Monolayer transition metal dichalcogenides (TMDs) hold great promise for optoelectronic devices, but their potential for successful application is limited by the various types of atomic defects that can exist [1] . Such defects can be introduced during growth, processing, and subsequent analysis, and their structures can be understood using aberration-corrected scanning transmission electron microscopy (STEM). In monolayer TMD films, line defects such as grain boundaries result from the merging

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... of misoriented islands during growth [2] . Optimization of growth parameters can potentially eliminate these grain boundaries, and a recent advance in film growth by metal-organic chemical vapor deposition has produced 2" wafer-scale single-orientation monolayer WS2 films on sapphire substrates [3] . However, annular darkfield (ADF-) STEM imaging reveals that these films still contain defect arrays that result from the coalescence of grains. Two types of linear defect arrays will be explored in this presentation. First, where oriented WS2 grains merge with a sub-unit cell offset, translational grain boundaries exist, which are analyzed using ADF-STEM imaging [4] . ReaxFF reactive force field molecular dynamics (MD) simulations show that their edge morphologies correspond to relatively fast growth, and dark-field (DF-) TEM also shows that these grain boundaries form networks throughout the monolayer films. Statistical analysis of the edge orientations shows their preferred angular distribution relative to the zigzag edge of WS2, which is related to the structure and periodicity of atomic units along the boundary. Second, where oriented WS2 grains merge in registry, W-vacancy arrays exist [5] . The experimentally observed morphologies of these metal-vacancy arrays will be discussed and explained in the context of their growth mechanisms. ReaxFF MD simulations support that the vacancy arrays form during film coalescence because of the steric and catalytic effects of the precursor molecules, substrate, and a nearby WS2 edge. Through this framework, mechanisms for different morphologies of W-vacancy arrays will be discussed. Finally, the in situ electron beam-induced atomic-scale dynamics of defect arrays will be explored. The existence of defect arrays in TMD films highlights the need to engineer film growth and defect formation/annihilation processes. In this case, the in situ atomic transformations that occur are able to provide valuable insights into the process of grain coalescence.