Ab Initio Modelling of Photoinduced Electron Dynamics in Nanostructures

  • Ryan J. McMillan

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

Nanostructured materials have recently attracted much interest due to their unique properties which make them appealing for the next generation of efficient and novel optical/electrical devices. In particular, two-dimensional (atomically thin) materials such as transition metal dichalcogenides (TMDs) have been extensively researched and shown to exhibit, e.g., strong optical absorption when compared to their bulk counterparts, with potential applications in photovoltaics for solar energy. The optical absorption in such materials may be further improved by decoration with metal nanoparticles (MNPs) due to a plasmonic enhancement effect. While such an effect has been demonstrated experimentally, it is hard to show theoretically from an ab initio point of view due to the computational demand of such calculations: the state-of- the-art approaches commonly used to determine the optical spectra of, e.g, the semiconducting TMD (i.e. density functional theory (DFT) and many-body perturbation theory) may not be applied to the full TMD-MNP structure as the MNP is often on a much larger scale than the semiconductor rendering the ab initio methods infeasible.

In this thesis, we develop a dynamical, hybrid approach (the projected equations of motion (PEOM) method) where a composite material is split into subsystems which are then coupled electromagnetically. The primary system (e.g., the TMD) may be treated using any time- dependent theory, while the secondary system (e.g., the MNP) is modelled entirely through its frequency-dependent polarisability which is fitted to obtain parameters which enter into the PEOMs.

As a proof of concept, the PEOM method is first applied to a semiconducting quantum dot- MNP system and the obtained energy absorption rates and population dynamics are compared with results from existing analytical approaches. Good agreement between the two methods is shown, while the PEOM proves to be an improvement when investigating ultrafast excitations in such systems. The method is then further tested by comparing absorption spectra in bilayers of hexagonal boron nitride and MoS2 obtained via the PEOM approach with those obtained by time-dependent DFT and the Bethe-Salpeter equation. Again, the two approaches are in good, qualitative agreement.The thesis is left open as a starting point towards investigating the TMD-MNP structures, as well as stating the limitations of the PEOM method and suggesting possible improvements.
Date of Award04 Nov 2017
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SupervisorMyrta Grüning (Supervisor) & Daniel Dundas (Supervisor)

Cite this

Ab Initio Modelling of Photoinduced Electron Dynamics in Nanostructures
McMillan, R. J. (Author). 04 Nov 2017

Student thesis: Doctoral ThesisDoctor of Philosophy