No description available.

No description available.

No description available.

abstract: This work is an investigation into the information provided by the concurrent use of in situ reflection high energy electron diffraction (RHEED) and reflection electron energy loss spectroscopy (REELS). The two analytical methods were employed during growth of metal, semiconductor and superconductor thin films by molecular beam epitaxy (MBE). Surface sensitivity of the REELS spectrometer was found to be less than 1 nm for 20 KeV electrons incident at a 2 degree angle to an atomically flat film surface, agreeing with the standard electron escape depth data when adjusted incident angle. Film surface topography was found to strongly influence the REELS spectra and this was correlated with in situ RHEED patterns and ex situ analysis by comparison with atomic force microscopy (AFM). It was observed in all the experimental results that from very smooth films the plasmon peak maxima did not fall at the predicted surface plasmon values but at slightly higher energies, even for nearly atomically flat films. This suggested the REELS plasmon loss spectra are always a combination of surface and bulk plasmon losses. The resulting summation of these two types of losses shifted the peak to below the bulk plasmon value but held its minimum to a higher energy than the pure surface plasmon value. Curve fitting supported this conclusion. / Dissertation/Thesis / Ph.D. Engineering Science 2012

abstract: Operando transmission electron microscopy (TEM) is an extension of in-situ TEM in which the performance of the material being observed is measured simultaneously. This is of great value, since structure-performance relationships lie at the heart of materials science. For catalyst materials, like the SiO2-supported Ru nanoparticles studied, the important performance metric, catalyst activity, is measured inside the microscope by determining the gas composition during imaging. This is accomplished by acquisition of electron energy loss spectra (EELS) of the gas in the environmental TEM while catalysis is taking place. In this work, automated methods for rapidly quantifying low-loss and core-loss EELS of gases were developed. A new sample preparation method was also established to increase catalytic conversion inside a differentially-pumped environmental TEM, and the maximum CO conversion observed was about 80%. A system for mixing gases and delivering them to the environmental TEM was designed and built, and a method for locating and imaging nanoparticles in zone axis orientations while minimizing electron dose rate was determined. After atomic resolution images of Ru nanoparticles observed during CO oxidation were obtained, the shape and surface structures of these particles was investigated. A Wulff model structure for Ru particles was compared to experimental images both by manually rotating the model, and by automatically determining a matching orientation using cross-correlation of shape signatures. From this analysis, it was determined that most Ru particles are close to Wulff-shaped during CO oxidation. While thick oxide layers were not observed to form on Ru during CO oxidation, thin RuO2 layers on the surface of Ru nanoparticles were imaged with atomic resolution for the first time. The activity of these layers is discussed in the context of the literature on the subject, which has thus far been inconclusive. We conclude that disordered oxidized ruthenium, rather than crystalline RuO2 is the most active species. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2016

abstract: Photocatalytic water splitting has been proposed as a promising way of generating carbon-neutral fuels from sunlight and water. In one approach, water decomposition is enabled by the use of functionalized nano-particulate photocatalyst composites. The atomic structures of the photocatalysts dictate their electronic and photonic structures, which are controlled by synthesis methods and may alter under reaction conditions. Characterizing these structures, especially the ones associated with photocatalysts’ surfaces, is essential because they determine the efficiencies of various reaction steps involved in photocatalytic water splitting. Due to its superior spatial resolution, (scanning) transmission electron microscopy (STEM/TEM), which includes various imaging and spectroscopic techniques, is a suitable tool for probing materials’ local atomic, electronic and optical structures. In this work, techniques specific for the study of photocatalysts are developed using model systems. Nano-level structure-reactivity relationships as well as deactivation mechanisms of Ni core-NiO shell co-catalysts loaded on Ta2O5 particles are studied using an aberration-corrected TEM. It is revealed that nanometer changes in the shell thickness lead to significant changes in the H2 production. Also, deactivation of this system is found to be related to a photo-driven process resulting in the loss of the Ni core. In addition, a special form of monochromated electron energy-loss spectroscopy (EELS), the so-called aloof beam EELS, is used to probe surface electronic states as well as light-particle interactions from model oxide nanoparticles. Surface states associated with hydrate species are analyzed using spectral simulations based on a dielectric theory and a density of states model. Geometry-induced optical-frequency resonant modes are excited using fast electrons in catalytically relevant oxides. Combing the spectral features detected in experiments with classical electrodynamics simulations, the underlying physics involved in this excitation process and the various influencing factors of the modes are investigated. Finally, an in situ light illumination system is developed for an aberration-corrected environmental TEM to enable direct observation of atomic structural transformations of model photocatalysts while they are exposed to near reaction conditions. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018

abstract: Topological insulators with conducting surface states yet insulating bulk states have generated a lot of interest amongst the physics community due to their varied characteristics and possible applications. Doped topological insulators have presented newer physical states of matter where topological order co&ndashexists; with other physical properties (like magnetic order). The electronic states of these materials are very intriguing and pose problems and the possible solutions to understanding their unique behaviors. In this work, we use Electron Energy Loss Spectroscopy (EELS) – an analytical TEM tool to study both core&ndashlevel; and valence&ndashlevel; excitations in Bi2Se3 and Cu(doped)Bi2Se3 topological insulators. We use this technique to retrieve information on the valence, bonding nature, co-ordination and lattice site occupancy of the undoped and the doped systems. Using the reference materials Cu(I)Se and Cu(II)Se we try to compare and understand the nature of doping that copper assumes in the lattice. And lastly we utilize the state of the art monochromated Nion UltraSTEM 100 to study electronic/vibrational excitations at a record energy resolution from sub-nm regions in the sample. / Dissertation/Thesis / M.S. Materials Science and Engineering 2013

abstract: Vibrational spectroscopy is a ubiquitous characterization tool in elucidating atomic structure at the bulk and nanoscale. The ability to perform high spatial resolution vibrational spectroscopy in a scanning transmission electron microscope (STEM) with electron energy-loss spectroscopy (EELS) has the potential to affect a variety of materials science problems. Since 2014, instrumentation development has pushed for incremental improvements in energy resolution, with the current best being 4.2 meV. Although this is poor in comparison to what is common in photon or neutron vibrational spectroscopies, the spatial resolution offered by vibrational EELS is equal to or better than the best of these other techniques. The major objective of this research program is to investigate the spatial resolution of the monochromated energy-loss signal in the transmission-beam mode and correlate it to the excitation mechanism of the associated vibrational mode. The spatial variation of dipole vibrational signals in SiO2 is investigated as the electron probe is scanned across an atomically abrupt SiO2/Si interface. The Si-O bond stretch signal has a spatial resolution of 2 – 20 nm, depending on whether the interface, bulk, or surface contribution is chosen. For typical TEM specimen thicknesses, coupled surface modes contribute strongly to the spectrum. These coupled surface modes are phonon polaritons, whose intensity and spectral positions are strongly specimen geometry dependent. In a SiO2 thin-film patterned with a 2x2 array, dielectric theory simulations predict the simultaneous excitation of parallel and uncoupled surface polaritons and a very weak excitation of the orthogonal polariton. It is demonstrated that atomic resolution can be achieved with impact vibrational signals from optical and acoustic phonons in a covalently bonded material like Si. Sub-nanometer resolution mapping of the Si-O symmetric bond stretch impact signal can also be performed in an ionic material like SiO2. The visibility of impact energy-loss signals from excitation of Brillouin zone boundary vibrational modes in hexagonal BN is seen to be a strong function of probe convergence, but not as strong a function of spectrometer collection angles. Some preliminary measurements to detect adsorbates on catalyst nanoparticle surfaces with minimum radiation damage in the aloof-beam mode are also presented. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

Detailed microstructural characterisation has been conducted in both ‘as-transformed’ and ‘hot deformed’ samples of 𝜏-MnAl-C using transmission electron microscopy. After hot deformation, true twins, dislocations, intrinsic stacking faults and precipitates of Mn3AlC are the main defects in the recrystallised grains. A significant fraction of non-recrystallised grains existed, which had microstructures based on combinations of high densities of true twins, dislocations, and deformation bands. The formation of the Mn3AlC precipitates was confirmed and related to the reduction of saturation magnetization and the increase in the Curie temperature of 𝜏-MnAl-C after hot deformation. Antiphase boundaries, which are believed to act as nucleation sites for reverse domains, were not observed in the hot deformed sample. Increasing structural disorder is expected for the tetragonal L10 𝜏-MnAl-C (c:a = 0.91) going from coherent true twin boundary to incoherent true twin boundary to the order twin boundary. This was demonstrated from the interface structure in the HAADF-HRSTEM images. The disorder at different types of twin boundaries is also associated with the degree of segregation of the constituent elements. By using STEM-EELS, higher Mn enrichment and Al deficiency was observed at the order twin boundary with a thickness about 6 nm, slightly Mn segregation was observed at incoherent true twin boundary with reduced thickness about 1.5-2 nm and no segregation was found at the coherent true twin boundary. In addition, the distribution of carbon is inhomogeneous across the twin boundary and carbon cluster was found at the twin boundary. Micromagnetic simulations and machine learning were conducted in an international collaboration with Danube University Krems, Austria, which enabled the quantification of the effect of twins on the magnetic properties of 𝜏-MnAl-C.

No description available.