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  • Evidence for phase transitions in CoFe 2 O 4 and NiCo 2 O 4 thin films in temperature-dependent X-ray photoelectron spectroscopy
Title: Evidence for phase transitions in CoFe 2 O 4 and NiCo 2 O 4 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 CoFe2O4and NiCo2O4thin 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 CoFe2O4and NiCo2O4thin films are very different. The XPS Co and Fe 2p3/2core levels for the CoFe2O4thin 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 2p3/2core levels, decreases with increasing temperature. The XPS core level binding energies of CoFe2O4thin film saturated at lower apparent binding energies above 455 K. This result shows that the prepared CoFe2O4thin film can be dielectric at room temperature but become more metallic at elevated temperatures. The dielectric nature of the CoFe2O4thin 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 NiCo2O4thin film demonstrated that annealing NiCo2O4thin film led to a more dielectric or insulating film. The original more metallic character of the NiCo2O4film was restored when the NiCo2O4was 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 CoFe2O4and NiCo2O4thin 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 NiCo2O4thin film that can undergo reversible metal-to-insulator transition with temperature.  more » « less
Award ID(s):
2044049
PAR ID:
10636554
Author(s) / Creator(s):
; ; ;
Publisher / Repository:
Purpose-Led Publishing
Date Published:
Journal Name:
Journal of Physics D: Applied Physics
Volume:
57
Issue:
49
ISSN:
0022-3727
Page Range / eLocation ID:
495301
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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