Two-dimensional superhard silicon nitrides with widely tunable bandgap, high carrier mobility and hole-doping-induced robust magnetism

The search for new forms of the traditional bulk materials to enrich their interactions and properties is an attractive subject in two-dimensional (2D) materials. In this work, novel tetra-hexa-mixed coordinated 2D silicon nitrides (Si₃N₄) and their analogues are systematically investigated via density functional theory. The results show the global minimum 2D structure, Si₃N₄ (T-aa), is a highly chemically and thermally stable superhard semiconductor with a wide indirect bandgap (about 6.0 eV), which is widely adjustable under both biaxial strain and vertical electric field. It also possesses anisotropic high carrier mobility, up to 5490 cm² V^-1 s^-1 at room temperature. Besides, its nitride analogues of group IV_A (Si, Ge, Sn, and Pb) exhibit diverse electronic structures with regular bandgap distribution. Remarkably, some nitride analogues display linearly increasing robust magnetism with hole doping. The theoretical Curie temperatures of Si₃N₄ and Sn₃N₄ with hole doping (1h⁺ per unit cell) are 298 and 180 K, respectively. The Si₃N₄ (T-aa) and its analogues have a variety of excellent properties to be potentially applied in various fields, e.g., semiconductor electronics, spintronics, high-temperature structural materials, and superhard materials.