Photoluminescence from Defects in Two-Dimensional Transition Metal Dichalcogenides

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.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Zhu, Yiru

Advisor dc:contributor.advisor

• Chhowalla, Manish

• Defect
• Photoluminescence
• Two-Dimensional Transition Metal Dichalcogenides

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/22cbeee4-d36f-4553-add2-54fe7cab4326/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.112297

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/374109