Browsing by Subject "TEM"
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Item Applications of Transmission Electron Microscopy on Free-standing and Embedded Two-dimensional Materials(2019-12) Wu, RyanIn the last decade, 2D nanosheets, more commonly referred to as 2D, layered, or van der Waals materials, have garnered significant scientific interest because of their novel material properties at the nanoscale regime compared to their bulk. Their rise in popularity is commonly attributed to the isolation and study of graphene by Geim and Novoselov in 2004 for which they were awarded the Nobel prize in physics in 2010. Since then, more than 1000 unique 2D chemical compounds have been at least theorized if not experimentally isolated. Many of these materials exhibit favorable mechanical, optical, or electronic properties that may also be tunable by controlling their number of layers. With novel materials being continuously synthesized and applied at such a feverish pace, there exists a critical need to characterize and understand the structures and properties of these novel materials that may have been nothing but theoretical predictions a mere decade ago. Herein, analytical scanning transmission electron microscopy (STEM) supported by computational methods is used to study the atomic and electronic structure of numerous free standing 2D materials as well as 2D materials embedded in devices with a spatial resolution of < 1 Å and an energy resolution of < 0.5 eV. Two computational applications are first presented to introduce and highlight the complexities of electron-sample interactions which can be used to extract additional information from experimental results. The first uses experimentally observed Moire patterns to correlate and understanding rotational misalignments of Bi2Se3; the second exploits the channeling of the electron beam in addition to sample tilt to determine the thickness of atomically thin MoS2. The thickness determination method is then experimentally proven using annular dark field-STEM imaging (ADF-STEM) and applied to MoS2 layers of various thicknesses to test the limits of measuring layer-dependent properties in the TEM using electron energy loss spectroscopy (EELS). Subsequently, the atomic and electronic structure of black phosphorus is thoroughly examined using STEM. Its crystal structure including its lattice parameters and stacking order is unambiguously determined by ADF-STEM. Its electronic structure including its conduction band density of states and plasmon excitations are measured using EELS and compared to density functional theory (DFT) calculations. Additionally, the effect of oxidation, a well-known phenomenon when using black phosphorus, on its properties is measured using a similar approach. The results, as measured using the aforementioned techniques in addition to energy dispersive x-ray spectroscopy (EDX), show that oxidation amorphizes black phosphorus transforming the semiconductor into an insulating oxide. Finally, STEM-EELS is applied to study 2D material embedded field effect transistors (FET) in cross-section. Using a layer-by-layer approach, the interactions between MoS2 and metal device contacts are measured to show the non-idealities of the contact/channel interface. These results, supplemented by DFT calculations, are used to understand the phenomenon of Fermi level pinning and the interaction of the metal contact with the MoS2 layers when deposited onto its surface. The results suggest that the chemistry of the metal-MoS2 bond is important in determining the efficacy of the FET and point toward the ultimate limits of which metals and alloys can and cannot be used when ultra-thin mono- and bi- layer MoS2 channels are desired.Item Atomic-scale investigations of multiwall carbon nanotube growth.(2010-06) Behr, Michael JohnThe fundamental processes of carbon nanotube (CNT) growth by plasma-enhanced chemical vapor deposition (PECVD) were investigated using a suite of characterization techniques, including attenuated total-reflection Fourier transform infrared spectroscopy (ATR-FTIR), optical emission spectroscopy (OES), Raman spectroscopy, convergent-beam electron diffraction (CBED), high-resolution transmission and scanning-transmission electron microscopy (TEM, STEM), energy dispersive x-ray spectroscopy, and electron energy-loss spectroscopy (EELS). It is found that hydrogen plays a critical role in determining the final CNT structure through controlling catalyst crystal phase and morphology. At low hydrogen concentrations in the plasma iron catalysts are converted to Fe3C, from which high-quality CNTs grow; however, catalyst particles remain as pure iron when hydrogen is in abundance, and produce highly defective CNTs with large diameters. The initially faceted and equiaxed catalyst nanocrystals are deformed by the surrounding CNT structure during growth. Although catalyst particles are single crystalline, they exhibit combinations of small-angle (~1-3 degree) rotations, twists, and bends along their axial length between adjacent locations. Fe3C catalyst nanoparticles that are located inside the base of well-graphitized CNTs of similar structure and diameter do not exhibit a preferred orientation relative to the nanotube axis, indicating that the graphene nanotube walls are not necessarily produced in an epitaxial process directly from Fe3C faces. Chemical processes occurring at the catalyst-CNT interface during growth were inferred by measuring, ex situ, changes in atomic bonding at an atomic scale with EELS. The observed variation in carbon concentration through the base of catalyst crystals reveals that carbon from the gas phase decomposes on Fe3C, near where the CNT walls terminate at the catalyst base. An amorphous carbon-rich layer at the catalyst base provides the source for CNT growth. These results suggest that what is required for CNT growth is a graphene seed and a source of decomposed carbon. Hydrogen atoms also interact with the graphene walls of CNTs. When the flux of H atoms is high, the continuous cylindrical nanotube walls are etched nonuniformly. Etch pits form at defective sites along the CNT, from which etching proceeds rapidly. It is determined that H etching occurs preferentially at graphene edges.Item Data supporting Holey Substrate-Directed Strain Pattering in Bilayer MoS2(2021-11-10) Zhang, Yichao; Choi, Moon-Ki; Haugstad, Greg; Tadmor, Ellad B; Flannigan, David J; flan0076@umn.edu; Flannigan, David JThis data set contains transmission electron microscopy (TEM), atomic force microscopy (AFM), and atomistic simulation data supporting "Holey Substrate-Directed Strain Pattering in Bilayer MoS2" manuscript cited in referenced by.Item Early stages of zeolite growth.(2010-08) Kumar, SandeepZeolites are crystalline nonporous aluminosilicates with important applications in separation, purification, and adsorption of liquid and gaseous molecules. However, an ability to tailor the zeolite microstructure, such as particle size/shape and pore-size, to make it benign for specific application requires control over nucleation and particle growth processes. But, the nucleation and crystallization mechanisms of zeolites are not fully understood. In this context, the synthesis of an all-silica zeolite with MFI-type framework has been studied extensively as a model system. Throughout chapters 2, 4 and 5, MFI growth process has been investigated by small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). Of fundamental importance is the role of nanoparticles (~5 nm), which are present in the precursor sol, in MFI nucleation and crystallization. Formation of amorphous aggregates and their internal restructuring are concluded as essential steps in MFI nucleation. Early stage zeolite particles have disordered and less crystalline regions within, which indicates the role of structurally distributed population of nanoparticles in growth. Faceting occurs after the depletion of nanoparticles. The chapter 6 presents growth studies in silica sols prepared by using a dimer of tertaprpylammonium (TPA) and reports that MFI nucleation and crystallization are delayed with a more pronounced delay in crystal growth.Item The Influence of 3D Interfaces on Mechanical Behavior of Nanolaminated Bimetallic Composites(2024-01) Cheng, JustinCu/Nb nanolaminates containing 3D interfaces (3D Cu/Nb) are used in this study to demonstrate the effects of controlled interface structure on mechanical behavior and unit deformation activity in nanostructured alloys. 3D interfaces are internal boundaries that exist on length scales relevant to unit deformation mechanisms and contain nanoscale chemical and crystallographic heterogeneities in all spatial dimensions. 3D interfaces are a new method to manipulate alloy microstructure whose effects on plastic deformation have not been previously explored in depth. Elucidation of the link between 3D interface structure and mechanical behavior will provide key insights into nanoscale metallic deformation allowing for materials that exhibit near-theoretical strengths while also being highly deformable. The exploration of these themes requires understanding of a wide range of topics in physical metallurgy, which is reflected in the structure of this thesis. Chapter 1 begins with a high-level overview of the motivation and methodology of this work. Chapters 2 introduces fundamental concepts of metallic deformation at the macroscale and the atomic scale. Chapter 3 explores the participation and influence of interfaces in atomic scale deformation and ties the nanoscale to the mesoscale by discussing previous findings about atomically sharp 2D interfaces on nanocrystalline alloy mechanical behavior. Chapter 4 introduces the experimental methods required to characterize 3D interfaces structurally and mechanically. Chapter 5 presents structural characterization results, while Chapter 6 presents mechanical characterization results. Chapter 6 contains findings from mechanical testing, while also providing discussion connecting 3D interface structure detailed in Chapter 5 to observed 3D Cu/Nb mechanical behavior. The information from these techniques are crucial to forming structure-behavior relationships detailing the effect of 3D interfaces on unit deformation, but they cannot probe the atomic scale alone, so synthesis of computational results with experimental results is also discussed in Chapter 6. Chapter 7 concludes with a summary of key findings of this and proposes future work addressing new scientific issues raised by this work.Item Molecular systematics and morphological congruence in the Pezizales and Neolectales (Ascomycota): three case studies(2013-07) Healy, Rosaria AnnA revolution in fungal systematics is underway due to the application of molecular phylogenetic analyses to previously intractable problems posed by unculturable fungi, unlinked lifecycle stages, and hidden diversity. Three chapters here detail how molecular phylogenetic analyses provided a taxonomic framework for early derived lineages of ascomycetes (Neolectales and Pezizales) upon which morphology could be re-examined for homology, linkage of lifecycle stages, and reconstruction of character states. Mitospores of fungi are rarely linked to meiospore stages in nature, unless they are temporally or spatially coordinated, or one form produces the other in culture. In the first chapter molecular phylogenetic analyses of ITS (a fungal barcode) and 28S rDNA are used to link mitotic sporemats that are produced on the soil surface with rarely cultured or unculturable ectomycorrhizal Pezizales. In this study, 48 OTUs representing six independent ectomycorrhizal lineages were delimited from 292 spore mats collected in Asia, Europe, South America, and North America. Most were truffle lineages, but one lineage included above ground cupuliform fungi, and one lineage had no detected meiospore stage. The discovery that a high diversity of mitospore-mat producing ectomycorrhizal Pezizales are common and widely distributed across the world implies that mitospores play an important role in the lifecycle of these organisms. Neolecta, the only fruit-body forming extant genus of the earliest derived lineage of ascomycetes, was previously determined to be phylogenetically related to non-ascoma forming Taphrinomycotina. The second chapter presents research on septal pore characters in Neolecta vitellina to investigate whether the ascoma in Neolecta is analogous or homologous to later derived lineages of ascoma forming Pezizomycotina. Two unique structures were associated with the septal pores in Neolecta: a vacuolar crystal that lodged within the septal pore of disrupted cells, and a membranous matrix that plugged the pores. The Neolecta crystal appears to be similar in function to the Woronin body of later derived lineages, but differs by the organelle in which it is formed, the numbers of crystals formed per organelle, and the association of vesicles with the crystal. Unlike Pezizomycete septal pore structures the membranous matrix of Neolecta septal pores is not confined to the septum. These two unique structures are presented as evidence of an independent evolution of the Neolecta ascoma. Truffles have evolved at least 16 times from cupuliform Pezizales. Previous phylogenetic analyses have inferred that reversals from truffles to cupuliform fruitbodies are unlikely. Chapter 3 details the phylogenetic analyses of multilocus alignments from world-wide collections of Pachyphloeus (truffles) and Scabropezia (cupuliform fungi). In these analyses, Scabropezia was inferred to be embedded within Pachyphloeus, and a truffle was reconstructed as the ancestral form of this lineage, perturbing the idea that cup fungi have not evolved from truffles. It is likely that taxa are missing from this study that could change these results. All described species in the lineage were transferred to Pachyphlodes to redress the illegitimacy of "Pachyphloeus." Eight lineages with 45 OTUs were delimited, expanding the diversity in this genus 3-fold. Spore wall development was useful for interpreting differences in spore ornamentation among the eight lineages.Item Probing the Crystallization Process and Morphology of Thin Films of Yttrium Iron Garnet on Non-Garnet Substrates with in situ TEM Methods(2018-10) Gage, ThomasThin films of yttrium iron garnet (YIG) are of high interest for promising photonics and spintronics applications. Integration challenges with current silicon processing technology have limited device geometries and caused reduced performance largely arising from crystallization issues of as-deposited films. In order to gain understanding of the amorphous to crystalline phase transformation of YIG thin films on non-garnet substrates, plan-view TEM and in situ laser annealing TEM methods were utilized. Thin YIG films were sputtered onto SiO2 TEM window membranes. These films were initially annealed ex situ using standard RTA annealing methods. A nanocrystalline matrix phase between YIG crystallites was discovered where previous studies had reported uncrystallized material. Preliminary in situ laser annealing led to the serendipitous discovery of a 2-step rapid thermal anneal which improved garnet phase formation in the films. To investigate YIG crystallization kinetics on SiO2, temperature dependent in situ laser annealing TEM diffraction experiments were conducted. Avrami constants and apparent activation energy for the nanocrystalline phase formation is reported. In situ bright-field TEM was also used to investigate the growth of the YIG crystallites and indicated they enter a stress limited growth phase after reaching a critical dimension. Additionally, considerable effort was put into instrument development for in situ TEM methods, including optimization of single-shot pump probe capability. A range for optimized cathode to Wehnelt aperture distance and photoelectron inducing laser fluence are reported. Demonstrations of single-shot capabilities in both diffraction and imaging modes with current equipment are shown.Item Simulation of dopant atom behavior in semiconducting nanocrystals(2015) Duncan, Samuel J. B.; Held, Jacob; Mkhoyan, K. AndreItem The study of oriented aggregation: a nonclassical nanocrystal growth mechanism(2013-02) Burrows, Nathan DennisOriented aggregation is a nonclassical crystal growth mechanism resulting in new secondary nanoparticles composed of crystallographically aligned primary crystallites. These secondary crystals often have unique and symmetry-defying morphologies, can be twinned, and can contain stacking faults and other significant defects. A wide range of important materials, such as titanium dioxide, iron oxides, selenides and sulfides, and metal oxyhydroxides, are known to grow by oriented aggregation under certain conditions. Evidence for oriented aggregation also has been observed in natural materials. However questions remain about what conditions are the most importing in facilitating purposeful control over nanoparticle size, size distribution, and morphology. Kinetic models for oriented aggregation point to important variables such as ionic strength, pH, temperature, and choice of dispersing solvent as being the key or keys to gaining control of this natural phenomenon and moving it towards a tool to be used in designing novel nanomaterials. The main technique used in this research is transmission electron microscopy with temporal resolution to characterize the population of growing nanocrystals. Cryogenic transmission electron microscopy is employed to observe the various stages of crystal growth. With extensive image analysis, it is possible to determine the kinetics of growth and the effects of systematically changing these key growth conditions. Additional complimentary techniques are employed, such as dynamic light scattering as well as various methods of characterization, such as powder X-ray diffraction. As our fundamental understanding of oriented aggregation improves, novel and complex functional materials are expected to emerge.Item Surprising microscopy subtleties: measuring picoscale thicknesses, visualizing core orbitals, and detecting charge transfer using the TEM(2015-11) Odlyzko, Michael50 years ago, Richard Feynman delivered a now-famous address outlining why there was ''plenty of room left at the bottom'': there remained much progress to be made in seeing and manipulating matter all the way down to the atomic scale. One of many means to that end, argued Feynman, was to make electron microscopes better. Why could not electrons with wavelengths of a few picometers not be used to clearly image atoms hundreds of picometers in size? Why could not electron beams be used to pattern miniscule wires a handful of metal atoms across? Over the course of decades, Feynman’s vision has been pursued zealously with rich reward, not least in the electron microscopy field. Enabled by the development of bright field-emission electron sources, high-resolution polepieces, and now aberration correctors, transmission electron microscopy (TEM) at atomic resolution has become routine. Seemingly, there is little room left at the bottom; after all, once you can clearly see atoms, what more is there left to do? Thankfully, there is plenty. Much of the hard work has been in the development of equipment that expands TEM to allow unprecedented spatially resolved analysis of elemental composition, inelastic scattering, and temporal processes. But there are also many opportunities to uncover new information using now widely available techniques and equipment. In the studies presented here, there has been some success in following the latter path. In tandem with careful computational analysis, selected-area electron diffraction allows not only determination of crystal symmetry, lattice parameter, and microstructure, but also measurements of material thickness on the scale of atomic layers. Supported by careful data processing and rigorous simulations, spatially resolved X-ray spectroscopy data is converted into real-space measurements of core-level electronic orbitals, in addition to providing routine atomic resolution chemical mapping. And aided by the development of novel bonding-inclusive TEM simulations, the detection of chemical bonding using nominally bonding-independent high-angle elastic scattering is both theoretically predicted and experimentally observed. Even once you have gone all the way down to the bottom, there is still a wide world of wonders left to explore.