Atomic scale studies of Perovskite transition metal oxides using STEM/EELS and DFT calculation

Abstract

Perovskite transition metal oxide materials have attracted a great deal of interest due to the various interesting properties such as ferromagnetism, ferroelectricity, piezoelectrics, metal to insulator transitions, superconductivity, etc.. Especially, recent advances in film growth technique enable creating artificial films or superlattice structures in a layer-by layer mode. Recently, a wide range of novel properties with artificially designed structures has been reported, such as enhanced ferromagnetic ordering, increased charge carrier density, two-dimensional electron gas, and increased optical absorption. These novel properties in the artificial ultrathin film or superlattice depend on the structure of the materials, especially, interface structure, intermixing, defects and strain. For a clear understanding of the novel properties on the perovskite transition metal oxides, microstructures should be studied in detail. In this thesis, atomic and electronic structure of the perovskite transition metal oxides are investigated by aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and density functional theory (DFT) calculation.

Aberration-corrected STEM achieves an atomic-scale real spacing imaging with directly interpretable atomic number contrast. This is realized by the improved aberration corrector that reduces the probe size down to ~ 1 Å. In addition, with the combination of EELS, it is possible to probe the electronic structure simultaneously with the imaging at atomic scale. The structure information required for the DFT calculations can be obtained from STEM imaging, and DFT calculation results provide theoretical electronic structure that is a reference for analyzing experimental EELS data, realizing an effective feedback each other. STEM/EELS combined with DFT analyses allow us to investigate a fundamental understanding of the properties of perovskite transition metal oxide thin films or superlattice and their relationship with interface or strain effects.