Prediction of Two-Dimensional Group IV Nitrides A x N y (A = Sn, Ge, or Si): Diverse Stoichiometric Ratios, Ferromagnetism, and Auxetic Mechanical Property.

In this work, we unveiled a new class of two-dimensional (2D) group IV nitride A x N y (A = Sn, Ge, or Si) prototypes, C 2/ m A 4 N, P 3̅ m 1 A 3 N, P 3 m 1 A 2 N, P 3̅ m 1 A 3 N 2 , P 6̅ m 2 AN, P 3̅ m 1 AN, P 6̅2 m A 3 N 4 , P 3 m 1 A 2 N 3 , P 4̅2 1 m AN 2 , and P 3̅ m 1 AN 3 , by using evolutionary algorithms combined with first-principles calculations. Using HSE06 functional calculations, a wide range of band gaps from metal to semiconductor (0.405-5.050 eV) and ultrahigh carrier mobilities (1-24 × 10 3 cm 2 V -1 s -1 ) were evidenced in these 2D structures. We found that 2D P 3 m 1 Sn 2 N 3 , Ge 2 N 3 , and Si 2 N 3 are intrinsic ferromagnetic semiconductors with gaps of 0.677, 1.285, and 2.321 eV, respectively. The lattice symmetry and Si-to-N 2 charge transfer upon strain lead to large anisotropic negative Poisson's ratios (-0.281 to -0.146) along whole in-plane directions in 2D P 4̅2 1 m SiN 2 . Our findings not only enrich the family of 2D nitrides but also highlight the promising optoelectronic and nanoauxetic applications of 2D group IV nitrides.