  Two-dimensional materials such as transitional metal dichalcogenides exhibit
unique optical and electrical properties. Here we report on the varying optical
properties of CVD grown MoS2 monolayer flakes with different shapes. In
particular, it is observed that the perimeter and the central region of the
flakes have non-uniform photoluminescence (PL) energy and intensity. We
quantified these effects systematically and propose that thermally induced
strain during growth is the origin. The strain relaxation after transfer of the
MoS2 flakes supports this explanation. Detailed investigations of the spatial
distribution of the PL energy reveal that depending on the shape of the MoS2
flakes, the width of the strain field is different. Thus, our results help to
elucidate the fundamental mechanisms responsible for the differences in PL and
Raman signals between the perimeter region and the center region of monolayer
MoS2 and suggest that the induced strain plays an important role in the growth
of monolayer materials.
