Strain-Engineered Ferroelectricity in 2H Bilayer MoS₂

The exploration of two-dimensional (2D) materials exhibiting out-of-plane ferroelectric and piezoelectric properties through interlayer twist/translation or strain, known as sliding ferroelectricity, has become a focal point in the quest for low-power electronic devices, capitalizing on weak van der Waals interactions. Herein, we delve into the behavior of strained bilayer molybdenum disulfide (2L-MoS₂) transferred onto a nanocone-patterned substrate. An intriguing observation is the emergence of unexpected vertical ferroelectricity in MoS₂, irrespective of whether it was prepared using chemical vapor deposition or mechanical exfoliation from the bulk crystal. Such an observation underscores the versatility and reproducibility of the emerging ferroelectricity across different preparation methods. Furthermore, the piezoelectric coefficients recorded are exceptionally high, with the values of 37.54 and 24.80 pm V^-1 for monolayer and bilayer MoS₂, respectively, outperforming most currently discovered 2D piezoelectrics. The presence of room-temperature out-of-plane ferroelectricity in strained 2L-MoS₂ is confirmed through first-principles calculations and piezoresponse force microscopy. This ferroelectric behavior can be attributed to the symmetry breaking and interlayer sliding within the strained 2L-MoS₂ structure. Our findings not only deepen the understanding of ferroelectricity in 2D materials but also offer insights for the design of 2D ferroelectrics, thereby enabling diverse functionalities and applications in ferroelectricity.