Fig. 1. Optical emission induced by adsorbates and engineered point defects in single-layer MoS₂.

a Schematic of as-exfoliated single layer MoS₂ on hBN (green) supported on a Si/SiO₂ substrate (gray). Black (yellow) dots denote molybdenum (sulfur) atoms. Red dots denote oxygen atoms, which are present either as part of adsorbates or as substitutional atoms on sulfur sites. b Heating the substrate to mild annealing temperatures (T_annealing < 500 K, orange) removes the absorbates. c Heating the substrates to high annealing temperatures (T_annealing > 500 K, red) creates sulfur vacancies by thermal desorption (arrows). d Schematic of vacancy generation in MoS₂ fully encapsulated in hBN through He-ion (He⁺) bombardment. e Evolution of low-temperature (T ∼20 K) photoluminescence spectra. As-exfoliated MoS₂ exhibits a broad sub-gap luminescence (L-band) due to adsorbates. Mild annealing at T_annealing =  500 K removes these adsorbates suppressing the L-band. Upon annealing at T_annealing =  800 K, a narrow defect luminescence at 1.75 eV emerges (X_L), due to the thermal generation of sulfur vacancies with well-defined in-gap states. Vertical, dashed lines indicate the emission energy of the neutral exciton (⁰X_A), the trion (⁻X_A), and X_L. The intensities are normalized to the exciton transition ⁰X_A. Helium-ion (He⁺) irradiation of MoS₂ encapsulated in hBN using a helium ion microscope (HIM) generates a similar defect feature at 1.75 eV (T = 4.2 K).