Browsing by Subject "Black arsenic"

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    Atomic and electronic structures of local crystalline defects in perovskite stannates studied using analytical STEM
    (2020-07) Yun, Hwanhui
    The past decades have seen the rapid advance of transmission electron microscopy (TEM) techniques from aberration correction of the electron probe for a supreme spatial resolution to development of various spectroscopy detectors for better detection efficiency and higher energy resolutions. Nowadays, seeing atoms has become a daily routine to electron microscopists, and atomic-scale characterization accompanied by spectroscopic techniques is widely performed to understand the physical and chemical properties of diverse materials. In particular, combination of scanning TEM (STEM) and spectroscopy, which is known as analytical STEM, is an optimal tool to explore atomically resolved crystalline, elemental, chemical, and electronic structures of materials. Analytical STEM includes STEM-energy dispersive X-ray (EDX) and STEM-electron energy-loss spectroscopy (EELS) and has been demonstrated to be ideal to study local crystalline structures such as the interface of materials, crystalline defects, etc. To well perform such analysis, adequate TEM modes and conditions need to be employed and correct interpretation of STEM images and spectroscopic data should be conducted, which is possible by integrating STEM experiments and related computational simulations. In this thesis, analytical STEM is employed to study the local atomic and electronic structures embedded in perovskite alkaline earth stannates, particularly BaSnO3, by means of various experimental and computational methods. First, in Ch. 2 and 3, micro- and atomic crystalline structures in BaSnO3 thin films are investigated via TEM images and diffraction patterns. Image simulations based on the Multislice theory assist to explain the observed complex image contrast. In Ch. 4, detailed electronic structures of BaSnO3 are explored by combination of EELS and ab initio calculations. Next, in Ch. 5 and Ch. 6, atomic and electronic structures of particular local crystalline defects in BaSnO3 thin films, new line defects (Ch. 5) and threading dislocations (TDs) (Ch. 6), are researched by combination of analytical STEM and ab initio calculations. Additionally, a study of black arsenic (BAs), a promising two-dimensional material, is also presented in Ch. 7. The individual projects that are included in this thesis are as follows: Ch. 2. Microstructure analysis of BaSnO3 thin films grown on different substrates Material properties of epitaxial thin films are directly affected by the microstructure of thin films, e.g. uniformity, orientation of grains, crystalline defects. Hence, in general, microstructure analysis is the most basic and essential step in the study of thin films. When various TEM techniques along with X-ray diffraction (XRD) are used, comprehensive and detailed micro- and atomic structures of a material can be obtained. Here, epitaxial La-doped BaSnO3 (La:BaSnO3) films grown on different perovskite substrates are analyzed via TEM to examine micro- and atomic structures in the films. Rotational disorders of columnar grains present in the thin films are visualized by conventional TEM under a two-beam condition, and the degree of the disorders is quantified by selected-area electron diffraction. Atomic structures near the film-substrate interfaces are inspected via high-resolution annular dark-field (ADF)-STEM images, and correlation between the lattice constant mismatch and the type and density of misfit dislocations (MDs) present at the interfaces is revealed. Ch. 3. Visualization of misfit dislocation network at the BaSnO3-substrate interface using ADF-STEM At the interface of two distinct materials, e.g. a film and a substrate, lattice mismatch between the two causes formation of MDs, which otherwise would form an epitaxially coherent interface. And, as shown in Ch. 2., the interfacial structures can have a significant impact on the overall properties of a film. Here, the MD network at the BaSnO3-LaAlO3 interface is visualized using plan-view ADF-STEM images from BaSnO3/LaAlO3 bilayers. ADF-STEM images of the bilayers are demonstrated to be sensitive to the electron beam direction, the thickness of each layer, and the defocus of the electron beam. To understand the effect of each parameter, STEM beam propagation through the bilayers is simulated, and the focal series of high-angle-ADF (HAADF)-STEM images are computed and compared with experimental data. The study provides a nice example of understanding complex contrast in ADF-STEM images in terms of channeling behavior. Ch. 4. Electronic structure of BaSnO3 analyzed using EELS and ab initio theory Along with crystalline structural analysis via STEM imaging, electronic structures of materials can be studied using EELS in STEM. In interpretation of complicated EELS data to extract useful information, introduction of ab initio calculations is an effective and powerful approach. In this chapter, experimental low-loss and core-loss EELS spectra obtained from bulk BaSnO3 are analyzed in detail with the electronic band structures of the material computed based on density functional theory (DFT). Low-loss EELS spectrum provides information of outer shell structures including the band gap, plasmon oscillation, and interband excitations. Core-loss EELS of O K, Ba M4,5, Sn M4,5, and Ba N4,5 edges with distinct fine structures are presented and spectral shape of O K edge is explained via the unoccupied density of states (DOS) in the conduction band of BaSnO3. This study demonstrates how to understand features in EELS by incorporating ab initio calculations. Ch. 5. Atomic and electronic structures of 1D line defects in BaSnO3 In the past decade, new kinds of atomic-scale crystalline defects have been discovered in perovskite-structured materials via sub-angstrom resolution STEM imaging. Here, a unique and newly found line defect in BaSnO3 thin films is reported and analyzed using analytical STEM and ab initio calculations. This line defect is aligned along the film growth direction and shows an atypical atomic configuration. Structure and composition of the line defects are investigated using atomic resolution STEM-EDX, which is supported by structural optimization using DFT-based simulations. The calculated electronic structure of the line defect reveals that the defect structure induces additional electronic bands that cross the Fermi level, which is distinct from the wide bandgap host BaSnO3. The nature of the defect is further explored via parametric simulations, and lastly, comparison of simulated and experimental O K edges from the line defect reaffirms the localized electronic structure modifications at the defect. Ch. 6. Dopant segregation around dislocations in La:BaSnO3 In epitaxially grown thin films, TDs are the most ubiquitous crystalline defects, and they alter the local atomic structures and modify physical and chemical properties of the material. In particular, TDs in BaSnO3 thin films are believed to be primarily responsible for the limited electronic transport properties of the material. However, detailed research on the properties of TDs in BaSnO3 is lacking. Here, local atomic, compositional, and electronic structures of dominant TDs in La:BaSnO3 thin films, [001]/(100) and [001]/(110) type edge dislocations, are explored. Core structures of dislocations show variations in their size and atomic configurations; especially, distinct core structures with Sn enrichment are also observed. La dopant segregation adjacent to TDs is monitored, and the observation is explained using a strain field map of TDs. Discussions in this chapter are from parts of the preliminary results of an ongoing project. Ch. 7. Dielectric response and stability study of black arsenic. Black arsenic (BAs) is a two-dimensional van der Waals layered material that has a puckered honeycomb structure. BAs has gained increased interest due to its anisotropic properties and promising performance in devices. Here, the physical and chemical properties of BAs are examined using analytical STEM. Three-dimensional crystalline structure and the degree of anisotropy are directly evaluated via STEM imaging, and the dielectric response of BAs is measured as a function of the number of layers using STEM-EELS. Lastly, the stability of BAs under different ambient environments is studied in detail, and its high sensitivity toward moisture in the air is discussed.