Uniaxial strain tuning of Raman spectra of a ReS₂ monolayer

In contrast to the intensively investigated transition metal dichalcogenides like MoS₂, ReS₂ crystals possess a reduced in-plane symmetry, leading to anisotropic optical properties. Here, we report on the impact of strain on the Raman response of a ReS2 monolayer. Since mechanical strain can be used to shift the Raman-active phonon frequencies, we apply uniaxial tensile strain of up to 0.74% along the Re-chain direction (x axis) of the atomically thin crystal and measure the Raman response with the scattered light polarized parallel and perpendicular to the strain direction along the x axis. Complementarily, we carry out ab initio calculations to determine the phonon energies and Raman intensities under strain. We find a shift to lower energies for all phonon modes when tensile strain is applied to the monolayer. The determined gauge factors/Grüneisen parameters are in good agreement between experiment and theory. Our study demonstrates that the optomechanical properties of ReS₂ can be tuned by external straining, which is of importance for potential future strain-sensor applications, e.g., in the biomedical sector.