Engineering Valley-Electronic Structures and Optical Properties of Monolayer WS₂ by Large Biaxial Strain

Monolayer tungsten disulfide (1L-WS2) is an atomically layered semiconductor with direct bandgap and novel physical and optical properties being extensively investigated by researchers worldwide. Here we present an in-depth investigation on tunability of valley-electronic and optical properties of 1L-WS2 under large biaxial tensile strain with in situ microreflectance and photoluminescence spectroscopic techniques. At room temperature, a red shift as large as 220.0 meV in the A-exciton luminescence energy and direct-to-indirect bandgap transition was observed in the 1L-WS2 sample under biaxial strain. Notably, the excitonic line width exhibits a strain-dependent dichotomic behaviors: below 2.5% strain, the line width narrows by 11.0 meV due to the enhanced KK-KQ valley separation, while above 2.5%, it broadens by 8.0 meV due to the activation of KΓ-mediated intervalley scattering. Reflectance spectroscopic measurements reveal that strain-driven renormalization of spin–orbit coupling splitting can be as large as 47.0 meV. Furthermore, the Stokes shift between the absorption and luminescence peaks can be effectively tuned by biaxial strain due to the regulation of exciton–phonon coupling and the bandgap transition from a direct gap to an indirect gap by the biaxial strain. This work establishes a framework for mechanically tailoring excitonic optical properties, spin-valley interactions, and band structures in 2D quantum materials, with implications for strain-tunable optoelectronic and valleytronic devices.