Solving the puzzle of higher photoluminescence yield at the edges of MoS2 monolayers grown by chemical vapor deposition

Higher photoluminescence yield from the boundaries as compared to the interiors in monolayer (1L) islands of transition metal dichalcogenides grown by chemical vapor deposition (CVD) has been frequently documented in the literature. However, the detailed understanding of this phenomenon is still lacking. Here, we investigate the effect observed in CVD grown 1L-MoS2 islands on c-sapphire substrates. The study reveals a blue shift of the A-excitonic feature from the interiors to the edges of the monolayers, suggesting the release of the tensile strain, which is resulting in the interiors due to lattice and/or thermal expansion coefficient mismatch between the layer and the substrate, toward the boundaries. The degree of valley polarization is also found to increase at the edges. However, when the as-grown monolayers are transferred on a SiO2 surface, the intensity, position, and valley polarization of the A-excitonic peak do not show any inhomogeneity over the surface. The study attributes the decrease in PL intensity and the valley polarization in the interiors as compared to the edges of these as-grown islands to the reduction of the energy gap between the Γ- and K-valley valence band maxima with the increase in the tensile strain in the layer. First principles density functional theory based calculations for the geometry optimization are performed on a 1L-MoS2 flake residing on a (0001) sapphire surface, which indeed shows the relaxation of tensile strain toward the edges.