Electron energy loss spectroscopy: an analytical technique for the investigation of optical and chemical properties in 2D materials and in-situ TEM holders

This PhD originally started under the title “Plasmon Tailoring of 2D Transition Metal Dichalcogenides Studied via Electron Energy Loss Spectroscopy”. The intended research goal was to investigate the plasmon properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors with electron energy loss spectroscopy (EELS) and then modify the material to change these properties in a controlled way. Throughout the course of the PhD research, this goal shifted to focus more on the technique of EELS and less on 2D materials.

An investigation into the plasmonic properties of pristine (unmodified) MoS2 lead to the publication of a first-author journal article in the Journal of Microscopy [1]. After this publication, the intent was to investigate different techniques for modifying MoS2 and other 2D TMDCs. The first approach was to investigate the effects of low energy ion implantation in creating substitutional implants in 2D MoS2 and MoSe2. Unfortunately, due to instrumental issues, sample cleanliness issues, and the effects of the Covid19 pandemic, this route did not produce any meaningful results towards this thesis. Contributions were made however to the PhD thesis of fellow student Dr. Michael Hennessy [2] and his publication in the journal Microscopy and Microanalysis [3]. The next avenue of investigation was rather than modifying the plasmons within the 2D TMDC itself, plasmonic metal nanostructures were placed on top of excitonic 2D TMDC substrates to study the coupling of excitons and plasmons. Again, the same struggles as before lead to a lack of meaningful results to contribute towards this thesis. During this time however, I was able to design and oversee the Final Year Project and MSc. Thesis of students, Neil Mulcahy and Liam Cregg respectively, on the topic of using the MNPBEM [4] simulation package to model optical and EEL spectra of metal nanostructures. These endeavours may not have contributed directly to this thesis but regardless, they were invaluable learning experiences in collaborative science and supervision of projects, while also developing skills related to electron microscopy and spectroscopy. Finally, a side project using EELS to study materials inside of an in-situ TEM holder, evolved to become a substantial investigation into the practical application of in-situ EELS, with a major focus on open access to data and open-source analysis tools.