  Intrinsic and extrinsic disorder from lattice imperfections, substrate and
environment has a strong effect on the local electronic structure and hence the
optical properties of atomically thin transition metal dichalcogenides that are
determined by strong Coulomb interaction. Here, we examine the role of the
substrate material and intrinsic defects in monolayer MoS2 crystals on SiO2 and
hBN substrates using a combination of scanning tunneling spectroscopy, scanning
tunneling microscopy, optical absorbance, and low-temperature photoluminescence
measurements. We find that the different substrates significantly impact the
optical properties and the local density of states near the conduction band
edge observed in tunneling spectra. While the SiO2 substrates induce a large
background doping with electrons and a substantial amount of band tail states
near the conduction band edge of MoS2, such states as well as the high doping
density are absent using high quality hBN substrates. By accounting for the
substrate effects we obtain a quasiparticle gap that is in excellent agreement
with optical absorbance spectra and we deduce an exciton binding energy of
about 480 meV. We identify several intrinsic lattice defects that are
ubiquitious in MoS2, but we find that on hBN substrates the impact of these
defects appears to be passivated. We conclude that the choice of substrate
controls both the effects of intrinsic defects and extrinsic disorder, and thus
the electronic and optical properties of MoS2. The correlation of substrate
induced disorder and defects on the electronic and optical properties of MoS2
contributes to an in-depth understanding of the role of the substrates on the
performance of 2D materials and will help to further improve the properties of
2D materials based quantum nanosystems.
