Browsing by Subject "Electron microscopy"

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    Linking morphology and reactivity: growth and ligand-assisted dissolution of cobalt oxyhydroxide.
    (2010-08) Myers, Jason C.
    Reactions at the interface of solid materials have a significant role in many fields of study, ranging from environmental science to industrial manufacturing. Identifying and quantifying the reactive surface area of these materials is vital to understanding the reactions in which they participate. The most basic effect of reactive surface area is governing the reaction rate at the surface but, in some cases, it is necessary to have a far more detailed understanding of the surface structure. Many reactions occur most efficiently, or even exclusively, at specific types of surface site. The ability to identify and measure these sites could dramatically improve the design of many applications, such as heterogeneous catalysts or waste remediation systems. One proposed method of measuring reactive surface area is the use of carefully selected probe molecules that are specifically reactive with the surface sites of interest. This work focuses on the development of a method for analyzing the surface characteristics of heterogenite (β-CoOOH) using the ligand iminodiacetic acid (IDA) as a probe. To investigate this system, first a range of model materials were necessary. The method of heterogenite synthesis was explored, revealing that a surprising amount of control can be exerted over the final particle morphology by altering simple factors such as reaction temperature or choice of oxidizing agent. The ligand-assisted dissolution of heterogenite by IDA produces a mixture of sfac and u-fac isomers of Co(IDA)2 –, and the relative amount of each isomer depends upon the surface characteristics of the heterogenite. When heterogenite particles were aged in suspension at room temperature, a rapid evolution of the number and type of surface site present was observed. This change was tracked by reacting the particles with IDA then separating and quantifying the resulting Co(IDA)2 – isomers. Through this method, it was found that the surface evolution occurs more slowly when aged in lower pH buffer. The connection between particle morphology and reactivity was strengthened when a link was found between the height of cylindrical heterogenite plates and the ratio of isomers formed during the dissolution reaction. From this, an empirical relationship between particle height and the relative amount of s-fac isomer was derived. This relationship allowed the tracking of particle growth via dissolution reactions rather than direct measurements. The final connection between morphology and reactivity was discovered when kinetic and thermodynamic studies were undertaken. The rate of reaction when the reactant concentrations are altered suggests that both surface diffusion and product desorption processes are involved in determining the overall reaction rate. Finally, it was hypothesized that the reactivity of some sites varies with temperature. Thus, the ratio of products produced depends not only on the number and type of surface site present, but also on the temperature of the reaction. Additional work is necessary to quantify this temperature dependence.
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    Structural Investigation of Electron-Beam Sensitive Zeolites and Metal-Organic-Frameworks Using Analytical Transmission Electron Microscopy
    (2018-08) Kumar, Prashant
    An interesting class of materials that has become ubiquitous in our daily lives is the family of zeolites, which are porous scaffolds made of silicon, oxygen and aluminum atoms. Zeolites act like a coffee filter paper with pores of molecular dimensions that can be tailored to separate molecules of different diameters by adjusting the sizes of the pore openings from 2 Å (1 Å = 0.0000000001 m) to 10 Å. For example, over 90% of commercially available detergents contain zeolites, which act as water softeners by selectively removing calcium and magnesium ions from water, while any product that can be traced back to a petrochemical refinery (like fuels, chemicals and pharmaceutics) contains molecules that have passed through selective zeolite pores numerous times. To date, 231 unique zeolite frameworks have been synthesized while computer simulations predict over 330,000 new structures. TEM imaging and electron diffraction has contributed in the crystal structure determination of many of the known 231 synthesized zeolite frameworks. However, the recent development of few-atom-thick zeolites (2-dimensional zeolite nanosheets) and new classes of porous materials based on metal-organic-frameworks (MOFs) pose new demands and create new opportunities for electron microscopy. The objective of this dissertation work is to determine the atomic arrangement in zeolite nanosheets and metal organic frameworks using transmission electron microscopy to develop a fundamental relationship between their crystal structure and performance as catalysts and membranes. Using electron diffraction, imaging, spectroscopy and digital image processing, TEM data acquisition and analysis routines have been developed to mitigate electron beam sensitivity of these materials. Implementation of the developed routines enabled crystal structure, growth and defect analysis down to the atomic scale, leading to novel findings and implications discussed in detail here.
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    A theoretical study of dopant atom detection and probe behavior in STEM
    (2013-12) Mittal, Anudha
    Very detailed information about the atomic and electronic structure of materials can be obtained via atomic-scale resolution scanning transmission electron microscopy (STEM). These experiments reach the limits of current microscopes, which means that optimal experimental design is a key ingredient in success. The step following experiment, extraction of information from experimental data is also complex. Comprehension of experimental data depends on comparison with simulated data and on fundamental understanding of aspects of scattering behavior. The research projects discussed in this thesis are formulated within three large concepts.1. Usage of simulation to suggest experimental technique for observation of a particular structural feature. Two specific structural features are explored. One is the characterization of a substitutional dopant atom in a crystal. Annular dark field scanning transmission electron microscope (ADF-STEM) images allow detection of individual dopant atoms in a crystal based on contrast between intensities of doped and non-doped column in the image. The magnitude of the said contrast is heavily influenced by specimen and microscope parameters. Analysis of multislice-based simulations of ADF-STEM images of crystals doped with one substitutional dopant atom for a wide range of crystal thicknesses, types and locations of dopant atom inside the crystal, and crystals with different atoms revealed trends and non-intuitive behaviors in visibility of the dopant atom. The results provide practical guidelines for the optimal experimental setup regarding both the microscope and specimen conditions in order to characterize the presence and location of a dopant atom. Furthermore, the simulations help in recognizing the cases where detecting a single dopant atom via ADF-STEM imaging is not possible. The second is a more specific case of detecting intrinsic twist in MoS2 nanotubes. Objective molecular dynamics simulations coupled with a density functional-based tight-binding model revealed that a stress-free single-walled (14,6) MoS2 nanotube has a torsional deformation of 0.87 °/nm. Comparison between simulated electron diffraction patterns and atomic-resolution ADF-STEM images of nanotubes with and without the small twist suggested that these experimental techniques are viable routes for detecting presence of the torsional deformation. 2. Development of theory to cast light on aspects of scattering behavior that affect STEM data. STEM probe intensity oscillates as the probe transmits through a crystalline sample. The oscillatory behavior of the probe is extremely similar during transmission through 3-D crystals and the hypothetical structure of an isolated column of atoms, a 1-D crystal. This indicates that the physical origin of oscillation in intensity is not due to scattering of electrons away from one atomic column and subsequent scattering back from neighboring columns. It leaves in question what the physical origin or intensity oscillation is. This question was answered here by analysis of electron beam behavior in isolated atomic columns, examined via multislice-based simulations. Two physical origins, changes in angular distribution of the probe and phase shift between the angular components, were shown to cause oscillation in beam intensity. Sensitivity of frequency of oscillation to different probe and sample parameters was used to better understand the influence of the two physical origins on probe oscillation. 3. Acquisition of atomic-scale STEM data to answer specific questions about a material. Graphene, due to its 2-Dimensionality, and due to its thermal, optical, electrical, and mechanical properties, which are conducive to providing a unique material for incorporation in devices, has gained a lot of interest in the research world and even spurred start-ups. There are several feasible routes of graphene synthesis, among which chemical exfoliation of graphite is a promising method for mass-scale, low-cost production of graphene. Chemical exfoliation of graphite to produce graphene is a two-step process: oxidation to exfoliate the graphite layers, which results in graphene oxide, and reduction of graphene oxide, to produce graphene as a final product. Here, we examined the atomic and electronic structure of graphene oxide and of the reduced sheets. Two different methods of reduction, thermal reduction in vacuum and aqueous reduction in atmosphere, were compared. TEM-based techniques were used for nanoscale characterization. GO was synthesized using the modified Hummer's method and presence of single layer sheets was confirmed by electron diffraction (ED). Non-uniform distribution of oxygen in GO was observed using Z-contrast imaging in STEM. Presence of sp2 and sp3 hybridized carbon bonds in GO was confirmed by examining the fine structure of carbon K-edge in electron energy loss spectra (EELS). Changes in oxygen distribution and electronic structure of carbon were monitored using the same techniques in situ during thermal reduction of GO to graphene. Change in oxygen level and carbon hybridization was gradual with increasing temperature, with complete conversion to oxygen-absent, sp2 hybridized carbon sheet at 1000 ̊C. Gradual change confirmed the ability to fine-tune the level of oxygen on carbon sheets using thermal reduction in vacuum. Instantaneous heating from room temperature to 1000 ̊C showed formation of holes in the graphene product. A several-hour gradual heating process was suggested to decrease perforation in graphene sheets. The second reduction process, aqueous thermal reduction in ambient pressure, did not lead to completely sp2 hybridized carbon sheets, observed using EELS. Presence of oxygen was also observed via x-ray photoemission spectra (XPS). Yet, electrical resistance of the product was 5 orders of magnitude less than the starting GO sheets. This property was explained by examining the atomic structure of the reduced GO. High resolution conventional TEM (CTEM) images of nano-scale section of the reduced GO showed randomly shaped crystalline areas and amorphous areas, with crystalline area being above the 2-D percolation threshold and thus explaining the conductive property.