Ferroelectricity in two-dimensional bilayers and multilayers of MgAl2S4

The promising applications of two-dimensional (2D) ferroelectrics have intrigued lots of studies. The van der Waals stacking as a successful method has been applied to obtain the 2D ferroelectrics from non-ferroelectric parent compounds. Applying this strategy to the new 2D MA2Z4 materials, We predict the sliding ferroelectricity in bilayer and multilayer MgAl2S4 structures by the first-principles calculations. The stacking order shows a significant impact on the physical properties, where staggered stacking in the bilayer or multilayer MgAl2S4 can induce ferroelectricity but direct stacking can not. The polarization of the bilayer or multilayer MgAl2S4 arises from the interlayer transfer of uncompensated charge between the faces and the outer layers, which can be switched by interlayer interface sliding. To manipulate the polarization strength, the external strain was applied and the material properties can be modulated. In addition, similar to the monolayer behavior in bulk ReS2, the electronic structures show little difference from monolayer to six-layer MgAl2S4. However, the polarization in MgAl2S4 is more than fifteen times as strong as that of the ReS2. Our work paves the way for further exploration into van der Waals semiconductor ferroelectric systems and the development of novel high-efficiency nonvolatile memory devices.