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Ni3+-induced semiconductor-to-metal transition in spinel nickel cobaltite thin films

DOI: 10.1103/PhysRevB.104.125136 DOI Help

Authors: X. C. Huang (Xiamen University) , W.-W. Li (Nanjing University of Aeronautics and Astronautics) , S. Zhang (University of Electronic Science and Technology of China) , F. E. Oropeza (IMDEA Energy Institute, Parque Tecnológico de Móstoles) , G. Gorni (CELLS-ALBA Synchrotron) , V. A. De La Pena-O'Shea (MDEA Energy Institute, Parque Tecnológico de Móstoles) , T.-L. Lee (Diamond Light Source) , M. Wu (Xiamen University) , L.-S. Wang (Xiamen University) , D.-C. Qi (Queensland University of Technology) , L. Qiao (University of Electronic Science and Technology of China) , J. Cheng (Xiamen University) , K. H. L. Zhang (Xiamen University)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Physical Review B , VOL 104

State: Published (Approved)
Published: September 2021
Diamond Proposal Number(s): 24219

Abstract: In this paper, we report insights into the local atomic and electronic structure of NiCo 2 O 4 epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined NiCo 2 O 4 epitaxial thin films with controlled properties on Mg Al 2 O 4 ( 001 ) substrates using pulsed laser deposition. Films grown at low temperatures ( < 400 ∘ C ) exhibit a ferrimagnetic and metallic behavior, while those grown at high temperatures are nonmagnetic semiconductors. The electronic structure and cation local atomic coordination of the respective films were investigated using a combination of resonant photoemission spectroscopy, x-ray absorption spectroscopy, and ab initio calculations. Our results unambiguously reveal that the Ni 3 + valence state promoted at low growth temperature introduces delocalized Ni 3 d -derived states at the Fermi level ( E F ), responsible for the metallic state in NiCo 2 O 4 , while the Co 3 d -related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and ∼ + 2 . Further structural and defect chemistry studies indicate that the formation of oxygen vacancies and secondary CoO phases at high growth temperature are responsible for the Ni 2 + valence state in NiCo 2 O 4 . The Ni 3 d -related state becomes localized away from E F , opening a band gap for a semiconducting state. The band gap of the semiconducting NiCo 2 O 4 is estimated to be < 0.8 eV , which is much smaller than the quoted values in the literature ranging from 1.1 to 2.58 eV. Despite the small band gap, its optical transition is d − d dipole forbidden, and therefore, the semiconducting NiCo 2 O 4 still shows reasonable transparency in the infrared-visible light region. The present insights into the role of Ni 3 + in determining the electronic structure and defect chemistry of NiCo 2 O 4 provide important guidance for use of NiCo 2 O 4 in electrocatalysis and opto-electronics.

Journal Keywords: Density of states; Electrical conductivity; Electronic structure; Cobaltates; Spinel; Thin films; Transparent conducting oxides; Density functional theory; Epitaxy; Laser ablation; Resistivity measurements; X-ray absorption spectroscopy; X-ray photoelectr

Diamond Keywords: Semiconductors

Subject Areas: Materials, Physics

Instruments: I09-Surface and Interface Structural Analysis

Other Facilities: Soft X-ray Spectroscopy beamline at the Australian Synchrotron; BL22-CLAESS at ALBA

Added On: 28/09/2021 10:19

Discipline Tags:

Surfaces Physics Hard condensed matter - structures Electronics Materials Science interfaces and thin films

Technical Tags:

Spectroscopy X-ray Photoelectron Spectroscopy (XPS) Hard X-ray Photoelectron Spectroscopy (HAXPES)