Strain-Engineered\nFerroelectricity in 2H Bilayer MoS<sub>2</sub>

The exploration of two-dimensional (2D) materials exhibiting\nout-of-plane\nferroelectric and piezoelectric properties through interlayer twist/translation\nor strain, known as sliding ferroelectricity, has become a focal point\nin the quest for low-power electronic devices, capitalizing on weak\nvan der Waals interactions. Herein, we delve into the behavior of\nstrained bilayer molybdenum disulfide (2L-MoS<sub>2</sub>) transferred\nonto a nanocone-patterned substrate. An intriguing observation is\nthe emergence of unexpected vertical ferroelectricity in MoS<sub>2</sub>, irrespective of whether it was prepared using chemical vapor deposition\nor mechanical exfoliation from the bulk crystal. Such an observation\nunderscores the versatility and reproducibility of the emerging ferroelectricity\nacross different preparation methods. Furthermore, the piezoelectric\ncoefficients recorded are exceptionally high, with the values of 37.54\nand 24.80 pm V<sup>–1</sup> for monolayer and bilayer MoS<sub>2</sub>, respectively, outperforming most currently discovered 2D\npiezoelectrics. The presence of room-temperature out-of-plane ferroelectricity\nin strained 2L-MoS<sub>2</sub> is confirmed through first-principles\ncalculations and piezoresponse force microscopy. This ferroelectric\nbehavior can be attributed to the symmetry breaking and interlayer\nsliding within the strained 2L-MoS<sub>2</sub> structure. Our findings\nnot only deepen the understanding of ferroelectricity in 2D materials\nbut also offer insights for the design of 2D ferroelectrics, thereby\nenabling diverse functionalities and applications in ferroelectricity.