Modified Oxygen Defect Chemistry at Transition Metal Oxide Heterostructures Probed by Hard X-ray Photoelectron Spectroscopy and X-ray Diffraction

Abstract

Transition metal oxide hetero-structures are interesting due to the distinctly different properties that can arise from their interfaces, such as superconductivity, high catalytic activity and magnetism. Oxygen point defects can play an important role at these interfaces in inducing potentially novel properties. The design of oxide hetero-structures in which the oxygen defects are manipulated to attain specific functionalities requires the ability to resolve the state and concentration of local oxygen defects across buried interfaces. In this work, we utilized a novel combination of hard x-ray photoelectron spectroscopy (HAXPES) and high resolution xray diffraction (HRXRD) to probe the local oxygen defect distribution across the buried interfaces of oxide heterolayers. This approach provides a non-destructive way to qualitatively probe locally the oxygen defects in transition metal oxide hetero-structures. We studied two trilayer structures as model systems - the La<sub>0.8</sub>Sr<sub>0.2</sub>CoO<sub>3-δ</sub>/(La<sub>0.5</sub>Sr<sub>0.5</sub>)<sub>2</sub>CoO<sub>4</sub>/La<sub>0.8</sub>Sr<sub>0.2</sub>CoO<sub>3-δ</sub> (LSC<sub>113</sub>/LSC<sub>214</sub>) and the La<sub>0.8</sub>Sr<sub>0.2</sub>CoO<sub>3-δ</sub>/La<sub>2</sub>NiO<sub>4+δ</sub>/La<sub>0.8</sub>Sr<sub>0.2</sub>CoO<sub>3-δ</sub> (LSC<sub>113</sub>/LNO<sub>214</sub>) on SrTiO<sub>3</sub>(001) single crystal substrates. We found that the oxygen defect chemistry of these transition metal oxides was strongly impacted by the presence of interfaces and the properties of the adjacent phases. Under reducing conditions, the LSC<sub>113</sub> in the LSC<sub>113</sub>/LNO<sub>214</sub> tri-layer had less oxygen vacancies than the LSC<sub>113</sub> in the LSC<sub>113</sub>/LSC<sub>214</sub> tri-layer and the LSC<sub>113</sub> single phase film. On the other hand, LSC<sub>214</sub> and LNO<sub>214</sub> were more reduced in the two tri-layer structures when in contact with the LSC<sub>113</sub> layer compared to their single phase counterparts. Furthermore, the results point out a potential way to modify the local oxygen defect states at oxide hetero-interfaces.