Abstract
The recently experimentally synthesized monolayer MoSi2N4 and WSi2N4 (Science 369, 670–674 (2020)) lack inversion symmetry, which allows them to become piezoelectric. In this work, based on ab initio calculations, we report structure effect on intrinsic piezoelectricity in septuple-atomic-layer MSi2N4 (M=Mo and W), and six structures (αi (i=1 to 6)) are considered with the same space group. These structures can connect to each other through translation, mirror and rotation operations of double layer unit Si2N2. It is found that MSi2N4 (M=Mo and W) with αi (i = 1 to 6) all are indirect band gap semiconductors. Calculated results show that MoSi2N4 and WSi2N4 monolayers have the same structural dependence on piezoelectric strain and stress coefficients (d11 and e11), together with the ionic and electronic contributions to e11. The α5 phase has largest d11 for both MoSi2N4 and WSi2N4, which are larger than 2.9 pm/V. Finally, we investigate the intrinsic piezoelectricity of monolayer MA2Z4 (M=Cr, Mo and W; A=Si and Ge; Z=N and P) withα1 and α2 phases expect CrGe2N4, because they all are semiconductors and their enthalpies of formation between α1 and α2 phases are very close. The most important result is that monolayer MA2Z4 containing P atom have more stronger piezoelectric polarization than one including N atom. The largest d11 among MA2N4 materials is 1.85 pm/V, which is close to the smallest d11 of 1.65 pm/V in MA2P4 monolayers. For MA2P4, the largest d11 is up to 6.12 pm/V. Among the 22 monolayers, α1-CrSi2P4,α1-MoSi2P4,α1-CrGe2P4,α1-MoGe2P4 and α2-CrGe2P4 have large d11, which are greater than or close to 5 pm/V, a typical value for bulk piezoelectric materials. These materials are recommended for experimental exploration. Our study reveals that the MA2Z4 family have the potential applications in piezoelectric field.
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