Understanding the role of chemical dopants is crucial to modulating
the optoelectronic properties of monolayer (ML) MoS2 and
realizing its optoelectronic applications such as photodetectors,
switching devices, and ultrathin transistors. Here, the effect of
the dopant phase of the same dopantsliquid and gaseouson
the optical properties of ML MoS2 has been investigated
using in situ Raman microspectroscopy with an environment-controlled
reaction cell. The results show that the gaseous n-type dopant, that
is, pyridine, completely quenched the PL intensity of ML MoS2, while liquid pyridine preserved 50% of the original PL intensity
attributed to its less effective charge transfer to MoS2 than the gaseous counterpart. Photoemission of the quenched MoS2 could not be recovered with mild annealing in nitrogen and
only partially recovered upon long-term exposure to air. The results
show that the dopant phase strongly impacts the PL properties of ML
MoS2.