Abstracts Physics

Quantum treatment of ultra-low loss electron energy loss spectroscopy with applications to molecular crystals

by Yue Wang

Continual advancements of experimental hardware for electron energy loss spectroscopy (EELS) - a material characterisation technique grounded in electron scattering - has opened up the avenue for studying vibrational excitations, as energy resolutions are now able to reach down to ~ 5meV. Although optical methods such as Infrared and Raman spectroscopy can be used to study phonon vibrations, EELS paired with scanning transmission electron microscopy (STEM) has the advantage of superior spatial resolution, making it an attractive alternative. An electron beam that is off-sample (aloof mode) reduces the sample beam damage, an attractive technique for better investigations of sensitive, organic molecular crystals such as pharmaceutical products. In this thesis, the commonly used theoretical derivations of ultra-low loss EELS are reviewed. Their shortcomings are identified, and an improved quantum formalism congruous with molecular crystals is presented. This modified ‘dipole’ theory dominates in magnitude over the current quantum treatment for aloof beam EELS, and is demonstrated to show improved relative intensity predictions for the molecular crystal of beta-guanine. It successfully distinguishes between two polymorphs of quinacridone, and reasonably quantifies the relative peak intensities. We also apply our theory to the bulk crystal of boron subphosphide by evaluating its performance over the other theoretical methods. These calculations require material properties determined from first principles using density functional theory, which will be expanded upon in the thesis.