  Two-dimensional (2D) Janus Transition Metal Dichalcogenides (TMDs) have
attracted much interest due to their exciting quantum properties arising from
their unique two-faced structure, broken-mirror symmetry, and consequent
colossal polarisation field within the monolayer. While efforts have been made
to achieve high-quality Janus monolayers, the existing methods rely on highly
energetic processes that introduce unwanted grain-boundary and point defects
with still unexplored effects on the material's structural and excitonic
properties Through High-resolution scanning transmission electron microscopy
(HRSTEM), density functional theory (DFT), and optical spectroscopy
measurements; this work introduces the most encountered and energetically
stable point defects. It establishes their impact on the material's optical
properties. HRSTEM studies show that the most energetically stable point
defects are single (Vs and Vse) and double chalcogen vacancy (Vs-Vse),
interstitial defects (Mi), and metal impurities (MW) and establish their
structural characteristics. DFT further establishes their formation energies
and related localized bands within the forbidden band. Cryogenic excitonic
studies on h-BN-encapsulated Janus monolayers offer a clear correlation between
these structural defects and observed emission features, which closely align
with the results of the theory. The overall results introduce the defect genome
of Janus TMDs as an essential guideline for assessing their structural quality
and device properties.
