Strain engineering has arisen as a powerful technique to tune the electronic and optical properties of two-dimensional semiconductors like molybdenum disulfide (MoS₂). Although several theoretical works predicted that biaxial strain would be more effective than uniaxial strain to tune the band structure of MoS₂, a direct experimental verification is still missing in the literature. Here we implemented a simple experimental setup that allows to apply biaxial strain through the bending of a cruciform polymer substrate. We used the setup to study the effect of biaxial strain on the differential reflectance spectra of 12 single-layer MoS₂ flakes finding a redshift of the excitonic features at a rate between −40 meV/% and −110 meV/% of biaxial tension. We also directly compare the effect of biaxial and uniaxial strain on the same single-layer MoS₂ finding that the biaxial strain gauge factor is 2.3 times larger than the uniaxial strain one.

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.

Here, we propose a method to determine the thickness of the most common transition metal dichalcogenides (TMDCs) placed on the surface of transparent stamps, used for the deterministic placement of two-dimensional materials, by analyzing the red, green and blue channels of transmission-mode optical microscopy images of the samples. In particular, the blue channel transmittance shows a large and monotonic thickness dependence, making it a very convenient probe of the flake thickness. The method proves to be robust given the small flake-to-flake variation and the insensitivity to doping changes of MoS₂. We also tested the method for MoSe₂, WS₂ and WSe₂. These results provide a reference guide to identify the number of layers of this family of materials on transparent substrates only using optical microscopy.