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2D transition metal dichalcogenides (TMDs) are leading materials for next-generation optoelectronics, but fundamental problems stand enroute to commercialization. These problems include firstly, the widely debated defect and strain-induced origins of intense low-energy broad luminescence peaks (L-peak) observed at low temperatures. Secondly, role of oxygen in tuning properties via chemisorption and physisorption is intriguing but challenging to understand. Thirdly, physical understanding of benefits of hBN encapsulation is inadequate. Using a series of samples, we decouple contributions of oxygen, defects, adsorbates, and strain on optical properties of monolayer MoS₂. Defect-origin of L-peak is confirmed by temperature and power-dependent photoluminescence (PL) measurements, with a dramatic redshift ~ 130 meV for oxygen-assisted chemical vapour deposition (O-CVD) samples (c.f. exfoliated). Anomalously, O-CVD samples show high A-exciton PL at room temperature (c.f. exfoliated), but reduced PL at low temperatures, attributed to strain-induced direct-to-indirect bandgap-crossover in low-defect O-CVD MoS₂. These observations are consistent with our density functional theory calculations, and supported by Raman spectroscopy. In exfoliated samples, charged O-adatoms are identified as thermodynamically favourable defects, and create in-gap states. Beneficial effect of encapsulation originates from reduction of charged O-adatoms and adsorbates. This experimental-theoretical study uncovers the type of defects in each sample, enables an understanding of the combined effect of defects, strain and oxygen on band structure, and enriches understanding of effects of encapsulation. This work proposes O-CVD for creating high-quality materials for optoelectronics.

Rights:

• https://creativecommons.org/licenses/by-nc-nd/3.0/
  

Keyword: MoS2, strain, defects, 2D materials, optical spectroscopy, DFT, oxygen

Date published: 2024-10-01

Publisher: IOP Publishing

Journal:

• 2D Materials (ISSN: 20531583) vol. 11 issue. 4 p. 45003-45003 045003

Funding:

• Prime Ministers Research Fellowship
• INSPIRE-DST
• Science and Engineering Research Board SRG-2020-000133
• Department of Science and Technology India
• Ministry of Electronics and Information technology
• National Supercomputing Mission
• Japan Society for the Promotion of Science 19H05790

Manuscript type: Author's version (Accepted manuscript)

MDR DOI:

First published URL: https://doi.org/10.1088/2053-1583/ad4e44

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Updated at: 2025-08-30 08:30:17 +0900

Published on MDR: 2025-08-30 08:17:29 +0900