Browsing by Subject "Transmission electron microscopy"

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    Dislocations in Magnetite: Experimental Observations of their Structural, Magnetic, and Low-temperature Effects
    (2013-09) Lindquist, Anna K.
    Magnetite (Fe3O4) is the most important mineral to the rock magnetic and paleomagnetic communities and is ubiquitous in igneous, sedimentary, and metamorphic rocks. Larger multidomain (MD) magnetite grains are more common than single domain grains, so understanding how they record paleomagnetic fields would be a boon to paleomagnetists. MD magnetite grains are divided into multiple domains, regions with uniform magnetization, separated by domain walls. Domain walls sweep through magnetite grains easily, so slight changes in ambient magnetic fields can alter the magnetization of MD magnetite. Because of this, MD magnetite is not considered reliable for paleomagnetic studies, and the mechanisms by which MD grains may record past magnetic fields are not well understood. Dislocations, linear crystallographic defects, may increase magnetic coercivity by pinning domain walls in place. This study, for the first time, experimentally investigates this pinning behavior by using a transmission electron microscope (TEM) to simultaneously image magnetic domain walls, dislocations, and low-temperature twin structures. Magnetite grains were deformed in the dislocation glide regime, which is active in natural magnetite grains. Dislocations were not uniformly distributed throughout the sample, but regions with more and longer dislocations pinned domain walls more strongly. First-order reversal curve diagrams demonstrate the presence of regions with pinning strengths of over 125 mT. The strength of domain wall pinning at dislocations was found experimentally and theoretically to be proportional to dislocation length, with longer dislocations pinning more strongly. Average pinning fields were around 0.2 mT. Magnetite grains with more uniformly distributed dislocations would likely have coercivities that were high enough to enable MD magnetite to record geomagnetic fields over geologic timescales. Further, low-temperature TEM and magnetic studies demonstrated that dislocations can affect twin growth in magnetite below the Verwey transition. Deformed magnetite samples had more soft-shouldered Verwey transitions and were able to retain more remanence after low-temperature demagnetization (LTD). Therefore, MD magnetite grains may be able to retain relevant magnetizations, even after LTD. Dislocation length, density, and distribution are then all important considerations when investigating the ways in which MD magnetite may retain a stable record of paleomagnetic field characteristics, even after LTD.
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    Inelastic scattering in STEM for studying structural and electronic properties of chalcogenide-based semiconductor nanocrystals
    (2013-09) Gunawan, Aloysius Andhika
    Transmission electron microscopy (TEM) relies upon elastic and inelastic scattering signals to perform imaging and analysis of materials. TEM images typically contain contributions from both types of scattering. The ability to separate the contributions from elastic and inelastic processes individually through energy filter or electron energy loss spectroscopy (EELS) allows unique analysis that is otherwise unachievable. Two prominent types of inelastic scattering probed by EELS, namely plasmon and core-loss excitations, are useful for elucidating structural and electronic properties of chalcogenide-based semiconductor nanocrystals. The elastic scattering, however, is still a critical part of the analysis and used in conjunction with the separated inelastic scattering signals. The capability of TEM operated in scanning mode (STEM) to perform localized atomic length scale analysis also permits the understanding of the nanocrystals unattainable by other techniques. Despite the pivotal role of inelastic scatterings, their contributions for STEM imaging, particularly high-angle annular dark field STEM (HAADF-STEM), are not completely understood. This is not surprising since it is currently impossible to experimentally separate the inelastic signals contributing to HAADF-STEM images although images obtained under bright-field TEM mode can be analyzed separately from their scattering contributions using energy-filtering devices. In order to circumvent such problem, analysis based on simulation was done. The existing TEM image simulation algorithm called Multislice method, however, only accounts for elastic scattering. The existing Multislice algorithm was modified to incorporate (bulk or volume) plasmon inelastic scattering. The results were verified based on data from convergent-beam electron diffraction (CBED), electron energy loss spectroscopy (EELS), and HAADF-STEM imaging as well as comparison to experimental data. Dopant atoms are crucial factors which control optical, electronic, and also magnetic properties of semiconductors. Their location inside the materials has become more important with the miniaturization of devices. The precise determination of the position, however, poses a great challenge. Imaging using HAADF-STEM has proven adequate for locating heavy dopant atoms buried in relatively light matrix, particularly using aberration-corrected microscopes. The imaging method has been unsuccessful in detecting dopant atoms with similar atomic number as the matrix. Inelastic core-loss or inner-shell electronic excitations using EELS offer a unique solution when simultaneous imaging and EELS acquisitions are performed. The dopant atoms that are invisible in the images due to the small atomic number differences can be detected via spatial correlation with EELS core-loss data. Three types of samples with varying concentration of Mn dopant atoms in ZnSe nanocrystals were used to confirm such method. Precise locations of the dopant atoms on planes perpendicular to electron beam propagation could be determined although not all of the dopant atoms were detected due to limitations in experimental conditions.Another important type of chalcogenide-based nanocrystals is PbSe which is useful for solar cells. Colloidal method commonly used to synthesize the nanocrystals leave oleic acid capping ligands as surface passivation and size stabilizer. These ligands have critical roles in controlling electrical and optical properties of an individual nanocrystal and their assembly. Deemed insulating due to long chains of carbons, oleic acid is typically treated with short ligands such as hydrazines to decrease the inter-nanocrystal distances and improve electronic coupling among the neighboring nanocrystals. Despite its apparent insulating behavior, oleic acid was shown to exhibit surface plasmon coupling under certain circumstances. The geometric arrangement of the ligands was first investigated by HAADF-STEM imaging. Under air exposure, PbSe nanocyrstals easily oxidize to form oxide shells that are responsible for p-type doping by introducing surface acceptor states. At early oxidation stage (partial oxidation), prior to the formation of uniform oxide shells, the nanocrystals appear to form links between neighbors. Localized EELS analysis shows that these links are made of carbon based materials, most likely modified form of oleic acid ligands consisting of conjugated double bonds. Such modification occurred through oxidative dehydrogenation of the oleic acid ligands that is facilitated by the growing oxide shells on the surface of nanocrystals.
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    The Power of Rust and Sand at the Nanoscale: Iron Oxide and Mesoporous Silica Nanoparticles for Biomedical Applications
    (2015-08) Hurley, Katherine
    Due to their high surface area and size-dependent properties, nanoparticles have seen use as biomedical devices in the past several decades. Magnetic nanoparticles are of particular interest as their properties allow for a variety of uses including separations, targeting, imaging, and therapy. The biological milieu is not a pristine environment, however. The complex medium presents many challenges for particle stability and reproducible performance. It even makes fundamental particle characterization more difficult. In this thesis, magnetic iron oxide nanoparticles are investigated as biomedical devices which provide diagnosis/imaging and therapy (theranostics). Innovative methods for characterizing these particles and observing their behavior over time in biologically relevant environments are also presented. Overall, this thesis aims to make the important point that magnetic nanoparticles are not stagnant objects but are in fact dynamic systems capable of vast changes upon exposure to in vitro or in vivo environments. Aggregation, oxidation, and dissolution all play a role in real-world nanoparticle performance. To mitigate and control some of these concerns, a functionalized mesoporous silica shell is employed as a protective layer around the iron oxide nanoparticle cores. This protective shell causes resistance to each of the above-mentioned factors, resulting in more stable and predictable performance appropriate for treatment planning and biological use. In chapter one, various methods for the characterization of magnetic nanoparticles in biological matrices are reviewed. Several case studies are presented to demonstrate the necessity for complementary techniques to obtain a complete picture of nanoparticle transformations. In chapter two, an early-phase iron oxide/mesoporous silica core/shell nanoparticle is presented, and the effects of synthetic parameters and long term storage conditions on particle performance are examined. In chapter three, a commercially-available iron oxide nanoparticle is studied in detail in various biological environments to understand how particle heating and imaging properties are related and how aggregation can affect them. In chapter four, a functionalized mesoporous silica shell is applied to the iron oxide core from chapter three. The new core/shell particle demonstrates a substantial reduction in aggregation and thus a stabilization of material properties in vitro and in vivo. Finally, chapter five details a variety of transmission electron microscopy (TEM) studies with a focus on visualizing the nano/bio interface in vitro. Dark field TEM is presented as a useful tool for locating and differentiating inorganic nanoparticles, including but not limited to iron oxide nanoparticles, from biological structures or stain artifacts.