  Assessing atomic defect states and their ramifications on the electronic
properties of two dimensional van der Waals semiconducting transition metal
dichalcogenides (SC TMDs) is the primary task to expedite multi disciplinary
efforts in the promotion of next generation electrical and optical device
applications utilizing these low dimensional materials. Here, with electron
tunneling and optical spectroscopy measurements with density functional theory,
we spectroscopically locate the midgap states from chalcogen atom vacancies in
four representative monolayer SC TMDs (MoS2, WS2, MoSe2, WSe2), and carefully
analyze the similarities and dissimilarities of the atomic defects in four
distinctive materials regarding the physical origins of the missing chalcogen
atoms and the implications to SC mTMD properties. In addition, we address both
quasiparticle and optical energy gaps of the SC mTMD films and find out many
body interactions significantly enlarge the quasiparticle energy gaps and
excitonic binding energies, when the semiconducting monolayers are encapsulated
by non interacting hexagonal boron nitride layers.
