Strain engineering in single-, bi- and tri-layer MoS₂, MoSe₂, WS₂ and WSe₂

Strain is a powerful tool to modify the optical properties of semiconducting transition metal dichalcogenides like MoS₂, MoSe₂, WS₂ and WSe₂. In this work we provide a thorough description of the technical details to perform uniaxial strain measurements on these two-dimensional semiconductors and we provide a straightforward calibration method to determine the amount of applied strain with high accuracy. We then employ reflectance spectroscopy to analyze the strain tunability of the electronic properties of single-, bi- and tri-layer MoS₂, MoSe₂, WS₂ and WSe₂. Finally, we quantify the flake-to-flake variability by analyzing 15 different single-layer MoS₂ flakes.