  An intense low-energy broad luminescence peak (L-peak) is usually observed in
2D transition metal dichalcogenides (TMDs) at low temperatures. L-peak has
earlier been attributed to bound excitons, but its origins are widely debated
with direct consequences on optoelectronic properties. To decouple the
contributions of physisorbed and chemisorbed oxygen, organic adsorbates, and
strain on L-peak, we measured a series of monolayer (ML) MoS2 samples
(mechanically exfoliated (ME), synthesized by oxygen-assisted chemical vapour
deposition (O-CVD), hexagonal boron nitride (hBN) covered and hBN
encapsulated). Emergence of L-peak below 150 K and saturation of
photoluminescence (PL) intensity with laser power confirm bound nature of
L-peak. Anomalously at room temperature, O-CVD samples show high A-exciton PL
(c.f. ME), but reduced PL at low temperatures, which is attributed to
strain-induced direct-to-indirect bandgap change in low defect O-CVD MoS2.
Further, L-peak redshifts dramatically ~ 130 meV for O-CVD samples (c.f. ME).
These observations are fully consistent with our predictions from density
functional theory (DFT) calculations, considering effects of both strain and
defects, and supported by Raman spectroscopy. In ME samples, charged oxygen
adatoms are identified as thermodynamically favourable defects which can create
in-gap states, and contribute to the L-peak. The useful effect of hBN is found
to originate from reduction of charged oxygen adatoms and hydrocarbon
complexes. This combined experimental-theoretical study allows an enriched
understanding of L-peak and beneficial impact of hBN, and motivates collective
studies of strain and defects with direct impact on optoelectronics and quantum
technologies.
