Strain engineering, as a powerful strategy to tune the optical and electrical properties of two-dimensional (2D) materials by deforming their crystal lattice, has attracted significant interest in recent years. 2D materials can sustain ultra-high strains, even up to 10%, due to the lack of dangling bonds on their surface, making them ideal brittle solids. This remarkable mechanical resilience, together with a strong strain-tunable band structure, endows 2D materials with a broad optical and electrical response upon strain. However, strain engineering based on 2D materials is restricted by their nanoscale and strain quantification troubles. In this study, we have modified a homebuilt three-points bending apparatus to transform it into a four-points bending apparatus that allows for the application of both compressive and tensile strains on 2D materials. This approach allows for the efficient and reproducible construction of a strain system and minimizes the buckling effect caused by the van der Waals interaction by adamantane encapsulation strategy. Our results demonstrate the feasibility of introducing compressive strain on 2D materials and the potential for tuning their optical and physical properties through this approach.

Single-layer MoS₂ produced by mechanical exfoliation is usually connected to thicker and multilayer regions. We show a facile laser trimming method to insulate single-layer MoS₂ regions from thicker ones. We demonstrate, through electrical characterization, that the laser trimming method can be used to pattern single-layer MoS₂ channels with regular geometry and electrically disconnected from the thicker areas. Scanning photocurrent microscope further confirms that in the as-deposited flake (connected to a multilayer area) most of the photocurrent is being generated in the thicker flake region. After laser trimming, scanning photocurrent microscopy shows how only the single-layer MoS₂ region contributes to the photocurrent generation. The presented method is a direct-write and lithography-free (no need of resist or wet chemicals) alternative to reactive ion etching process to pattern the flakes that can be easily adopted by many research groups fabricating devices with MoS₂ and similar two-dimensional materials.

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.