Electronic, Optical and Electrochemical Properties of Epitaxial Complex Oxide Heterostructures

Complex transition metal oxides have enough structural flexibility to produce a wide range of functional properties for electronics and energy storage applications. To control these properties, researchers grow thin films of these materials, seeking to control their crystal structure. Controlling crystal structure opens the door for tuning electronic structure and possibly achieving novel properties. Electronic structure in oxides is largely controlled by the charges on the cations. Ni⁴⁺ ions are energetically unstable and rarely found in chemical compounds. We have stabilized Ni⁴⁺ in a carefully designed superlattice of SrNiO₃ and LaFeO₃. The cubic perovskite SrNiO₃, which is not observed in bulk form, was shown to form in these superlattices, but only at the thickness of one crystalline unit, or unit cell.

We deposited alternating layers of SrNiO₃ and LaFeO₃ with varying thicknesses by means of molecular beam epitaxy and found that well-ordered, cubic SrNiO₃ nucleated only when a single unit cell of SrNiO₃ was included. By controlling the thickness of the adjacent LaFeO₃ layers, they could control the charges on the Fe and Ni cations.

 If a single unit cell of LaFeO₃ was used, resulting in a (SrNiO₃)₁/(LaFeO₃)₁ superlattice, the Ni was trivalent (Ni³⁺) and the Fe was tetravalent (Fe⁴⁺). However, as the LaFeO₃ layer thickness increased, the Ni became tetravalent (Ni⁴⁺) and the Fe trivalent (Fe⁴⁺). It was found that additional layers of LaFeO₃ induced larger rotations of the FeO₆ octahedral cages in the LaFeO₃ layers. The result was that the structure of the LaFeO₃ layers approached that of bulk LaFeO₃. This structural change modified the densities of electronic states such that holes could localize in the SrNiO₃ layer, producing Ni⁴⁺ in the (SrNiO₃)₁/(LaFeO₃)₅ superlattice.