The origin of the electrical and optical properties of Cu-substituted NiO (Cu:NiO) polycrystalline bulks synthesized via a solid-state reaction is reported. The partial substitution of Ni sites with Cu led to a drastic decrease of the electrical resistivity from 7.73 × 108 to 6.51 × 104 Ω·cm and a reduction in the energy for the self-trapping barrier from 0.58 to 0.24 eV in accordance with small polaron hopping conduction. The well-sustained band gap of 3.1 eV and antiferromagnetic transition temperature of 453 K demonstrate that the strength of the electron correlation in NiO can persist even at a high Cu concentration up to 22 atomic percent. Density functional theory calculations confirm that the Cu 3d orbital encourages d-p hybridization between metal cations and oxygen anions at the valence band maximum. As a consequence, this hybridization plays a critical role in improving the polaron hopping efficiency without much suppression of the Mott-Hubbard interaction and thus retaining the wide band gap nature.
Bibliographical noteFunding Information:
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2019R1A6A1A11055660). It was also partly supported by the MEXT, Element Strategy Initiative to form a research core. M. S. O. acknowledges the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and ICT for the First-Mover Program for Accelerating Disruptive Technology Development (NRF-2018M3C1B9088458). S. L. was supported by the National Research Foundation of Korea through NRF-2019M3F3A1A02072175. We also thank Prof. H. Lei for valuable discussions.
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All Science Journal Classification (ASJC) codes
- Inorganic Chemistry