Zhou, Dan (2016)
Aberration-Corrected Analytical Transmission Electron Microscopy of Light Elements in Complex Oxides: Application and Methodology.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

The advent of aberration correctors for electron optical lenses at the end of 20th century has brought atomic resolution analysis of the materials into a new era. In this thesis, the new possibilities of application and methodology on aberration-corrected analytical transmission electron microscopy (TEM) of light elements in complex oxides are explored by experiments and image simulations, with the emphasis on annular bright-field (ABF) imaging. The arrangement and bonding of light elements, like lithium (Li) and oxygen (O), in complex oxides plays a crucial rule in the material’s properties, however the characterization of the materials remains challenging. In recent years ABF imaging has become a popular imaging technique owing to its ability to map both light and heavy elements. I start from the application of ABF on qualitatively determining O’s distribution in ZrO2-La2/3Sr1/3MnO3 (LSMO) pillar–matrix thin films, together with the application of high-angle annular dark-field (HAADF) and electron energy-loss spectroscopy (EELS) to obtain a fuller picture of the investigated complex oxide. After that, the methodology study of ABF imaging, concerning the quantitative determination of atom column position and concentration, is presented. The accuracy of atom column position determination is of great importance for investigating atomic structure defects like elastic and plastic strains. Atomic-scale control of the synthesis of complex oxide materials envisages the atomic-scale properties and requires the knowledge of atomic-scale characterization. The ZrO2-LSMO pillar–matrix thin films were found to show anomalous magnetic and electron transport properties controlled by the amount of ZrO2. With the application of an aberration–corrected analytical transmission electron microscope (TEM), structure and interfacial chemistry of the system, especially of the pillar–matrix interface were revealed at atomic resolution. In addition, three types of Mn segregated antiphase boundaries (APBs) connecting ZrO2 pillars were investigated by HAADF and ABF imaging. The local atomic structure, chemical composition, cation valence and electric field were determined at atomic-scale. These results provide detailed information for future studies of macroscopic properties of these materials. Moreover, a consequence of aberration correctors is the high electron dose rate in the scanning mode. This can lead to radiation-induced modifications of materials. I studied the electron beam induced reconstruction of three types of APBs. With the utilization of HAADF scanning transmission electron microscopy (STEM), ABF STEM and EELS, the motion of both heavy and light element columns under moderate electron beam irradiation are revealed at atomic resolution. Besides, Mn segregated in the APBs was found to have reduced valence states, which can be directly correlated with oxygen loss. Charge states of the APBs are finally discussed based on these experimental results. This study provides support for the design of radiation engineering solid-oxide fuel cell materials. The determination of atom positions from atomically resolved transmission electron micrographs is fundamental for the analysis of crystal defects and strain. Contrast formation in ABF is partially governed by the phase of the electron wave, which renders the technique more sensitive to the tilt of the electron beam with respect to the crystal zone axis than in high-angle annular dark-field (HAADF) imaging. I show this sensitivity experimentally and use image simulations to quantify this effect. This is essential for future quantitative ABF studies including error estimation. Another aspect of quantification is the number of atoms in an atom column. The attempt to quantify Li concentration by ABF imaging has been done by simulations. The influences of convergence semi-angle, collection semi-angle, and defocus are explored, while direct correlation with experimental results need more theoretical investigations in this area. Semi-quantification of the Li amount was studied by EELS in case of the particle-size dependent delithiation process of LiFePO4. From the core-loss region and low-loss region analysis it is found that the sample with particle size of 25 nm delithiates homogeneously over the whole particle, whereas the 70 nm and 150 nm particles form an FePO4 core and a LiFePO4 shell. The practical considerations, like radiation damage, delocalization, interface effects and so on are also discussed.

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