Janus structure derivatives SnP-InS, GeP-GaS and SiP-AlS monolayers with in-plane and out-of-plane piezoelectric performance

The piezoelectricity of two-dimensional (2D) materials has triggered the development in a wide variety of fields. Since the symmetry of materials has a close relationship with piezoelectricity, a series of derivatives of the Janus structure was investigated on the piezoelectric effect of GeP-GaS, SiP-AlS, and SnP-InS. Density functional theory (DFT) was employed to explore the stiffness and the piezoelectric tensors. The charge density distribution was calculated to explain the polarization and chemical bonds. Compared with monolayer group III monochalcogenides, such as SiN and SnP who only possess the in-plane direction piezoelectricity, the results show that all three systems have the out-of-plane piezoelectricity. The emergence of out-of-plane piezoelectricity is caused by the unmirrored structure of GeP-GaS, SiP-AlS, and SnP-InS. Simultaneously, the measured d(31) of SnP-InS is 9.69 p.m./V and is higher than that of the other two systems. Therefore, the high energy conversion efficiency between the electrical and the mechanical domain shows their great potential in applications. Moreover, the intrinsic piezoelectricity of the Janus structure in the out-of-plane direction also suggests that replacing the atoms is an effective method to explore novel piezoelectric materials.