The defect modes of MoS₂ : indirect double resonance Raman spectroscopy in transition metal dichalcogenides : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Nanoscience at Massey University, Manawatū, New Zealand

Molybdenum disulfide (MoS₂) is a layered two-dimensional (2D) semiconducting crystal which has been a focal point of 2D materials research since the isolation and characterisation of graphene in 2005, owing to its relative abundance and a wide range of potential applications in thin film electronics, photovoltaics, information storage, and catalysis to name a few. It has unique layer-dependent electronic properties, demonstrating intense photoluminescence in single-layers and tightly-bound excitons at room temperature. The excitonic properties of MoS₂ lead to an optical phenomenon known as double resonance Raman (DRR), whereby the excited state charge carriers of excitons can interact with lattice vibrations (phonons) to scatter throughout the Brillouin zone during optical absorption and Raman spectroscopy measurements. These processes reveal signatures of electron and hole scattering dynamics that govern the complex electronic and physical properties of the material. At the nanoscale, the edges of MoS₂ have significant influence over the material’s properties, altering the electronic structure and contributing to doping effects for semiconductor performance and photoluminescence tuning. In addition, these edges (and defects) are also the sites of catalytic reactions, leading to intense efforts in literature to optimise MoS₂ as a catalyst for the production of clean hydrogen fuels. Consequently, understanding the edges of these materials is of great interest. This work demonstrates the design of a home-built low-frequency scanning Raman microscope, used to probe DRR features in MoS₂, revealing for the first time a defect-mediated indirect DRR mechanism which allows the quantification of defects and determination of zigzag and armchair edge structures in MoS₂ materials. This mechanism reveals indirect scattering pathways in the Brillouin zone, and appears to be applicable to similar materials in the family of transition metal dichalcogenides (TMDs). This indirect resonance mechanism has immediate application in facilitating the development of catalytic materials and novel nanomaterial architectures, as well as potential applications in the characterisation of exotic TMD materials and next-generation spin-valley coupled information storage devices