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

In this paper, we report insights into the local atomic and electronic structure of ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ epitaxial thin films with controlled properties on $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}(001)$ substrates using pulsed laser deposition. Films grown at low temperatures ($<400{\phantom{\rule{0.28em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$) 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 ${\mathrm{Ni}}^{3+}$ valence state promoted at low growth temperature introduces delocalized $\mathrm{Ni}\phantom{\rule{0.28em}{0ex}}3d$-derived states at the Fermi level (${E}_{F}$), responsible for the metallic state in ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$, while the $\mathrm{Co}\phantom{\rule{0.28em}{0ex}}3d$-related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and $\ensuremath{\sim}+2$. 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 ${\mathrm{Ni}}^{2+}$ valence state in ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$. The $\mathrm{Ni}\phantom{\rule{0.28em}{0ex}}3d$-related state becomes localized away from ${E}_{F}$, opening a band gap for a semiconducting state. The band gap of the semiconducting ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ is estimated to be $<0.8\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, 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 $d\text{\ensuremath{-}}d$ dipole forbidden, and therefore, the semiconducting ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ still shows reasonable transparency in the infrared-visible light region. The present insights into the role of ${\mathrm{Ni}}^{3+}$ in determining the electronic structure and defect chemistry of ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ provide important guidance for use of ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ in electrocatalysis and opto-electronics.