Evidence for phase transitions in CoFe2O4 and NiCo2O4 thin films in temperature-dependent X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy (XPS) shows that dramatic changes in the core level binding energies can provide strong indications of transitions between more dielectric and more metallic CoFe 2 O 4 and NiCo 2 O 4 thin films. These significant variations in the XPS core level binding energies are possible with a combination of annealing and oxygen exposure; however, the behaviors of the CoFe 2 O 4 and NiCo 2 O 4 thin films are very different. The XPS Co and Fe 2 p 3/2 core levels for the CoFe 2 O 4 thin film at room temperature show large photovoltaic surface charging, leading to binding energy shifts, characteristic of a highly dielectric (or insulating) surface at room temperature. The photovoltaic charging, observed in the XPS binding energies of the Co and Fe 2 p 3/2 core levels, decreases with increasing temperature. The XPS core level binding energies of CoFe 2 O 4 thin film saturated at lower apparent binding energies above 455 K. This result shows that the prepared CoFe 2 O 4 thin film can be dielectric at room temperature but become more metallic at elevated temperatures. The dielectric nature of the CoFe 2 O 4 thin film was restored only when the film was annealed in sufficient oxygen, indicating that oxygen vacancies play an important role in the transition of the film from dielectric (or insulating) to metallic. In contrast, the XPS studies of initially metallic NiCo 2 O 4 thin film demonstrated that annealing NiCo 2 O 4 thin film led to a more dielectric or insulating film. The original more metallic character of the NiCo 2 O 4 film was restored when the NiCo 2 O 4 was annealed in sufficient oxygen. Effective activation energies are estimated for the carriers from a modified Arrhenius-type model applied to the core level binding energy changes of the CoFe 2 O 4 and NiCo 2 O 4 thin films, as a function of temperature. The origin of the carriers, however, is not uniquely identified. This work illustrates routes to regulate the surface metal-to-insulator transition of dielectric oxides, especially in the case of insulating NiCo 2 O 4 thin film that can undergo reversible metal-to-insulator transition with temperature.