Evidence for Phase Transitions in CoFe₂O₄ and NiCo₂O₄ Thin Films in Temperature Dependent X-Ray Photoelectron Spectroscopy

The temperature dependent X-ray photoelectron spectroscopy (XPS) of the CoFe₂O₄ thin film showed that core level binding energies decreased with increasing temperature. The large binding energy shifts of the Co 2p_3/2 and Fe 2p_3/2 core levels of CoFe₂O₄ thin film, observed at room temperature, are due to large photovoltaic surface charging. The large binding energy shifts of the Co 2p_3/2 and Fe 2p_3/2 core levels, in the X-ray photoelectron spectroscopy of CoFe₂O₄ thin film, decreased with increasing temperature. However, above 455 K, during annealing of the sample, shifts in the core level binding energies ceased to decrease. This shows that the prepared CoFe₂O₄ thin film can be dielectric at room temperature but more metallic at elevated temperatures. The dielectric nature of the film was restored only when the film was annealed in sufficient oxygen, indicating that the oxygen vacancies play a role in the transition of the film from dielectric (or insulating) to conducting. In contrast, similar studies on NiCo₂O₄ thin film showed that annealing of NiCo₂O₄ thin film, which was observed to be conducting, could make NiCo₂O₄ thin film insulating, and the original more metallic character of the NiCo₂O₄ thin film could be restored only when the sample was annealed in sufficient oxygen. A model that governs the core level binding energy changes, as a function of temperature, is proposed. Furthermore, restoration of the original properties or phases of the thin films after undergoing a metal-to-insulator transition illustrates routes to regulate the surface metal-to-insulator transition, especially in the case of insulating NiCo₂O₄ thin film which can undergo reversible metal-to-insulator transition with temperature. This work provides a better fundamental understanding of defect mediated surface phases for thin film oxides and opens avenues for defect assisted and/or temperature dependent future beyond CMOS devices.