  The piezoelectricity of group IV monochalcogenides (MXs, with M = Ge, Sn and
X = S, 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 MXs with the ferroelectric AA stacking configuration, which
has recently been stabilized in experiments. We found that AA-stacked MXs 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.
