Liquid cell electron microscopy has been proven to provide high spatial and temporal resolution for studying liquid layers and the solid-liquid interface at the micro and nano scale. The in situ environment allows for spatial and spectral characterization and quantification of the dynamics and kinetics involved with structural and chemical changes of nanostructures, which has seen application is fields of materials science, electrochemistry, corrosion, biomaterials, and nanophysics. The rapid growth of in situ liquid cell electron microscopy has motivated the fabrication of a custom liquid cell for improved control over the experimental conditions, including cell dimensions and materials. In this work, the process flow and micro-fabrication of a custom liquid cell system are proposed and executed, with proof of operation through the in situ imaging of suspended gold nanoparticles and electrochemical characterization. The in situ TEM system is improved upon by forming 1μm diameter holes through the viewing membranes, removing the background noise contribution from imaging electrons through the windows. This allows for high resolution liquid cell imaging. This improved system is used to study the oxidative etching conditions for palladium nanocrystals, which are commonly used as catalysis for hydrogen fuel cells. The dendritic etching is studied through native etching under exposure to hydrochloric acid without the presence of oxidizing species, followed by the radiolytic generation of oxidizing radicals via the microscope electron beam, and finally by the application of electrical biasing. / Thesis / Master of Applied Science (MASc) / Characterization of nanomaterials has been available for several decades and has aided in the improvement of material design, such as steel strength and corrosion resistance, electrical systems such as those involved in computers and smartphones, and biological sensing and detection. Observation of dynamic process which occur at the interface between solid and liquid phases, or purely within liquid layers, has always been a challenging topic due to the difficulty of finding a stable environment for both solids and liquids to exist at the nano scale within a measurement device. In situ liquid cell electron microscopy offers the ability to image this interface with real-time data acquisition for recording of dynamics and kinetics at the nano scale. Previous work has shown the liquid cell to provide high spatial and temporal resolution of systems in an environment which mimics their native operating conditions. This thesis addresses current developments for in situ systems and works to develop a custom liquid cell to further the applications of the liquid cell and provide improved control over experimental conditions. The work then aims to improve on current technology by increasing the spatial resolution obtainable. Finally, the technique is applied to study the structural changes of nanocrystals under various etching conditions as a demonstration of its’ capabilities.

The major focus of the research in our laboratory is the investigation of the role of the nitrogenase component, theMoFe protein, in the catalytic mechanism of biological nitrogen fixation. This dissertation research centers on the role(s) of the P cluster, one of the two unique FeS clusters of the MoFe protein, in the electron transfer mechanism of nitrogenase. Prior to the solution of the x-ray crystal structure of the Azotobacter vinelandii MoFe protein, it was had been determined which of the highly conserved cysteinyl residues of this protein were likely P cluster ligands. After elucidation of the crystal structure, it became evident that cysteine-88 of the a-subunit (a-Cys88) and cysteine-95 of the b-subunit (b-Cys95) could play important roles in maintaining and/or perturbing the conformation of the double-cubane structure by virtue of their bridging positions. It was found that three out of ten bacterial strains with substitutions at the a-Cys88 ligand retained significant catalytic activity. We investigated the effects of these substitutions on the overall structural, kinetic and spectroscopic parameters. The results of prior studies suggested a role for the P clusters in accepting, storing, and then delivering the electrons received from the Fe protein. Therefore, we asked whether a-Cys88 substitution resulted in perturbed functioning of the overall catalytic mechanism and more importantly what these differences reveal about the normal mechanism of nitrogenase. Alterations in the bridging cysteine a-88 affected the rate of substrate reduction which can be explained in part by production of cluster-less MoFe protein. Electron flux, NaCl concentration, and reductant concentration titration assays revealed a significant uncoupling of the ATP hydrolysis rate from the substrate reduction rate in the a-88Cys-to-Gly MoFe protein. Rapid kinetic analysis revealed decreased electron tranfer rates in all three of the a-Cys88 altered MoFe proteins when compared to the wild-type MoFe protein. The intermolecular electron transfer rate was lowered in the a-88Cys-to-Asp MoFe protein, while the intramolecular electron transfer rate was limiting in the a-88Cys-to-Gly and a-88Cys-to-Thr MoFe proteins. These results indicated a role for the a-88 position in controlling electron flow through the P cluster. Another significant finding centers on the spectroscopic signals derived from one of these a-88Cys substituted MoFe proteins. The a-88Cys-to-Gly MoFe protein possesses a unique S=1/2 EPR signal in the native, dithionite-reduced state that was shown to be due to a one-electron-oxidized P cluster. This new paramagnetic center was evidence for the dramatic perturbation of the electromagnetic properties of the P cluster by the a-88Cys-to-Gly substitution. Additionally, both Mössbauer and magnetic circular dichroism spectroscopies have also demonstrated significant changes in the electromagnetic environments of the P clusters of these a-88Cys altered MoFe proteins and that each substitution affected the P cluster differently. The novel EPR signal was exploited in order to follow the sequence of electron transfer events in the nitrogenase reaction. Finally, altered nitrogenase component proteins were combined and analyzed in an attempt to distinguish which particular step(s) are perturbed in the overall enzymatic reaction. / Ph. D.

Since the first observations of neutrino oscillation, neutrino experiments have come a long way toward precise measurements of the neutrino oscillation parameters, but some obstacles still remain. The next generation of oscillation experiments, including the Deep Underground Neutrino Experiment (DUNE), will be using the Liquid Argon Time Projection Chambers (LArTPCs) with natural argon as the neutrino target material. A precise model of the neutrino cross section on argon does not exist, which is a source of significant uncertainty in such experiments. The E12-14-012 experiment at Jefferson Lab seeks to help remedy this via electron scattering measurements on argon and titanium targets. The experiment collected both inclusive (e,e') and exclusive (e,e'p) data at a wide range of kinematics with the intent to measure the electron-nucleus cross section and thus derive a spectral function for argon that can be used with neutrino experiments. The use of titanium in this experiment stems from the equivalent shell structure that its protons share with the neutrons in argon, which will be crucial in oscillation experiments but cannot be measured directly in electron scattering. This thesis collects several papers which present results from the E12-14-012 experiment. These results include the inclusive (e,e') cross sections for carbon, titanium, argon, and aluminum at a beam energy of 2.22 GeV and a scattering angle of 15.54 deg with uncertainty of less than 5%. Also included are the first results of the exclusive (e,e'p) cross section on argon and titanium. / Doctor of Philosophy / Since the first observations of neutrino oscillation, neutrino experiments have come a long way toward precise measurements of the neutrino oscillation parameters, but some of the properties of neutrinos still remain uncertain. The next generation of neutrino oscillation experiments, including the Deep Underground Neutrino Experiment (DUNE), will be using the Liquid Argon Time Projection Chambers (LArTPCs) with natural argon as the neutrino target material. A precise model of the neutrino cross section on argon does not exist, which will reduce the opportunity for DUNE to measure the neutrino properties with high precision. The E12-14-012 experiment at Jefferson Lab seeks to help remedy this via electron scattering measurements on argon and titanium targets. The experiment collected both inclusive (e,e') and exclusive (e,e'p) data.The goal of this experiment is measure the electron-nucleus interactions and provide a nuclear model for electron and neutrino interactions. This Ph.D. thesis collects several papers which present the results from the Jefferson lab E12-14-012 experiment.

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No description available.

The free electron laser (FEL) is proposed to meet the Navy's need for a speed-of-light high energy laser weapon capable of engaging a variety of targets including anti-ship cruise missiles, small boats, and theater ballistic missiles. A key attribute of FELs is good optical beam quality; in other words, they operate in only a few of the lowest-order transverse Gaussian modes. For weapons applications, a good mode quality is desired because it delivers the highest intensity on target ensuring a high level of lethality. A few higher-order modes can arise from the interaction of the electron beam with the optical beam, or from misalignments of the electron beam or resonator mirrors. High intensity on FEL optics can lead to mirror distortion due to heating and insufficient cooling of the mirror substrate. Mirror distortions, including astigmatism, can cause higher-order modes to appear affecting FEL performance. Therefore, it is important to quantify these higher-order modes because doing so uniquely identifies the optical field and may allow for corrective optics to single out the best modes for FEL lethality. This thesis will review free electron laser theory, and for the first time develop analytical solutions to quantify Hermite-Gaussian higher-order modes, develop a diagnostic for modal analysis, and determine the tolerance limits on mirror distortions.

Modern semiconductor fabrication techniques allow for the fabrication of semiconductor heterostructures which host electron transport with a minimum of scattering sites. In such devices, electrons populate a two-dimensional electron gas (2DEG) in which electrons propagate in exactly two dimensions, and may be further confined by potential barriers to form electron billiards. At sub-Kelvin temperatures, electron trajectories are determined largely by reflections from the billiard walls, while net conduction through the device depends on quantum mechanical wave interference. Measurements of magnetoconductance fluctuations (MCF) serve as a probe of dynamics within the electron billiard. Many prior studies have utilized heterostructures employing the modulation doping architecture, in which the 2DEG is spatially removed from the donor atoms to minimize electron scattering. Theoretical studies have claimed that MCF will be fractal when the confinement potential defining the billiard is soft-walled, regardless of the presence of smooth potentials within the billiard such as those introduced by remote ionized donors. The small-angle scattering sites resulting from these potentials are often disregarded as negligible; we use MCF measurements to investigate such claims. To probe the effect of remote ionized donor scattering on the phase space in electron billiards, we compare MCF measured on billiards in a modulation-doped heterostructure to those measured on billiards in an undoped heterostructure, in which this potential landscape is believed to be absent. Fractal studies are performed on these MCF traces, and we find that MCF measured on the undoped billiards do not exhibit measurably different fractal characteristics than those measured on the modulation-doped billiards. Having confirmed that the potential landscapes in modulation-doped heterostructures do not affect the electron phase space, we then investigate the effect of these impurities on the distribution of electron trajectories through the billiards. By employing thermal cycling experiments, we demonstrate that this distribution is highly sensitive to the precise potential landscape within the billiard, suggesting that modulation-doped heterostructures do not support fully ballistic electron transport. We compare our MCF correlation data with the dynamics of charge transfer within heterostructure systems to make qualitative conclusions regarding these dynamics.

Apresentamos aqui resultados de propriedades eletrônicas relacionadas à defeitos complexos em GaAs. Os cálculos autoconsistentes da estrutura eletrônica foram feitos usando o método do espalhamento múltiplo X a junto com o modelo do aglomerado molecular e com os orbitais de superfície saturados com a esfera de Watson. / We repor! resu/ts for e/ectronic properties related to complex defects in GaAs. The self-consistent-fie/d electronic structure ca/culations were performed by using the multiplescattering X a method within framework of Watson-sphere-terminated molecular cluster model. We have simulated, first, the electronic states of the 26-atom clusters: 26Td/1As (tetrahedral interstitial As centered cluster), 26Td/1Ga (tetrahedral interslitial Ga cenlered clusle!) and 26C3v (trigonal cluster centered in the middle of a Ga-As bond).

The design of high quality electron generators to meet specific requirements is important in the application of these devices to a variety of materials processing systems (including welding, cutting and additive manufacture), X-ray tubes for medical, scientific and industrial applications, microscopy and lithography. Designs can be analysed by field solvers, and electron trajectories plotted to provide an indication of the beam quality. Incremental improvement of designs has normally been executed by trial and error, and this can be a time consuming activity requiring expert intervention for each iteration of the design process. The unique contribution made to knowledge by this work is the application of optimisation techniques to the design of electron guns to produce beams with the required optical properties. This thesis presents a review of the design of electron guns, including a discussion of thermionic cathode material properties and their suitability for use in electron guns for processing materials, the influence of space-charge on gun design and the derivation of salient beam metrics to characterise the beam. Beam quality metrics have been developed that allow quantification of electron beam characteristics, allowing objectives to be set for the optimisation process. Additionally, a method is presented that enables real world measurements to be directly compared with modelled beams. Various optimisation methods are reviewed. A genetic algorithm was selected, which would use gun modelling and beam characterisation calculations as the objective function, as a suitable method for application to this problem. However, it was recognised that selections for the best evolutionary parameters, the population size, number of parents, the mutation rate and mutation scale, were not readily determined from published work. An investigation is presented where a range of evolutionary parameters was tested for a set of geometrical problems, which had some similarity to electron gun design but could be computed sufficiently quickly to enable an extensive survey, and the most efficient combination of parameters was identified. Detail is given of the customisation of a genetic evolutionary optimisation method for the design of electron guns. Examples are presented of electron gun design optimisation processes to meet specified beam requirements within defined geometric and electrical constraints. The results of this work show that optimum evolutionary parameter settings for the geometric problem vary with the complexity of the problem and trends have been identified. Application of these parameters to an electron gun optimisation has been successful. The derived beam parameter metrics have been applied to electron guns as an objective function. Comparisons of modelled predictions of the beam characteristics with the measured real world values have been shown to be reasonable.

Apresentamos aqui resultados de propriedades eletrônicas relacionadas à defeitos complexos em GaAs. Os cálculos autoconsistentes da estrutura eletrônica foram feitos usando o método do espalhamento múltiplo X a junto com o modelo do aglomerado molecular e com os orbitais de superfície saturados com a esfera de Watson. / We repor! resu/ts for e/ectronic properties related to complex defects in GaAs. The self-consistent-fie/d electronic structure ca/culations were performed by using the multiplescattering X a method within framework of Watson-sphere-terminated molecular cluster model. We have simulated, first, the electronic states of the 26-atom clusters: 26Td/1As (tetrahedral interstitial As centered cluster), 26Td/1Ga (tetrahedral interslitial Ga cenlered clusle!) and 26C3v (trigonal cluster centered in the middle of a Ga-As bond).