Moreover, the amount of experimental works studying more generalized strain cases, such as compressive (i) hydrostatic, (ii) in-plane, or (iii) out-of-plane strain, is scarce. (31) Owing to intrinsic difficulties in estimate strain values in the different methodologies, dispersive values were reported for the gauge factors of the band gap, spanning from 4 meV/% (28) to 124 meV/% (32) for biaxial strained monolayer MoS 2. While most of the works have been devoted to the study of particular case of the in-plane uniaxial tensile for monolayers, (9,10,7,11−19) the amount of works studying monolayers and multilayers in the biaxial tensile strain case is significantly reduced, and its methodologies are varied, including investigation of bubbles present in the layers, (20−23) indirectly tuning the strain through thermally expanding the substrate, (24−28) electromechanically controlled piezoelectric substrates, (29) textured substrates, (30) or mechanically bending the substrate. (8) At such high values, the band gap can be effectively red-shifted by ∼100 meV. (6,7) Moreover, substrate-induced tensile strain up to 2.6% has been reported without fracturing. Importantly, it has been shown that it is possible to reversibly apply 1% uniaxial tensile strain on TMDCs monolayers for over 20 cycles. So far, intensive research has been focused on the effects of substrate and tensile strain on the optical and electrical properties of 2D materials. Finally, we discuss the pressure evolution of an optical transition closely lying to the A exciton for bulk WS 2 as well as the direct-to-indirect transition of the monolayer upon compression. The exceptionally large in-plane gauge factor confirms transition metal dichalcogenides as very promising candidates for flexible functionalities. The corresponding experimentally determined out-of-plane and in-plane stress gauge factors for WS 2 monolayers are −8 and 24 meV/GPa, respectively. We provide a useful model to describe effect of strain on the optical gap energy. Our results show that WS 2 remains fully adhered to the substrate at least up to a −0.6% in-plane compressive strain for a wide range of substrate materials. In the present work we combine first-principles calculations based on density functional theory and the effective Bethe–Salpeter equation with high-pressure optical measurements to thoroughly describe the effect of strain and dielectric environment onto the electronic band structure and optical properties of a few-layered transition-metal dichalcogenide. The optical properties of two-dimensional materials can be effectively tuned by strain induced from a deformable substrate.
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