Effects of the surface termination and the oxygen vacancy position on LaNiO3 ultra-thin films: First-principles study

While ultra-thin layers of the ${\mathrm{LaNiO}}_{3}$ film exhibit a remarkable metal-insulator transition as the film thickness becomes smaller than a few unit cell (u.c.), the formation of possible oxygen vacancies and their effects on the correlated electronic structure have been rarely studied using first principles. Here, we investigate the effects of the surface termination and the oxygen vacancy position on the electronic properties and vacancy energetics of ${\mathrm{LaNiO}}_{3}$ ultra-thin films under the compressive strain using density functional theory plus U $(\mathrm{DFT}+\mathrm{U})$. We find that oxygen vacancies can be easily formed in the Ni layers with the ${\mathrm{NiO}}_{2}$ terminated surface (0.5 u.c. and 1.5 u.c. thickness) compared to the structures with the LaO terminated surface and the in-plane vacancy is energetically favored than the out-of-plane vacancy. When two vacancy sites are allowed, the Ni square plane geometry is energetically more stable in most cases as two oxygen vacancies tend to stay near a Ni ion. Strong anisotropy between the in-plane and out-of-plane vacancy formation as well as the layer and orbital dependent electronic structure occur due to strain, surface termination, charge reconstruction, and quantum confinement effects. The in-plane vacancy of the ${\mathrm{NiO}}_{2}$ terminated structure is favored since the released charge due to the oxygen vacancy can be easily accommodated in the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital, which is less occupied than the ${d}_{{z}^{2}}$ orbital. Remarkably, the oxygen vacancy structure containing the Ni square-plane geometry becomes an insulating state in $\mathrm{DFT}+\mathrm{U}$ with a sizable band gap of 1.2 eV because the large crystal field splitting between ${d}_{{z}^{2}}$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals in the square-plane favors an insulating state and the Mott insulating state is induced in other Ni sites due to strong electronic correlations. In the thin-film structure without oxygen vacancies, the strong correlation effect in $\mathrm{DFT}+\mathrm{U}$ drives a pseudogap ground state at the Fermi energy, similarly as the experimental photo-emission spectra; however, the variation of the $\mathrm{DFT}+\mathrm{U}$ electronic structure depending on the surface termination becomes weaker compared to those obtained in DFT.