Giant piezoelectricity in group-IV monochalcogenides with ferroelectric AA layer stacking

The piezoelectricity of group-IV monochalcogenides ($\mathit{MX}\mathrm{s}$, with $M=\mathrm{Ge},\mathrm{Sn}$ and $X=\mathrm{S},\mathrm{Se}$) has attracted much attention due to their substantially higher piezoelectric coefficients compared to other 2D materials. However, with increasing layer number, their piezoelectricity rapidly disappears due to the antiferroelectric stacking order, severely limiting their practical applications. Using first-principles calculations, we investigated the piezoelectricity of $\mathit{MX}\mathrm{s}$ with the ferroelectric AA stacking configuration, which has recently been stabilized in experiments. We found that AA-stacked $\mathit{MX}\mathrm{s}$ have a ferroelectric ground state with the smallest lattice constant among other stacking configurations, resulting in a giant piezoelectric coefficient, which is the first demonstration of a strategy where the piezoelectric coefficients can increase with the number of layers. This can be attributed to a strong negative correlation between the lattice constant along the armchair direction and the piezoelectric coefficient, and spontaneous compressive strain stabilized in ferroelectric AA stacking configuration.