Strain- and Ar/H₂ Annealing-Induced SulfurDefects in Monolayer MoS₂ Probed by Raman Spectroscopy

Defects and strain in monolayer transition metal dichalcogenides (TMDs) can significantly affect their physical properties. By applying strain through the substrate and generating a large number of sulfur vacancies via annealing, we observe the Raman-forbidden LA(M) mode in monolayer MoS₂. We further investigate factors influencing the LA(M) mode, including the number of layers, the substrate, and the annealing temperature. Our findings show that combining the intrinsic <i>E</i>_2g, <i>A</i>_1g, and LA(M) modes of Raman spectroscopy can quantify the density of sulfur defects and the degree of strain. Additionally, the overtone 2LA(M) mode exhibits almost no dependence on the LA(M) mode and can appear in samples under different conditions. The activation of the LA(M) mode is correlated with the disorder induced by both the density of sulfur vacancies and strain from the gold (Au)-coated substrate. These findings offer crucial insights into the controlled manipulation of defects in TMDs.