and 6 × 6 × 1 MonkhorstPack k-point grid, respectively are used. The elastic stiness coecients (C ij ) are obtained using a nite dierence method as implemented in the VASP code. DFPT is used to calculate Born effective charges (Z ij ) and ionic and electronic parts of piezoelectric (e ij ) tensors. A 4 × 4 × 1 supercell is used for the phonon dispersion calculations of the monolayers, which is obtained by PHONOPY code using DFPT method.