Browsing by Subject "Characterization"
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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 Characterization and loss modeling of silicon carbide based power electronic converters(2015-04) Ravi, LakshmiSilicon Carbide (SiC) based power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are great candidates for high-voltage, high-frequency and high-temperature power switching applications because of their favorable material properties when compared with Silicon (Si) power MOSFETs. In this thesis, the design, characterization, and modeling of a power electronic converter based around SiC MOSFETs is investigated. The test converter circuit is designed to be general enough that it can represent a half bridge converter, a DC chopper circuit or an output phase of an inverter for flexibility in testing. A practical characterization procedure is proposed which takes a circuit-level approach, as opposed to a device-level approach, using only the actual power electronic circuit under study and no additional test circuitry. Therefore this study takes into account the inherent parasitic impedances associated with the test circuit and its influence on the SiC devices' high-speed switching behavior. The hardware setup is operated at frequencies up to 200 kHz and efficiencies up to approximately 99% were recorded.Based on the characterization data and analysis, a model is constructed using MATLAB (a mathematical modeling software) for predicting converter and gate driver losses at different load currents, DC bus voltages, and operating temperatures (for both a DC-DC synchronous buck converter and a DC-AC three phase, two-level Voltage Source Inverter). Good agreements are obtained between the model outputs and experimental results. Possible future extensions to the work are discussed.Item Characterization of Field Pennycress (Thlaspi arvense L.) Germplasm for Use as a Cover Crop and Biofuel Feedstock(2017-01) Altendorf, KaylaField pennycress (Thlaspi arvense L.) is a winter annual oilseed crop currently under investigation for use as a feedstock for domestic biofuel production. As an undomesticated species, pennycress has a variety of undesirable traits and the species has seen very limited formal selection or genetic improvement. This research seeks to characterize a collection of 42 wild, winter type accessions for morphological and yield component traits (Chapter 2) as well as seed chemistry traits (Chapter 3). This data will be used to guide the breeding and development of improved germplasm and eventual variety release. The germplasm collection was grown in five unique Minnesota environments in 2013/2014 and 2014/2015. Mixed effects models were used to estimate best linear unbiased estimates (BLUEs) for each of the accessions and traits, which were used in subsequent analyses. Within the morphological traits and yield component traits, relationships between traits were assessed using Pearson’s correlation coefficients and estimates of heritability were calculated for each trait. Hierarchical clustering was used to identify groups of accessions based on similarity of trait values. Significant variation for accession was detected in 13 of the 19 trait models for morphological and yield component traits at P < 0.05, and for 1 of 19 at P < 0.10 (Table 2.7). Pairwise differences after adjusting for multiple comparisons using Tukey’s Honest Significant Difference (HSD) resulted in more than one grouping in 9 of the 13 models in which accession was significant (P < 0.05). For seed chemistry traits, significant variation (P < 0.05) for accession was detected for nine of the ten fatty acids detected and oil percentage. Significant variation was observed for many of the traits evaluated, but to make sizable gains in selection for certain traits, additional genetic variation in the form of mutants and additional collections is required.Item Characterizing the multicomponent density structure of galaxies(2012-12) Dhar, Barun KumarAbridged) Quantifying the 3D spatial structure of galaxies forms a key element in furthering our understanding of the formation and evolution of galaxies and consequently the universe. What we observe, however, is a 2D projection (on the plane of the sky) of the intensity of light of galaxies. Inferring the intrinsic 3D distribution is critical for the understanding of galaxies, but technically difficult. Despite 80 years of research, we still do not have a consensus on a model that can describe the 2D projected light of galaxies over their entire spatial extent.In the first part of this work, I summarize existing models and then show how one can efficiently model the high resolution light profiles of galaxies to quantify both the 2D and the 3D distributions and also infer the existence of multiple components in a galaxy. Using data of 23 galaxies from the Virgo Cluster, I present models of their light profiles with mean residuals of ~ 3% (that is consistent with measurement errors) over a radial range spanning an order of up to 10^5 -- an unprecedented accuracy over such large spatial extent. Consequences of such models are discussed, especially an evidence for an universality in the functional form that describes both the baryonic and dark matter distributions. In the second part of this work, I demonstrate a novel way of obtaining projections of intrinsic quantities when the standard method does not yield analytical solutions, i.e. a procedure to perform otherwise difficult analytical integrations.Item Design, Synthesis, and Characterization of Novel Photosensitizers for Use in Molecular Machines(2021-08) Bayard, BrandonPhosphorus porphyrin-naphthyl compounds were synthesized and evaluated for future use as an electron acceptor/donor system within a photoactivated molecular shuttle. This evaluation consisted of characterizing the redox and optical properties of the systems, as well as determining the formation constant of the complex formed between the naphthyl group in each compound and a previously reported macrocycle, cyclobis(paraquat-p-phenylene) (CBPQT), with affinity towards various naphthyl derivatives. This data was holistically reviewed and used to evaluate the potential use of phosphorus porphyrin-naphthyl compounds as an electron donor/acceptor system within a molecular shuttle. Furthermore, an informed design for such a molecular shuttle was produced, and steps towards its synthesis were taken. Two other projects in which novel photosensitizers were synthesized and characterized are detailed here. The purpose of these two projects is to extensively identify their optical and redox properties such that they may be used in future systems such as molecular machines or artificial photosynthesis. The first project explores a series of phosphorus tetraphenylporphyrin derivatives in which the phenyl rings of the porphyrins have various amounts of methoxy groups adorning them. The second project explores a novel phosphorus porphyrin/aluminum porphyrin heterodimer. The goal of which is to characterize the interactions of the chromophores when no exciton coupling is observed.Item Development and Characterization of Magnetostrictive GaFe and Plasmonic Gold Thin Films(2015-04) Estrine, EliotAs device sizes continue to shrink into the nano-scale, material development becomes increasingly important. This presents new deposition and characterization challenges which must be overcome to produce the next generation of devices. Magnetostrictive GaFe (galfenol) is one such material in which development of deposition and characterization techniques is necessary to enable new MEMS devices. In addition, plasmonic gold Near Field Transducers (NFTs) used in Heat Assisted Magnetic Recording (HAMR) require new characterization options to understand device failure modes as well as new gold deposition processes to improve device reliability. While these applications are very different, the underlying material deposition and characterization challenges involving thin film crystallinity are very similar. Magnetostriction measurements of electrodeposited galfenol show that it is possible to achieve thin films of this material over a wide range of compositions using electrodeposition. In addition, grain refinement in gold was achieved through alloying which shows the potential to create more robust thin films while maintaining gold's desirable plasmonic properties. Finally, advanced characterization processes using Electron Back Scatter Diffraction (EBSD) were also developed to analyze thin film crystal structure and its role in NFT stability. These results will further progress in the fields of MEMS and HAMR as well as provide the basis for identifying and solving materials challenges in the future.Item Directed synthesis and characterization of zeolite nanoparticles(2013-10) Zhang, XueyiZeolites are a class of materials with ordered micropores (smaller than 2 nm), that can be used for gas separation, catalysis, and adsorption. Structurally, zeolites are composed of SiO4 tetrahedra sharing corners in an ordered manner. The numerous arrangements of these SiO4 tetrahedra give zeolites micropores in the forms of channels and cages. Although the applications of each zeolite depends on the spatial arrangements and the sizes of these micropores, size and morphology of zeolite particles are equally important. By preparing zeolite nanoparticles, diffusion paths of zeolite particles can be shortened, total surface area of zeolite particles can be increased, which are beneficial to reducing energy consumption in gas separation, reducing deactivation in catalytic reactions, and increasing adsorption capacities. This dissertation introduces various methods to prepare zeolite nanoparticles. Zeolite nanoparticles prepared with the help of mesoporous carbon templates (hard template) are firstly introduced where the shape and size of zeolite particles are imprinted from the templates. In addition to hard templates, the use of bifunctional surfactant (soft templates) to prepare ultra-small zeolite nanoparticles and lamellar zeolite membrane is also introduced. With only one structure-directing agent that is less intuitive than hard or soft templates, the preparation of hierarchical lamellar zeolite with 2 nm lamellae, the self-pillard pentasil (SPP) zeolite, is introduced, where the intrinsic growth patterns of the crystal played an important role. Finally, template-free synthesis of zeolite nanoparticles, where zeolite formation is totally driven by the intrinsic growth patterns, is introduced. In addition to the preparation methods, a series of computational methods to determine and study the structures of zeolite nanoparticles are also introduced.Item Electromechanical Switches Fabricated by Electrophoretic Deposition of Single Wall Carbon Nanotube Films(2015-08) Lim, Jun YoungPower dissipation is a critical problem of CMOS devices especially for mobile applications. Many efforts have been made to solve the problem, but there are still major issues associated with scaling the device size. Micro electromechanical (MEMS) and nano electromechanical (NEMS) devices are one candidate to solve the problems because of their excellent standby leakage. However, the switches have a tradeoff between low operating power and high device speed. Suspended beams with low mass density and good mechanical properties provide a way to optimize the device. Carbon nanotubes (CNTs) have the low mass density and excellent mechanical properties to enable high performance MEMS/NEMS devices. However, the high temperature required for the direct synthesis for CNTs makes it difficult for them to be compatible with a substrate containing transistors. Therefore, continuous film deposition techniques are investigated with low temperature (< 300 C). Electrophoretic deposition (EPD) is a simple and versatile processing method to deposit carbon nanotubes on the substrate at room temperature. The movement of the charged CNTs in suspension occurs by an applied electric field. The deposited CNT film thickness can be controlled through the applied voltage and process time. We demonstrate the use of an EPD process to deposit various thicknesses of CNT films. Film thicknesses are studied as a function of, deposition time, electric field strength, and suspension concentration. The deposition mechanism of the EPD process for carbon nanotube layers was explained with experimental data. We determined the film mass density and electrical/optical properties of SWCNT films. Rutherford backscattering spectroscopy was used to determine the film mass density. Films created in this manner had a mass density that varies with thickness from 0.12 to 0.54 g/cm3 and a resistivity of 2.1410-3 Ω∙cm. For the mechanical property measurements, we describe a technique to fabricate free-standing thin films using modified Langmuir-Blodgett method. Then we extracted the Young’s modulus of the film from the load-displacement data from nanoindentation using the appropriate modeling. The Young’s modulus had a range of 4.72 to 5.67 GPa, independent of deposited thickness. We fabricated two-terminal fixed beam switches with SWCNT thin films using the EPD process. Device pull-in voltages under 1V were achieved by decreasing the air-gap. The pull-in voltages were compared with the calculated results using the device geometry and extracted Young’s modulus from nanoindentation. Generally good agreement was observed. Also, we found a range of 2.4 to 3.5 MHz resonant frequency. However, we encountered several problems with the device including a gradual turn-on, hysteresis between pull-in and pull-out voltage, changes in the pull-in voltages with repeated on-off cycling, and early failure due to moisture absorption during testing in the air. Mechanisms for these observations are postulated. Further work is needed to improve device performance and reliability.Item Genetic analysis and characterization of variegation in hybrid grape populations (Vitis spp.)(2020-09) Olson, JackVariegation is a plant trait defined by “plants which develop patches of different colors in the vegetative parts”, although variegation can express in reproductive parts of plants as well (Kirk and Tilney-Bassett, 1978). Variegation is a common trait found among a wide variety of plant species and has been reported in V. vinifera, sport mutations, and segregating in hybrid grape populations (Reisch and Watson, 1984; Filler et al., 1994; personal observation). Variegation is highly desirable in ornamental plant breeding for its showy colors and patterns, but in grape seedlings, it has been observed to have deleterious effects in the form of shorter and less vigorous plants than their wild-type siblings. Mapping the locus or loci associated with variegation would allow for the development of markers to identify parents that may carry the recessive allele for the trait, allowing for more informative decisions on population size and parental combinations when making crosses (Chapter 2). Three different mapping approaches - bulked segregant analysis (BSA), Genome-wide association study (GWAS), and genetic mapping - were utilized to detect and validate associated loci. A total of 9 hybrid grape populations were utilized in mapping, of which all 9 were used in BSA, 3 (GE1642, GE1703, GE1895) were used in GWAS and 2 (GE1642, GE1703) were used in QTL genetic mapping. BSA detected four highly significant SNP markers on chromosome 14 between the physical positions of 21,425,721 to 21,425,734 Mbp. GWAS identified 24 significantly associated markers on chromosome 14 from 27.1 to 30.1 Mbp in GE1642 and GE1895; however, 9 markers on chromosome 11 from 12.1 to 18.4 Mbp were significantly associated with variegation in GE1703. Genetic mapping of GE1642 and GE1703 mapped the variegation the same regions, which validated the region identified in GWAS. Thus, two major loci on chromosomes 11 and 14 were associated with variegation in separate hybrid grape populations. Candidate genes for variegation were identified in the two locus regions for future studies. The effects of variegation on hybrid grape were examined in a variety of experiments in which it was discovered that variegation resulted in a reduction in photosystem II efficiency; reduced leaf chlorophyll and carotenoid concentration; altered leaf palisade mesophyll structure; and had significant reductions in plant growth-related traits (Chapter 3).Item Hydrothermal stability of hierarchically-structured two-dimensional MFI zeolite nanosheets(2019-06) Guefrachi, YasmineTwo-dimensional (< two-unit-cell-thick) zeolites are an important class of two-dimensional nanoporous materials. They possess the intrinsic properties of the conventional three-dimensional crystalline silicate/aluminosilicate zeolites and when synthesized in a pillared or intergrown form they exhibit a hierarchical porosity. Despite the tremendous effort directed towards their synthesis, very limited knowledge is available on their hydrothermal stability under industrially-relevant conditions. The hydrothermal stability of the thin crystalline domains governs their enhanced use as selective catalysts, adsorbents and in thin membranes’ preparation. This dissertation focuses on developing a fundamental understanding of the response of these novel thin crystallites under hydrothermal environments towards an advanced design and engineering of their synthesis and industrial applications. Self-Pillared Pentasil zeolite was utilized as a model system of hierarchically-structured two-dimensional MFI zeolite nanosheets in the structural and catalytic stability investigations explored in this dissertation. The nanostructural evolution of purely-siliceous nanosheets in presence of water under different atmospheres was studied via a thorough structural characterization analysis which involved the use of high-resolution and three-dimensional tomography bright-field transmission electron microscopy (the tilt series acquired in this analysis are enclosed as supplementary media files in this dissertation). The kinetic and thermodynamic driving forces behind the captured structural changes were investigated. The acidity modifications and the catalytic consequences resultant from the hydrotreatment structural transformation were explored using the aluminosilicate nanosheets counterparts.Item Investigation of Hot Mix Asphalt Mixtures at Mn/ROAD(Minnesota Department of Transportation, 1997-02) Stroup-Gardiner, Mary; Newcomb, David E.This report presents the material characterization for the Minnesota Road Research Project (Mn/ROAD) bituminous materials. This effort will provide the historical base line information on properties needed for the validation of future pavement evaluation and design models. The objectives of the work were to 1) Document construction of Mn/ROAD, 2) Establish a series of test methods for characterizing the materials and 3) Develop a data base of material properties to develop mechanistic pavement design procedures. Documentation on construction included mixture design, construction techniques and a summary of test results. The laboratory test methods represent a wide variety of tests developed by the Strategic Highway Research Program, the National Cooperative Highway Research Program and the Federal Highway Administration. The materials represent those tested during the mixture design, construction and post construction phases of Mn/ROAD.Item Minnesota Taconite Workers Health Study: Environmental Study of Airborne Particulate Matter in Mesabi Iron Range Communities and Taconite Processing Plants - A Characterization of the Mineral Component of Particulate Matter(University of Minnesota Duluth, 2019-12) Monson Geerts, Stephen D; Hudak, George J; Marple, Virgil; Lundgren, Dale; Zanko, Lawrence M; Olson, Bernard; Bandli, BryanThe Minnesota Taconite Workers Health Study (MTWHS) was initiated in 2008 and included a multicomponent study to further understand taconite worker health issues on the Mesabi Iron Range (MIR) in northeastern Minnesota. Approximately $4.9 million funding was provided by the Minnesota Legislature to conduct five separate studies related to this initiative, including: An Occupational Exposure Assessment, conducted by the University of Minnesota School of Public Health (SPH); A Mortality (Cause of Death) study, conducted by the University of Minnesota SPH; Incidence studies, conducted by the University of Minnesota SPH; A Respiratory Survey of Taconite Workers and Spouses, conducted by the University of Minnesota SPH; and An Environmental Study of Airborne Particulate Matter, conducted by the Natural Resources Research Institute (NRRI) at the University of Minnesota Duluth (UMD). Results of the four studies conducted by the University of Minnesota SPH can be found on the Taconite Workers Health Study website (http://taconiteworkers.umn.edu/news/documents/Taconite_FinalReport_120114.pdf). NRRI’s “Environmental Study of Airborne Particulate Matter” comprises a multi-faceted characterization of size-fractionated airborne particulate matter (PM) from MIR community “rooftop” locations, background sites, and all taconite processing facilities active between 2008 and 2014. Characterization includes gravimetric determinations, chemical characterization, mineralogical characterization, and morphological characterization. This report specifically discusses the mineralogy and morphology of EMPs collected from the rooftops of five communities located within the MIR, three reference or background locations, and the six taconite processing plants. The samples were collected between 2008 and 2011.Item Optical Tweezers: Characterization and systems approach to high bandwidth force estimation.(2010-04) Sehgal, HullasIn recent times, the hard boundaries between classical fields of sciences have almost disappeared. There is a cross-pollination of ideas between sciences, engineering and mathematics. This work investigates a modern tool of micro-manipulation of microscopic particles that is used primarily by bio-physicists and bio-chemists for single cell, single molecule studies. This tool called the Optical Tweezers can trap microscopic dielectric particles using radiation pressure of light. Optical tweezers is increasingly being used in bio-assays as it provides a means to observe bio-molecules non invasively and offers a spatial resolution in nanometers and force resolution in femto-Newtons at millisecond timescales. In this work, physics governing the operating principle behind optical tweezers is presented, followed by a step by step procedure to build an optical tweezers system having measurement and actuation capability along with a controller logic for feedback implementation. The working of optical tweezers system is presented using a spring mass damper model and the traditional methods of optical tweezers characterization are discussed. A comprehensive view of Optical tweezers is then presented from a system theoretic perspective, underlying the limitations of traditional methods of tweezers characterization that are based on the first principle. The role of feedback in Optical tweezers is presented along with the fundamental limitations that the plant model imposes on optical tweezers performance to be used as a force sensor for fast dynamics input force. The purpose of optical tweezers as a pico-newton force probe is emphasized and a classical controls based method to improve the bandwidth of force estimation using an ad-hoc approach of system inversion is presented. The efficacy of system inversion based method in improving the force probe capability of feedback enhanced optical tweezers is validated by experimental results. It is shown experimentally that the system inversion method results in an order of magnitude improvement in the bandwidth of external force estimation. Finally, a robust control strategy is presented, where the problem of estimation of high bandwidth force is casted as an H-infinity optimization problem along with other performance objectives. This strategy is then compared with the traditional method using PI-controllers and experimental results presented. The robust control strategy is found to further improve the ability of optical tweezers as a force sensor for fast changing force profile by approximately three times over the system inversion approach.Item Synthetic Control and Characterization of NU-1000(2019-12) Webber, TomThe production and release of greenhouse gasses has become a major issue in today’s society. Methane is a powerful greenhouse gas and is the main component of natural gas. Natural gas is often collected and transported to be used as a fuel, but leaks result in release of some of that methane into the atmosphere. Work is underway to develop an efficient catalyst capable of selective oxidation of methane to methanol. Metal-organic frameworks have become popular candidates for catalysts and catalyst scaffolds. The Zr-based metal-organic framework NU-1000 is a robust, mesoporous material that can be used in a variety of applications, including catalysis, sensing, gas separation and storage, and scaffolds. It can be synthesized by combining a solution of hexazirconium nodes ([Zr6O16H16]8+) and organic acid modulator in N,N-dimethylformamide with a solution of linker (1,3,6,8-tetrakis(p-benzoic acid)pyrene) and aging at elevated temperature. The typical product is 1-3 μm crystals that are primarily composed of NU-1000 but that contain domains of an impurity phase called NU-901 that is a polymorph of NU-1000. The ideal NU-1000 synthesis will yield phase-pure particles and enable control over crystal size. The structural differences between NU-1000 and NU-901 lead to a hypothesis that changing the organic acid modulator from benzoic acid to a larger and more rigid carboxylic acid might lead to steric interactions between the modulator coordinating on the node and linkers bound to nodes, inhibiting growth of the more dense NU-901-like material and resulting in phase-pure NU-1000. Side-by-side reactions comparing the products of synthesis using benzoic acid or biphenyl-4-carboxylic acid as organic acid modulator produce structurally heterogeneous crystals and phase-pure NU-1000 crystals, respectively. NU-1000 particles synthesized in the range of 1-3 μm, while useful for many applications, are not large enough for single-crystal X-ray diffraction and are not small enough for nanomaterial applications like drug delivery. The synthesis of NU-1000 provides a variety of experimental handles that can be tuned to produce a wide range of particle sizes. For example, the rate of nucleation and growth is closely tied to the concentration of modulator. This is because NU-1000 is formed via a competitive reaction between modulator and linker molecules for the binding sites on the hexa-Zr nodes. By changing the concentration of the linker, modulator, and any additives, the nucleation and growth rates can be altered to produce the desired particle size. The choice of Zr precursor between ZrOCl2 • 8 H2O and ZrCl4 also plays a significant role in determining the resulting particle size. People acquire a wide range of data like crystal size and morphology, crystallographic information, and elemental quantification and distribution using techniques like transmission electron microscopy and energy-dispersive X-ray spectroscopy. The characterization of size, size distribution, crystallinity, and chemical composition are critical to studying the catalytic properties of product materials. However, due to the delicate nature of MOFs, gathering this data can be very challenging. MOFs commonly undergo radiation damage under a focused electron beam causing a loss of crystallinity. While various techniques can circumvent this damage like cryogenic transmission electron microscopy and low-dose electron microscopy, this dissertation focuses on analyzing the damage and ensuring the data collected remains reliable.Item A theoretical study of dopant atom detection and probe behavior in STEM(2013-12) Mittal, AnudhaVery 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.