Two-dimensional semiconductors XY 2 (X = Ge,Sn;Y = S,Se) with promising piezoelectric properties

With the help of first-principles calculations, we proposed a new type of group IV-VI Two dimensional (2D) semiconductors (XY2, where X = Ge, Sn and Y = S, Se) with promising piezoelectronic properties. Under special point group limit, the spontaneous polarization can be produced perpendicular to the two-dimensional plane by applying in-plane shear strain. More importantly, the electronic structures of them can be effectively modulated by external strain and stacking. Considering their two-dimensional semiconductor nature, they are promising candidates for future miniaturized electronic and optoelectronic devices.