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 MoSi2N4 family by breaking the out-of-plane (OOP) structural symmetry through Z (P/As) substitution of N. Despite the semiconducting nature of MoSi2X4 (X=N/P/As), four Janus MoSi2NxZ4−x 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 OOP electric polarizations around 10−203 pC/m are determined by the asymmetric OOP charge distribution due to the non-centrosymmetric Janus structure. The corresponding OOP piezoelectricity is further revealed as high as 39−153 pC/m and 0.10−0.31 pm/V for piezoelectric strain and stress coefficients, respectively. The results demonstrate polar metallicity and high OOP piezoelectricity in Janus MoSi2NxZ4−x monolayers and open new vistas for exploiting unusual coexisting properties in monolayers derived from MoSi2N4 family.
@article{arxiv.2304.04209,
title = {Monolayer polar metals with large piezoelectricity derived from MoSi$_2$N$_4$},
author = {Yan Yin and Qihua Gong and Min Yi and Wanlin Guo},
journal= {arXiv preprint arXiv:2304.04209},
year = {2023}
}