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Photoluminescence from Defects in Two-Dimensional Transition Metal Dichalcogenides https://doi.org/10.17863/CAM.112297 Zhu, Yiru Chhowalla, Manish Atomic defects in monolayer transition metal dichalcogenides (TMDs) such as chalcogen vacancies significantly affect their properties. In this thesis, I provide a reproducible and facile strategy to rationally induce chalcogen vacancies in monolayer MoS₂ by annealing at 600 °C in an argon/hydrogen (95 vol.%/5 vol.%) atmosphere. At sulfur vacancy densities of ~1.8×10¹⁴ cm⁻², I observe defect-mediated photoluminescence (PL) at ~1.72 eV (referred to as LX_D) at room temperature. The LX_D emission is attributed to excitons trapped at defect-induced in-gap states and is typically observed only at low temperatures (≤77 K). Time-resolved PL measurements reveal that the lifetime of defect-mediated LX_D emission is longer than band edge excitons, both at room and low temperatures (~2.44 ns at 8 K). My results provide insight into how excitonic and defect-mediated PL emission in MoS₂ are influenced by sulfur vacancies at room and low temperatures. I also show that the LX_D peak can be suppressed by annealing the defective MoS₂ in sulfur or selenium vapor, which indicates that it is possible to passivate the vacancies. At annealing temperatures ≥600 °C in selenium vapor, I observed the filling of sulfur vacancies by selenium. I extended the defect generation strategy to monolayer MoS₂ on various dielectric substrates to investigate the influence of dielectric substrates on defect-mediated emission. The LX_D emission after annealing was observed on SiO₂ and hBN, but not on HfO₂. When the MoS₂ is first annealed on SiO₂ and then transferred onto HfO₂, the LX_D emission is present on untreated HfO₂, but not on annealed HfO₂. These results suggest that surface states in dielectric substrates can influence the defect-mediated emission. University of Cambridge 2024-03-30 Thesis Doctoral Doctor of Philosophy (PhD) eng https://www.repository.cam.ac.uk/handle/1810/374109 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/22cbeee4-d36f-4553-add2-54fe7cab4326/download 87eda9de84448d1f82354d60eee3eb5f https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/eedc9bdc-919e-45c2-bc90-73b064695c52/download b1d41e3267599a4dfb3d1c96803ffd4e https://www.rioxx.net/licenses/all-rights-reserved/ Defect Photoluminescence Two-Dimensional Transition Metal Dichalcogenides