| Summary: | <p>Furthering the integration of theoretical and computational electronic structure methods with electron microscopy techniques is the overarch- ing theme of this thesis. In this work, theoretical and computational tools are developed and applied to several specific experimental problems in electron microscopy.</p> <p>Hartree-Fock theory is combined with methods for calculating damped dispersion and short-range correlation energies and used to calculate the blue shift in the 1s<sup>2</sup> → 1s2p(<sup>1</sup>P) excitation energy as a function of the density of confined helium atoms under extreme pressures. This is then compared to existing electron energy loss spectroscopy measurements on these systems. We find that, though the trends vary widely between different experiments, our theoretical predictions do not match very closely with any of them. We conclude that this is due either to the difficulty of the experimental technique or physical effects that are not accounted for in our model or the interpretation of the data.</p> <p>A general approach for obtaining projected electrostatic potentials from pseudopotential plane wave (PPW) density functional theory (DFT) is presented in detail. Projected potentials are the crucial physical quantity of interest in the simulation of (scanning) transmission electron microscopy ((S)TEM) images, and the incorporation of more accurate electronic structure theory into the calculation of these potentials improves the accuracy of (S)TEM simulations. A novel solution to the problems arising in handling the singularities due to nuclei in the projected potentials is also presented and shown to improve the precision of image simulations while simultaneously reducing the complexity of obtaining projected potentials from DFT for the user. Through its implementation in CASTEP, a PPW code, this method of obtaining projected potentials is then used to demonstrate the sensitivity of the experimental technique of (S)TEM ptychography to the electronic structure of monolayer hexagonal boron nitride.</p>
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