Prediction of Two-Dimensional Group IV Nitrides AN (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 AN (A = Sn, Ge, or Si) prototypes, 2/ AN, 3̅1 AN, 31 AN, 3̅1 AN, 6̅2 AN, 3̅1 AN, 6̅2 AN, 31 AN, 4̅2 AN, and 3̅1 AN, 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 cm V s) were evidenced in these 2D structures. We found that 2D 31 SnN, GeN, and SiN are intrinsic ferromagnetic semiconductors with gaps of 0.677, 1.285, and 2.321 eV, respectively. The lattice symmetry and Si-to-N charge transfer upon strain lead to large anisotropic negative Poisson's ratios (-0.281 to -0.146) along whole in-plane directions in 2D 4̅2 SiN. Our findings not only enrich the family of 2D nitrides but also highlight the promising optoelectronic and nanoauxetic applications of 2D group IV nitrides.