Coexistence of in- and out-of-plane piezoelectricity in Janus XSSiN2 (X = Cr, Mo, W) monolayers

Recently, owing to the asymmetric structures, the 2D Janus piezoelectric materials have attracted much attention. However, the piezoelectric property of Janus materials related to a new 2D family named MA(2)Z(4) is rarely noticed. To explore it, a Janus XSSiN2 (X = Cr, Mo, W) structure was constructed and the corresponding piezoelectric property was systematically investigated for the first time by first-principles calculations. It is found that the CrSSiN2 MoSSiN2 and WSSiN2 possess in-plane piezoelectric coefficients d(11) of 2.51, 1.87, and 1.35 pm/V, respectively. Simultaneously, the out-of-plane piezoelectric coefficients d31 of CrSSiN2, MoSSiN2, and WSSiN2 are 0.28, 0.19, and 0.16 pm/V, respectively, which are found to be nonzero due to the vertical mirror asymmetry. In addition, the Bader charge analysis and charge density difference present that the strength of covalent or ionic bonds in CrSSiN2 is weaker than those in MoSSiN2 and WSSiN2, leading to the small elastic stiffness coefficients (C-11 - C-12 and C-11 + C-12) of CrSSiN2. Both the inversion asymmetry and vertical mirror asymmetry together with the flexible elastic property lead to great piezoelectric coefficients of CrSSiN2. This work provides a theoretical basis for piezoelectricity of new 2D Janus XSSiN2 monolayers and their application in electronic devices.