论文标题
表面终止和氧空位位置以及对LANIO $ _ {3} $超薄电影的影响：第一原理研究
Effects of the surface termination and oxygen vacancy positions and on LaNiO$_{3}$ ultra-thin films: First-principles study
论文作者
论文摘要
尽管随着膜的厚度变得小于几个单位单元（U.C.），但lanio $ _3 $薄膜的超薄层表现出了显着的金属 - 绝缘体过渡，但很少研究氧空位及其对相关电子结构的影响。在这里，我们研究了使用密度功能理论加U（DFT+U）的LANIO $ _3 $ Ultra-Thin膜的电子性质和空位能量的影响的表面终止和氧空位位置的影响。我们发现，与LAO终止表面的结构相比，NiO $ _2 $终止表面（0.5 U.C.和1.5 U.C.厚度）可以轻松地在Ni层中形成氧空位，而平面空位的能量比面积外空置偏爱。当允许两个空位位点时，在大多数情况下，NI方平面的几何形状在能量上更加稳定，因为两个氧气空位倾向于保持在ni离子附近。 Nio $ _2 $终止结构的平面空缺受到青睐，因为由于氧气空位而释放的费用很容易容纳在$ d_ {x^2-y^2} $ orbital中，而不是$ d_ {z^2} $ orbital的占用。 Remarkably, the oxygen vacancy structure containing the Ni square-plane geometry becomes an insulating state in DFT+U with a sizable band gap of 1.2eV because the large crystal field splitting between $d_{z^2}$ and $d_{x^2-y^2}$ orbitals in the square-plane favors an insulating state and the Mott insulating state is induced in other Ni sites due to strong电子相关性。
While ultra-thin layers of the 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 oxygen vacancies and their effects on the correlated electronic structure have been rarely studied. Here, we investigate the effects of the surface termination and the oxygen vacancy position on the electronic properties and vacancy energetics of LaNiO$_3$ ultra-thin films using density functional theory plus U (DFT+U). We find that oxygen vacancies can be easily formed in the Ni layers with the 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. The in-plane vacancy of the NiO$_2$ terminated structure is favored since the released charge due to the oxygen vacancy can be easily accommodated in the $d_{x^2-y^2}$ 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 DFT+U with a sizable band gap of 1.2eV because the large crystal field splitting between $d_{z^2}$ and $d_{x^2-y^2}$ 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.
