Kieczka, Daria; (2025) Defects and defect clusters on surfaces of Transition Metal Dichalcogenides MoS₂ and WS₂. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Transition Metal Dichalcogenides (TMDs), a class of van der Waals materials, exhibit diverse optoelectronic properties that vary with layer thickness, making them attractive for electronic applications such as field-effect transistors (FETs). However, their performance is often altered by the presence of defects. This thesis provides a comprehensive investigation of point and cluster defects on the surfaces of TMDs, focusing on monolayer MoS2, monolayer WS2, and surface of bulk WS2. 4D scanning transmission electron microscopy (STEM) was used to analyse defect clustering in monolayer MoS2, revealing that larger sulphur (S) vacancy clusters generate continuous empty defect states near the conduction and valance bands that have the potential to form one-dimensional (1D) metallic channels. In contrast, rhenium line clusters exhibit potential for high-spin states. Our study proposes mechanisms for vacancy clustering driven by local vacancy rearrangement in the high defect density regime and electron/hole accumulation within clusters. For S vacancy formation, it was found that Frenkel pair formation poses a significant barrier in pristine structures, though this barrier is reduced in charged or rhenium-doped systems. Through density functional theory (DFT) calculations, several point defects were identified and characterised in monolayer WS2, demonstrating that generalised gradient approximation (GGA) functionals are effective for defects with the lowest formation energies, while hybrid functionals are necessary for defect states near the conduction and valence bands. The oxidation of TMDs was explored, a critical degradation pathway, under varying oxygen pressures. X-ray photoelectron spectroscopy (XPS) experiments on highly defective WS2 surfaces show that oxidation rates are minimal at low oxygen pressures due to the reduced likelihood of defect-oxygen collisions. Defects were identified as active sites for oxidation reactions and further investigated oxygen-surface interactions using DFT. Finally, the effects of sputtering at different energies and durations were evaluated with respect to surface defect creation, using XPS to monitor changes in defect concentrations before and after atmospheric oxidation. These results quantitatively link defect concentration to oxidation rates, demonstrating that surfaces with higher defect densities induced by sputtering oxidise more rapidly. This thesis provides new insights into the electronic structure modifications and oxidation behaviours of TMDs, advancing our understanding of the dynamics of defects and their impact on material stability and performance.

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