title: Characterization of Atomic Structure and Electrostatic Characteristic in Complex Oxides by 4D-STEM and STEM-EELS
Creators: name: Yang, Chao
Publisher: name: UNSPECIFIED
Year: 2024

description: Complex oxides exhibit a wide range of intriguing physical properties resulting from intricate atomic arrangements and complex electronic interactions. Accurate characterization of the atomic structure and electrostatic characteristics of these materials is crucial for elucidating their functional properties and enabling the design of novel devices. In this study, we employ advanced electron microscopy techniques, specifically four-dimensional scanning transmission electron microscopy (4D-STEM) and electron energy loss spectroscopy (EELS), to investigate the atomic structure and electrostatic properties of complex oxides. 


The 4D-STEM technique enables the acquisition of rich information from electron diffraction patterns, allowing the precise recording of atomic positions and the analysis of complex electron interactions within the crystal lattice. High-resolution images of the atomic structure can be reconstructed, enabling the identification of defects, interfaces, and subtle structural distortions present in the complex oxides. The electrostatic field or charge distribution within a thin sample can also be extracted by calculating the momentum transfer of the electron beam. By combining 4D-STEM with STEM-EELS, one can obtain a comprehensive understanding of the local atomic and electronic structure, as well as the charge distribution and their interactions within specific regions of interest in complex oxides, e.g., defects, interfaces, surfaces, and grain boundaries. 
 

In this thesis, we firstly studied the atomic structure and electrostatic characteristics at a grain boundary (GB) in a SrTiO3 bicrystal. We demonstrate that the Σ5 GB is rich in Ti and poor in Sr. We investigate possible effects on the variation of the atomic electrostatic field, including oxygen vacancies, Ti valence change, and accumulation of cations. A negative charge resulting from a space charge zone in SrTiO3 compensates for a positive charge accumulated at the GB, which is in agreement with the double-Schottky-barrier model. It demonstrates the feasibility of characterizing the electrostatic properties at the nanometer scale by 4D-STEM, which provides comprehensive insights into understanding the GB structure and its associated effects on the electrostatic properties.


We also investigated the atomic structures and electrostatic characteristics at specific regions surrounding a Ruddlesden-Popper fault in a NdNiO3 film, the interfaces between SrTiO3 and NdNiO2, and the surface of the partially reduced nickelate films. The NdNiO3 system has received considerable attention due to the discovery of superconductivity in Nd0.8Sr0.2NiO2. In rare earth nickelates, Ruddlesden-Popper (RP) faults play a significant role in the functional properties, motivating us to investigate their microstructural characteristics and electronic structure. Therefore, we employed aberration-corrected scanning transmission electron microscopy and spectroscopy to study a NdNiO3 film grown by layer-by-layer molecular beam epitaxy (MBE). We found RP faults with multiple configurations in high-angle annular dark-field images. Quantitative analysis of the variation in lattice constants indicates that large strains exist around the substrate-film interface. We demonstrate that the Ni valence change around RP faults is associated with strain and structure variation. This work provides insight into the microstructure and electronic-structure modifications around RP faults in nickelates. 


In addition, interface polarity plays a vital role in the physical properties of oxide heterointerfaces, as it can cause specific modifications of the electronic and atomic structure. Reconstruction due to the strong polarity of the NdNiO2/SrTiO3 interface in recently discovered superconducting nickelate films may play an important role, since no superconductivity has been observed in the bulk. Thus, we investigated the effects of oxygen distribution, polyhedral distortion, elemental intermixing, and dimensionality in NdNiO2/SrTiO3 superlattices grown on SrTiO3 (001) substrates. Oxygen distribution maps show a gradual variation of the oxygen content in the nickelate layer. Remarkably, we demonstrate a thickness-dependent interface reconstruction due to a polar discontinuity. An average cation displacement of ∼0.025 nm at interfaces in 8NdNiO2/4SrTiO3 superlattices is twice as large as that in 4NdNiO2/2SrTiO3 superlattices. 


Furthermore, the polarity of a surface can also affect the electronic and structural properties of oxide thin films through electrostatic effects. Understanding the mechanism behind these effects requires knowledge of the atomic structure and electrostatic characteristics at the surface. We used annular bright-field (ABF) imaging to investigate the surface structure of a Pr0.8Sr0.2NiO2+x (0 &lt; x &lt; 1) film. We observed a polar distortion coupled with octahedral rotations in a fully oxidized Pr0.8Sr0.2NiO3 sample and a stronger polar distortion in a partially reduced sample. Its spatial depth is about three unit cells from the surface. Additionally, we used 4D-STEM to directly image the local atomic electric field surrounding Ni atoms near the surface and discovered distinct valence variations of Ni atoms, which were confirmed by EELS. Our results suggest that the strong surface reconstruction in the reduced sample is closely related to the formation of oxygen vacancies by topochemical reduction. Benefited from the advantages of 4D-STEM and STEM-EELS, this thesis presents the detailed studies of the atomic structure and electrostatic characteristic at defects, interfaces, surfaces, and grain boundaries in complex oxides, providing insights into understanding the mechanism of the physical behaviors at the atomic scale.