Proper spectroscopic analysis of transition metal oxides using ex-situ X-ray photoelectron spectroscopy

Transition metal oxides (TMOs) are one of the most exciting classes of materials due to their emergent phenomena over the past few decades. In general, the emergent phenomena in TMOs are driven by the chemical state of the TMOs. Therefore, it is vital to understand the correlation between the chemical state and the physical properties of the TMOs. X-ray photoelectron spectroscopy (XPS) is the most widely used method for analyzing the chemical state of materials. However, when using XPS to investigate the chemical properties of TMOs, a lack of clear theoretical explanations for the interpretation, including discussions of oxygen vacancies, inaccurate XPS peak fitting, and inaccurate calibration, often leads to misinterpretation. In this review, we present a brief introduction to XPS, the peak fitting/deconvolution method for analyzing the chemical state of TMOs, and several case studies that use XPS to correlate the chemical state and the physical properties of TMOs.