Publication date: 10th April 2024
The experimental identification of the charge transition levels is of high interest, as it promotes the comprehension of the relationship between electronic structure and material properties. This serves as a foundation for quantitative defect models. The charge transition levels within the energy gap of transition metal oxides can be directly probed by means of X-ray photoelectron spectroscopy (XPS). They are directly connected to the oxidation state of the elements, dependent on the oxygen activity.
In this presentation, I will describe the recent work on identifying the charge transition level of Co2+/3+ in (La,Sr)CoO3-δ (LSCO) samples via X-ray photoelectron spectroscopy. All perovskite-type LSCO thin film samples have been deposited by RF sputtering. Post-deposition reducing surface treatments with water vapor and interface experiments to BaTiO3 (BTO) and Nb-doped SrTiO3 (STO) could reveal the Co2+/3+ charge transition levels, which are positioned at 0.01 eV above the valence band maximum. Therefore, the Fermi level in LSCO will be below the valence band maximum if Co is in the 3+ oxidation state. This is in good agreement with the high electronic conductivity of LSCO. Further oxidation of LSCO is expected to occur on the oxygen sites, as the presence of Co4+ states is not likely. The measured Co2+/3+ charge transition level of LSCO is consistent with the respective level in Co-doped BiFeO3 [1], assuming that the charge transition levels are aligned in perovskite-type oxides.
The presented work has been supported by the German Research Foundation (DFG) within the collaborative research centre (CRC) 1548 FLAIR (Fermi level engineering applied to oxide electroceramics).
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