Single Layer Molybdenum Disulfide under Direct Out-of-Plane Compression: Low-Stress Band-Gap Engineering.

Tuning the electronic structure of 2D materials is a very powerful asset toward tailoring their properties to suit the demands of future applications in optoelectronics. Strain engineering is one of the most promising methods in this regard. We demonstrate that even very small out-of-plane axial compression readily modifies the electronic structure of monolayer MoS2. As we show through in situ resonant and nonresonant Raman spectroscopy and photoluminescence measurements combined with theoretical calculations, the transition from direct to indirect band gap semiconductor takes place at ∼0.5 GPa, and the transition to a semimetal occurs at stress smaller than 3 GPa.