  Few- and single-layer MoS2 host substantial densities of defects. They are
thought to influence the doping level, the crystal structure, and the binding
of electron-hole pairs. We disentangle the concomitant spectroscopic expression
of all three effects and identify to what extent they are intrinsic to the
material or extrinsic to it, i.e., related to its local environment. We do so
by using different sources of MoS2 -- a natural one and one prepared at high
pressure and high temperature -- and different substrates bringing varying
amounts of charged impurities and by separating the contributions of internal
strain and doping in Raman spectra. Photoluminescence unveils various optically
active excitonic complexes. We discover a defect-bound state having a low
binding energy of 20 meV that does not appear sensitive to strain and doping,
unlike charged excitons. Conversely, the defect does not significantly dope or
strain MoS2. Scanning tunneling microscopy and density functional theory
simulations point to substitutional atoms, presumably individual nitrogen atoms
at the sulfur site. Our work shows the way to a systematic understanding of the
effect of external and internal fields on the optical properties of
two-dimensional materials.
