Strain Engineering and Raman Spectroscopy of Monolayer Transition Metal Dichalcogenides

We describe a facile technique based on polymer encapsulation to apply several percent (>5%) controllable strains to monolayer and few-layer transition metal dichalcogenides (TMDs). We use this technique to study the lattice response to strain via polarized Raman spectroscopy in monolayer WSe₂ and WS₂. The application of strain causes mode-dependent red shifts, with larger shift rates observed for in-plane modes. We observe a splitting of the degeneracy of the in-plane E′ modes in both materials and measure the Grüneisen parameters. At large strain, we observe that the reduction of crystal symmetry can lead to a change in the polarization response of the A′ mode in WS₂. While both WSe₂ and WS₂ exhibit similar qualitative changes in the phonon structure with strain, we observe much larger changes in mode positions and intensities with strain in WS₂. These differences can be explained simply by the degree of iconicity of the metal-chalcogen bond.