STEM investigation of complex oxides at the atomic scale

Date: 19 January 2015

Venue: Campus Groenenborger - Room U0.25 - Groenenborgerlaan 171 - 2020 Antwerpen

Time: 4:00 PM

Organization / co-organization: Department of Physics

PhD candidate: Ricardo Egoavil

Principal investigator: J. Verbeeck & G. Van Tendeloo

Short description: Ph.D. defense of Ricardo Egoavil - Department of Physics

Abstract: The content of this thesis is devoted to exploiting the excellent capabilities of scanning transmission electron microcopy (STEM) to investigate complex oxide materials down to the atomic scale. A combination of different techniques like high-angle annular dark field scanning transmission electron microcopy (HAADF-STEM), electron energy loss spectroscopy (EELS) and energy-dispersive x-ray spectroscopy (EDS) were suitably chosen to obtain fundamental information in order to study the highly important physical phenomena of different oxide-based hetero-structures, with potential technological applications. Special interest is paid to correlate the exact morphology, crystal structure, chemical composition, as well as the electronic fine structure of the constituent elements to the corresponding physical properties. The major experimental results of this thesis are given from chapter 3 to 6. Chapter 3 describes the study of a ferroelectric thin film of PbTiO3 grown on self-organized SrRuO3 nanowires deposited on a mixed termination DyScO3 substrate. This is a prototype structure proposed to obtain a precise control of the ferroelectric domain walls (DW) in PbTiO3. Our study reveals that (i) the nanowires grow only on the Sc-terminated terraces of the substrate, and (ii) the size and the location of the DWs are strongly related to the nanowire positions. Chapter 4 provides direct imaging at the atomic scale of the B-site ordering in a double perovskite La2CoMnO6 thin film. The electronic structure of Co and Mn are confirmed in the ordered state, while the presence of antiphase boundaries (APBs) breaking the long range order is used to describe the apparent disordered state. Chapter 5 studies the electronic reconstruction at the interface between LaxSr1-xMnO3 and SrTiO3. EELS results reveal that significant amount of La interdiffusion into the SrTiO3 can be attributed as the main cause of obtaining reduced conductivity and magnetization at the interfaces of these class of materials. Chapter 6 explores the possibility of mapping phonon excitations in STEM-EELS experiments. A new EELS spectra normalization method is developed to bring to light spectral features with atomic resolution in the very low energy-loss regime opening the opportunities to perform vibrational spectroscopy. Finally, chapter 7 summarizes the above-mentioned research contributions in this thesis and discusses the direction for future work.