  Control on spatial location and density of defects in 2D materials can be
achieved using electron beam irradiation. Conversely, ultralow accelerating
voltages (less than or equal to 5kV) are used to measure surface morphology,
with no expected defect creation. We find clear signatures of defect creation
in monolayer (ML) MoS2 at these voltages. Evolution of E' and A1' Raman modes
with electron dose, and appearance of defect activated peaks indicate defect
formation. To simulate Raman spectra of MoS2 at realistic defect distributions,
while retaining density-functional theory accuracy, we combine machine-learning
force fields for phonons and eigenmode projection approach for Raman tensors.
Simulated spectra agree with experiments, with sulphur vacancies as suggested
defects. We decouple defects, doping and carbonaceous contamination using
control (hBN covered and encapsulated MoS2) samples. We observe cryogenic PL
quenching and defect peaks, and find that carbonaceous contamination does not
affect defect creation. These studies have applications in photonics and
quantum emitters.
