Analytical Transmission Electron Microscopy for Nanostructure Interpretation in High-Entropy Oxides and Beyond
Restricted (Penn State Only)
- Author:
- Miao, Leixin
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 11, 2022
- Committee Members:
- John Mauro, Program Head/Chair
Nasim Alem, Chair & Dissertation Advisor
Jon-Paul Maria, Major Field Member
Zhiqiang Mao, Outside Unit & Field Member
Ismaila Dabo, Major Field Member - Keywords:
- Transmission Electron Mircoscopy
High Entropy Oxides
Ferroelectric
Domain Structure
Nanostructure
Image Processing
Ferrovalley Materials
Hybrid Improper Ferroelectric - Abstract:
- Understanding the Structure-property relationship, which is the connection between the underlying structure and the performance of the materials, is critical in the field of materials science and engineering. In particular, the nanostructures in the materials play an essential role in determining real-world performance. The nanostructures in the materials significantly impact the mechanical, optical, electrical, and chemical properties of the materials. Consequently, it is critical to understand the physics and chemistry of nanostructures. On the other hand, most areas of scientific research focused on the macroscopic properties and structures of newly discovered materials but paid less attention to the nanostructures and defects that could impact the properties for real-world applications. Transmission electron microscopy is one of the most powerful tools for exploring nanoscopic structures in materials. Modern aberration-corrected transmission electron microscopy has achieved sub-angstrom spatial resolution combined with high energy resolution for atomically resolved imaging (AC-STEM), hyperspectral imaging (STEM-EELS), and high-speed scanning nano-diffraction (4D-STEM). Advanced instrumentation has been further enhanced by the development of data-oriented analytical methods, especially the various machine learning algorithms. As a result, analytical transmission electron microscopy enabled the direct visualization and in-depth interpretation of the nanostructures in many classes of crystalline materials. This dissertation focuses on the characterization and interpretation of the microstructures in high-entropy oxides, hybrid improper ferroelectric oxides, and quantum materials with analytical transmission electron microscopy. The present work leverages advanced data processing techniques in electron microscopy to explore the nanostructures in the materials. The in-house developed algorithms for the atomic position tracking in the atomically resolved STEM images are first introduced. We integrated the algorithm into an open-source MatLab application named EASY-STEM that we developed. EASY-STEM allows the users to process the STEM images using a mouse cursor without the need to write codes. This algorithm can achieve picometer precision atomic position tracking and is robust for most crystalline materials systems. This work further demonstrates the potential application of unsupervised machine learning on the interpretation of the short-range ordering structures in a van der Waal crystal. The analysis reveals potential structures of the short-range ordering to be the triangular-shaped Te vacancy clusters propagating along multiple crystallographic axes. The algorithms mentioned above are employed for interpreting the nanostructures in high entropy oxide (HEO) thin films synthesized using pulsed laser deposition (PLD). The effect of the substrate temperature during the PLD synthesis is investigated. The analytical transmission electron microscopy was performed on a stacking HEO thin film grown at different substrate temperatures by PLD. The film grown at different substrate temperature show different lattice parameters while retaining the pristine rocksalt structure. The structural change can be correlated with the chemical environment shift of the Co ions caused by the deficiency of the constituent cations. Furthermore, the possibility of the engineering of nanostructures via controlling the PLD synthesis conditions is explored. A series of HEO thin films with varied film thickness and growth speed were investigated. Two distinct types of nanostructures, tweeds and spinel nano-cuboids, are discovered and characterized. The formation of the tweed and spinel nano-cuboids was linked with slow growth rate and large film thickness, and both nanostructures can be generated in the same film by combining the two synthesis conditions. The nature of the coexistence of the polar and nonpolar phases in the Ruddlesden-Popper phase perovskite oxide (Ca, Sr)3Mn2O7 (CSMO) was investigated at the atomic scale. A randomly distributed and densely populated nanostructure was discovered in the CSMO. The atomically resolved STEM imaging reveals the nanostructure to be the double-bilayer polar nanoregions (db-PNRs), and two types of db-PNRs, representing the polar phase viewing from two main crystallographic axes, are discovered. In-situ heating TEM shows the polar nanoregions, and nonpolar matrix can coexist up to 650 °C, indicating a stabilization mechanism. The stabilization mechanism is determined with atomic resolution monochromated EELS to be the Mn antisite defects on the Ca sites between the perovskite layers. Lastly, the domain structures in the ferrovalley van der Waals crystal VAgP2Se6 were investigated. The reduced symmetry of the ferrovalley materials can profoundly affect the formation of the orientational domains and impact the electronic structure and magnetic properties. In this work, we determine the domain morphology, the broken inversion, and the time-reversal symmetry of VAgP2Se6 using the combination of aberration-corrected scanning/transmission electron microscopy (AC-S/TEM), optical second harmonic generation (SHG) polarimetry, and magnetic hysteresis measurement. Through atomically resolved AC-STEM imaging conjointly with group theory symmetry analysis, we reveal and identify all the possible orientational domain variants stacking along the c-axis and three distinct types of domain boundaries in VAgP2Se6. In summary, this dissertation investigates the nanostructures in various crystalline material systems and explores the application of advanced electron microscopy techniques, including AC-STEM, in-situ TEM, monochromated EELS, 4D-STEM, and the advanced analytical algorithms employed to extract information from the TEM data and assist in the interpretation of the nanostructures. The chapters in this dissertation provide a detailed picture of the nanostructure characterization and show how analytical electron microscopy can shed light on the novel nanostructure discovery and engineering.