Depth-Dependent Chemical and Structural Profiling of Oxide Thin Films with Scanning Transmission Electron Microscopy.

다국어 초록 (Multilingual Abstract)

Complex oxide perovksites are among the most versatile materials from a chemical, structural, and applications perspective. The perovskite lattice can expand or contract to fit the structure of a substrate due to the flexible oxygen octahedral network, opening up epitaxial thin film perovskites to be widely studied. Novel phases of materials are formed when grown with thin film morphology, having tunable properties through interfacial engineering. Metal oxide heterostrucutres are additionally a testbed for fundamental studies on the chemical, atomic, and electronic reconstructions at complex heterointerfaces. The thin film systems of interest presented here are La1−xSrxMnO3 (LSMO), multilayered LSMO-(La1−xSrxCrO3, and SrRuO3 (SRO) grown on SrTiO3. Each of these systems are of interest for their attractive magnetic properties. LSMO is a colossal magnetoresistive ferromagnet that has been the focus of many thin film studies, but stymied in application due to the loss of magnetic character in ultra-thin films. Most recently, an investigation into multilayered LSMO-(La1−xSrxCrO3, or LSCO) heterostructures has uncovered new possibilities for ultra-thin LSMO. SRO, on the other hand, is an itinerant ferromagnet that has also been the subject of extensive study, having properties extremely sensitive to growth conditions and resultant film structure. Here, these films are investigated with scanning transmission electron microscopy (STEM) imaging in combination with electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy. The studies included here serve two purposes. First, to produce material results on with the STEM. In each case, advanced electron microscopy is used to characterize the films to determine structure-property relationships as a function of film depth to understand interfacial phenomena. Second, the materials are used to develop STEM image and EELS analysis techniques as well as isolate potential shortcomings of STEM analysis due to sample preparation effects. From a structural perspective, both LSMO and SRO thin films are model systems to develop analysis methods, having similar cationic, but different oxygen sublattice structure as well as depth-dependent structure that varies with proximity to the interface.