Abstract
In this paper, we report insights into the local atomic and electronic structure of epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined epitaxial thin films with controlled properties on substrates using pulsed laser deposition. Films grown at low temperatures () 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 valence state promoted at low growth temperature introduces delocalized -derived states at the Fermi level (), responsible for the metallic state in , while the -related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and . 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 valence state in . The -related state becomes localized away from , opening a band gap for a semiconducting state. The band gap of the semiconducting is estimated to be , 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 dipole forbidden, and therefore, the semiconducting still shows reasonable transparency in the infrared-visible light region. The present insights into the role of in determining the electronic structure and defect chemistry of provide important guidance for use of in electrocatalysis and opto-electronics.
2 More- Received 2 June 2021
- Revised 16 August 2021
- Accepted 8 September 2021
DOI:https://doi.org/10.1103/PhysRevB.104.125136
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