  Defects usually play an important role in tuning and modifying various
properties of semiconducting or insulating materials. Therefore we study the
impact of point and line defects on the electronic structure and optical
properties of MoS2 monolayers using density-functional methods. The different
types of defects form electronic states that are spatially localized on the
defect. The strongly localized nature is reflected in weak electronic
interactions between individual point or line defect and a weak dependence of
the defect formation energy on the defect concentration or line defect
separation. In the electronic energy spectrum the defect states occur as deep
levels in the band gap, as shallow levels very close to the band edges, as well
as levels in-between the bulk states. Due to their strongly localized nature,
all states of point defects are sharply peaked in energy. Periodic line defects
form nearly dispersionless one-dimensional band structures and the related
spectral features are also strongly peaked. The electronic structure of the
monolayer system is quite robust and it is well preserved for point defect
concentrations of up to 6%. The impact of point defects on the optical
absorption for concentrations of 1% and below is found to be very small. For
higher defect concentrations molybdenum vacancies were found to quench the
overall absorption and sulfur defects lead to sharp absorption peaks below the
absorption edge of the ideal monolayer. For line defects, we did not find a
considerable impact on the absorption spectrum. These results support recent
experiments on defective transition metal chalcogenides.
