Atomically resolved EELS mapping of the interfacial structure of epitaxially strainedLaNiO3/LaAlO3superlattices

The interfacial atomic structure of a metallic ${\mathrm{LaNiO}}_{3}/{\mathrm{LaAlO}}_{3}$ superlattice grown on a ${\mathrm{LaSrAlO}}_{4}$ substrate was investigated using a combination of atomically resolved electron energy loss spectroscopy (EELS) at the Al $K$, Al ${L}_{2,3}$, Sr ${L}_{2,3}$, Ni ${L}_{2,3}$, La ${M}_{4,5}$, and O $K$ edges as well as hybridization mapping of selected features of the O $K$-edge fine structure. We observe an additional ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Al}}_{1\ensuremath{-}y}{\mathrm{Ni}}_{y}{\mathrm{O}}_{3}$ layer at the substrate-superlattice interface, possibly linked to diffusion of Al and Sr into the growing film or a surface reconstruction due to Sr segregation. The roughness of the ${\mathrm{LaNiO}}_{3}/{\mathrm{LaAlO}}_{3}$ interfaces is found to be on average around one pseudocubic unit cell. The O $K$-edge EELS spectra revealed reduced spectral weight of the prepeak derived from Ni-O hybridized states in the ${\mathrm{LaNiO}}_{3}$ layers. We rule out oxygen nonstoichiometry of the ${\mathrm{LaNiO}}_{3}$ layers and discuss changes in the Ni-O hybridization due to heterostructuring as possible origin.