  Molybdenum disulfide (MoS2) is a particularly interesting member of the
family of two-dimensional (2D) materials due to its semiconducting and tunable
electronic properties. Currently, the most reliable method for obtaining
high-quality industrial scale amounts of 2D materials is chemical vapor
deposition (CVD), which results in polycrystalline samples. As grain boundaries
(GBs) are intrinsic defect lines within CVD-grown 2D materials, their atomic
structure is of paramount importance. Here, through atomic-scale analysis of
micrometer-long GBs, we show that covalently bound boundaries in 2D MoS2 tend
to be decorated by nanopores. Such boundaries occur when differently oriented
MoS2 grains merge during growth, whereas the overlap of grains leads to
boundaries with bilayer areas. Our results suggest that the nanopore formation
is related to stress release in areas with a high concentration of dislocation
cores at the grain boundaries, and that the interlayer interaction leads to
intrinsic rippling at the overlap regions. This provides insights for the
controlled fabrication of large-scale MoS 2 samples with desired structural
properties for applications.
