Two-dimensional MSi2N4 monolayers and van der Waals heterostructures: Promising spintronic properties and band alignments

A class of septuple-atomic-layer two-dimensional (2D) materials, $\mathrm{Mo}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ and $\mathrm{W}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$, was fabricated recently, and the three constituent atoms may be replaced by other elements in the Periodic Table, thus forming a large family. In the current work, using density functional theory calculations, we systematically investigate 18 $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ (where $M$ denotes groups IV-B, V-B, and VI-B transition metals) monolayers in both H- and T-phases for their stabilities and their electronic, magnetic, and spintronic properties, and we highlight the spintronic properties and their van der Waals heterostructures. The electronic and magnetic properties of these 2D monolayers are correlated with the $d$-levels splitting (from crystal field and exchange field) and electron filling. Spin-dependent band alignment of $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ heterostructures shows that most of them are type II heterostructures for both spin channels. In addition, the valence-band edge of seven $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ monolayers resides at the Brillouin zone center ($\mathrm{\ensuremath{\Gamma}}$-point), which may form momentum-matched heterostructures by stacking with other 2D semiconductors. The ferromagnetic semiconductors (H-$\mathrm{V}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$, H-$\mathrm{Nb}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$, and H-$\mathrm{Ta}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$) and the heterostructures composed of them (e.g., H-$\mathrm{Mo}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$/H-$\mathrm{V}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ or H-$\mathrm{W}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$/H-$\mathrm{V}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$) can be a half-semiconductor or half-metal. These results, in addition to the literature reporting high Curie temperatures, indicate that $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ and their heterostructures are promising for room-temperature spintronics and optoelectronics.