous or recognizing that large islands have formed on the surface. However, due to a lack of theoretical understanding of the diffraction patterns, finer, more precise levels of observations are challenging. To address these limitations, we implement new data analytics techniques in the growth of three LaFeO3 samples on Nb-doped SrTiO3 by MBE. These techniques improve our ability to perform unsupervised machine learning using principal component analysis (PCA) and k-means clustering by using drift correction to overcome sample or stage motion during growth and intensity transformations that highlight more subtle features in the images such as Kikuchi bands. With this approach, we enable the first demonstration of PCA and k-means across multiple samples, allowing for quantitative comparison of RHEED videos for two LaFeO3 film samples. These capabilities set the stage for real-time processing of RHEED data during growth to enable machine learning-accelerated film synthesis. 

Iron and nickel-based perovskite oxides have proven promising for the oxygen evolution reaction (OER) in alkaline environments, as their catalytic overpotentials rival precious metal catalysts when the band alignment is tuned through substitutional doping or alloying. Here, we report the engineering of band alignment in LaFeO3/LaNiO3 (LFO/LNO) heterostructures via interfacial doping that yields greatly enhanced catalytic performance. The 0.2 eV offset (VBO) between the Fermi level in metallic LNO and the valence band in semiconducting LFO that we predict using density functional theory makes LFO a p-type semiconductor, resulting in significantly lower barriers for hole transport through LFO compared to the intrinsic material. Experimental band alignment measured with in situ x-ray photoelectron spectroscopy of epitaxial LFO/LNO heterostructures confirms these predictions, producing a measured VBO of 0.3(1) eV. Furthermore, OER catalytic measurements on these samples in the alkaline solution show an increase in catalytic current density by a factor of ∼275 compared to LFO grown on n-type Nb-doped SrTiO3. These results demonstrate the power of tuning band alignments through interfacial band engineering for improved catalytic performance of oxides. 

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 LaFeO 3 , are particularly promising due to the favorable energy alignment of the valence and conduction bands comprised of Fe 3+ cations and the visible light band gap of such materials. In this work, we examine the role of band alignment on the electrocatalytic oxygen evolution reaction (OER) in the intrinsic semiconductor LaFeO 3 by growing epitaxial films of varying thicknesses on Nb-doped SrTiO 3 . Using cyclic voltammetry, we find that there is a strong thickness dependence on the efficiency of electrocatalysis for OER. These measurements are understood based on interfacial band alignment in the system as well as catalytically active surface defect states as confirmed by layer-resolved electron energy loss spectroscopy, electrochemical impedance spectroscopy, and Mott–Schottky measurements. Our results demonstrate the importance of band engineering for the rational design of thin film electrocatalysts for renewable energy sources. 

Abstract 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.