Probing surfaces and interfaces in complex oxide films via in situ X-ray photoelectron spectroscopy

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.

Oxygen Reduction Electrocatalysis with Epitaxially Grown Spinel MnFe2O4 and Fe3O4

“…The conductivity of the Nb:STO substrates negated the need for an electron emission neutralizer for sample charge compensation. All spectra were shifted accordingly to place their Fe 2p 3/2 peaks at a 711 eV binding energy. , Atomic force microscopy (AFM) was used to acquire images of film topography and was acquired using a Park XE7 AFM in a noncontact mode. Out-of-plane high-resolution X-ray diffraction (HRXRD) and X-ray reflectivity (XRR) were performed using a Rigaku SmartLab system (Cu Kα source) with a hybrid pixel area detector in a 0D mode.…”

High Mobility Two-Dimensional Electron Gas at the BaSnO3/SrNbO3 Interface

“…Such measurements have also been employed in the past for a variety of oxide heterostructures to examine cation intermixing and electron accumulation at an interface from charge transfer. 48 More sophisticated modeling of the electron concentration within each unit cell of the BSO film below the interface can also be used to estimate the electron density within the BSO layer. By assuming a reduction in the electron concentration of 33% per BSO unit cell moving away from the interface, which mimics the DFT results above, we can derive a model to estimate the Sn 3+ distribution in BSO (Figure 5b).…”

“…This result shows that the Sn 3+ charge state is greater toward the surface of the heterostructure, consistent with an accumulation of electrons at the BSO/SNO interface. Such measurements have also been employed in the past for a variety of oxide heterostructures to examine cation intermixing and electron accumulation at an interface from charge transfer . More sophisticated modeling of the electron concentration within each unit cell of the BSO film below the interface can also be used to estimate the electron density within the BSO layer.…”

Regio-isomerism directed electrocatalysis for energy efficient zinc-air battery

“…This is reflected in the X-ray photoelectron spectroscopy (XPS) data of these molecules, ( Figure 2 D) wherein the binding energy peak corresponding to Co 3+ (2p 3/2 ) species demonstrated a positive binding energy shift of 0.6 eV in the β-isomeric molecule compared to the α-isomeric molecule. These suggest that the contribution of oxidized cobalt species is noticeably enriched in the β-isomer molecule compared to the corresponding α counterpart, ( Figure 2 D), which in turn indicates a positive shift in the redox energy of Co 3+ /Co 2+ in the former ( Li et al., 2019a ) ( Gu et al., 2016 ) ( Rodríguez-Fernández et al., 2019 ) ( Thapa et al., 2021 ). This should lead to a better overlap of Co 3+ /Co 2+ redox energy level with the oxygen redox level in the β-isomer molecule which can amplify the oxygen redox reactions.…”