Intrinsic piezoelectricity in monolayer $\mathrm{XSi_2N_4}$ (X=Ti, Zr, Hf, Cr, Mo and W)

TL;DR

This study uses density functional theory to reveal intrinsic piezoelectricity in monolayer XSi2N4 materials, showing how strain and elemental substitution can significantly enhance their piezoelectric coefficients for nanoscale device applications.

Contribution

It is the first computational investigation of piezoelectricity in XSi2N4 monolayers, demonstrating how strain and element substitution improve piezoelectric properties.

Findings

CrSi2N4 has the highest $d_{11}$ of 1.24 pm/V among studied monolayers.

Strain engineering can increase $d_{11}$ of MoSi2N4 by 107% at 4% tensile biaxial strain.

Substituting N with P or As in MoSi2N4 greatly enhances $d_{11}$, reaching up to 6.23 pm/V.

Abstract

Motived by experimentally synthesized $\mathrm{MoSi_2N_4}$ (\textcolor[rgb]{0.00,0.00,1.00}{Science 369, 670-674 (2020})), the intrinsic piezoelectricity in monolayer $\mathrm{XSi_2N_4}$ (X=Ti, Zr, Hf, Cr, Mo and W) are studied by density functional theory (DFT). Among the six monolayers, the $\mathrm{CrSi_2N_4}$ has the best piezoelectric strain coefficient $d_{11}$ of 1.24 pm/V, and the second is 1.15 pm/V for $\mathrm{MoSi_2N_4}$. Taking $\mathrm{MoSi_2N_4}$ as a example, strain engineering is applied to improve $d_{11}$. It is found that tensile biaxial strain can enhance $d_{11}$ of $\mathrm{MoSi_2N_4}$, and the $d_{11}$ at 4\% can improve by 107\% with respect to unstrained one. By replacing the N by P or As in $\mathrm{MoSi_2N_4}$, the $d_{11}$ can be raise substantially. For $\mathrm{MoSi_2P_4}$ and $\mathrm{MoSi_2As_4}$, the $d_{11}$ is as high as 4.93 pm/V and 6.23 pm/V, which is mainly due to smaller $C_{11}-C_{12}$ and very small minus or positive ionic contribution to piezoelectric stress coefficient $e_{11}$ with respect to $\mathrm{MoSi_2N_4}$. The discovery of this piezoelectricity in monolayer $\mathrm{XSi_2N_4}$ enables active sensing, actuating and new electronic components for nanoscale devices, and is recommended for experimental exploration.