Biaxial substrate strain tuning of excitons in monolayer MoS₂ and MoSe₂

We report the excitonic properties of monolayer TMDs grown by high temperature physical vapor deposition (PVD) on SiO₂ and Si₃N₄. By using photoluminescence (PL) measurements, the exciton transition energy is observed to redshift by 66.0 meV (MoS₂) and 34.0 meV (MoSe₂) on SiO₂, while 35.0 meV (MoS₂) and 22.0 meV (MoSe₂) on Si₃N₄ substrates. We explain this result by biaxial tensile strain originating from thermal expansion coefficient mismatch on two substrates. The expected strain levels were consistent with the amount of phonon softening observed by Raman spectroscopy, while polarization-resolved second harmonic generation confirmed the biaxial nature of strain. Spatial mapping of PL shed further light on the nature of substrate-induced strain.