Angular Emission Properties of Strained Transition-Metal Dichalcogenides in the Low Strain Regime

Monolayers of transition-metal dichalcogenides have shown that uniaxial strain changes both the photoluminescence emission energy and intensity. The changes are attributed to the band-structure evolution under tensile strain, where both the bandgap decreases and a direct-to-indirect transition occurs. This was shown for relatively high strains, whereas this is not the case at low strain values <1% in which, in this work, we observe nonmonotonic dependency of the photoluminescence intensity at low strain values as a function of strain. We find that in the regime of low excitation power and low strain, the dominant physical property is the dependence of the optical-dipole emission on the curvature of the substrate and not the direct-to-indirect transition, which is more dominant at high strain values. We validate the behavior of the photoluminescence intensity with experimental angular emission spectroscopy (k-space imaging). These findings are supported by finite-difference time-domain simulations, in agreement with the experimental data. Our findings present the importance of choosing the right substrate for flexible devices based on transition-metal dichalcogenides.