X-ray photoelectron spectroscopy of some inverse spinel oxide thin films and nanoparticles

Despite the numerous X-ray photoelectron spectroscopy (XPS) studies that have been carried out on inverse spinel oxide materials, there has been a lack of agreement on the physical interpretation of the components of the 2p_3/2 core level XPS spectra of the cations in inverse spinel oxides of type AB₂O₄. In this study, we have been able to establish that the 2p_3/2 core level XPS spectra of AB₂O₄ materials represent surface and bulk weighted components, implying surface-to-bulk core level shifts in the binding energies of the core levels [1]. The ratio of surface weighted components to bulk weighted components of the Ni and Fe core levels in NiFe₂O₄ thin films significantly depend on the emission angle, with respect to the surface normal, in angle resolved X-ray photoelectron spectroscopy (ARXPS). For nanoparticles representing a range of alloys between Ni_xCo_1-xFe₂O₄ (x=0.2, 0.5, 0.8, 1), the behavior observed in X-ray photoelectron spectroscopy (XPS) resembles the surface of the NiFe₂O₄ thin film regardless of composition. Surface-to-bulk core level shifts observed in some inverse spinel thin films are difficult to capture in the XPS of the nanoparticles. Moreover, our study shows that the nature of surface contamination could affect the degree to which the ratio of surface weighted to bulk weighted components depends on the photoemission angle.

[1] A. Subedi et al., J. Vac. Sci. Technol. A 40, 023201 (2022).