Monolayer polar metals with large piezoelectricity derived from MoSi₂N₄

The advancement of two-dimensional polar metals tends to be limited by the incompatibility between electric polarity and metallicity as well as dimension reduction. Here, we report polar and metallic Janus monolayers of the MoSi₂N₄ family by breaking the out-of-plane structural symmetry through Z (P/As) substitution of N. Despite the semiconducting nature of MoSi₂X₄ (X = N/P/As), four Janus MoSi₂NZ_4- monolayers are found to be polar metals owing to the weak coupling between the conducting electrons and electric polarity. The metallicity is originated from the Z substitution induced delocalization of occupied electrons in Mo-d orbitals. The out-of-plane electric polarizations around 1.5-15.7 pC m^-1 are determined by the asymmetric out-of-plane charge distribution due to the non-centrosymmetric Janus structure. The corresponding out-of-plane piezoelectricity is further revealed as high as 18.7-73.3 pC m^-1 and 0.05-0.25 pm V^-1 for the piezoelectric strain and stress coefficients, respectively. The results demonstrate polar metallicity and high out-of-plane piezoelectricity in Janus MoSi₂NZ_4- monolayers and open new vistas for exploiting unusual coexisting properties in monolayers derived from the MoSi₂N₄ family.