Orientation dependent interlayer stacking structure in bilayer MoS domains.

We have studied the atomic structure of small secondary domains that nucleate on monolayer MoS grown by chemical vapour deposition (CVD), which form the basis of bilayer MoS. The small secondary bilayer domains have a faceted geometry with three-fold symmetry and adopt two distinct orientations with 60° rotation relative to an underlying monolayer MoS single crystal sheet. The two distinct orientations are associated with the 2H and 3R stacking configuration for bilayer MoS. Atomic resolution images have been recorded using annular dark field scanning transmission electron microscopy (ADF-STEM) that show the edge termination, lattice orientation and stacking sequence of the bilayer domains relative to the underlying monolayer MoS. These results provide important insights that bilayer MoS growth from 60° rotated small nuclei on the surface of monolayer MoS could lead to defective boundaries when merged to form larger continuous bilayer regions and that pure AA' or AB bilayer stacking may be challenging unless from a single seed.