Prediction of Two-Dimensional Group IV Nitrides A_xN_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 AxNy (A = Sn, Ge, or Si) prototypes, C2/m A4N, P3̅m1 A3N, P3m1 A2N, P3̅m1 A3N2, P6̅m2 AN, P3̅m1 AN, P6̅2m A3N4, P3m1 A2N3, P4̅21m AN2, and P3̅m1 AN3, 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 × 103 cm2 V-1 s-1) were evidenced in these 2D structures. We found that 2D P3m1 Sn2N3, Ge2N3, and Si2N3 are intrinsic ferromagnetic semiconductors with gaps of 0.677, 1.285, and 2.321 eV, respectively. The lattice symmetry and Si-to-N2 charge transfer upon strain lead to large anisotropic negative Poisson's ratios (-0.281 to -0.146) along whole in-plane directions in 2D P4̅21m SiN2. Our findings not only enrich the family of 2D nitrides but also highlight the promising optoelectronic and nanoauxetic applications of 2D group IV nitrides.