Computational Exploration of Stable 4d/5d Transition-Metal MSi2N4 (M = Y–Cd and Hf–Hg) Nanosheets and Their Versatile Electronic and Magnetic Properties

Motivated by the recent discovery of MA2Z4 family materials, we perform a systematic study on the structural stability and electronic properties of MSi2N4 nanosheets with 4d and 5d transition metals. We have identified 12 stable MSi2N4 nanosheets with trigonal prismatic (H-phase) or octahedral (T-phase) geometries, which have robust dynamic, mechanical, and thermal stabilities. It is found that most of the stable MSi2N4 nanosheets concentrate in the early transition-metal systems. Both the H- and T-phase geometries are stable in groups IIIB and IVB metal systems, while only the H-phase is stable for groups VB and VIB ones. Regarding the late transition-metal systems, only the PdSi2N4 and PtSi2N4 nanosheets with a T-phase geometry are stable. These MSi2N4 nanosheets exhibit versatile electronic properties depending on the number of valence electrons. Both the H- and T-YSi2N4 nanosheets present a half-metallic behavior, while the H-NbSi2N4 one is a promising ferrovalley material with a large valley polarization. The H-MoSi2N4, H-WSi2N4, T-PdSi2N4, and T-PtSi2N4 nanosheets have appropriate band edge energies, which are suitable for water splitting under a pH neutral environment. The semiconducting MSi2N4 nanosheets can even form diverse band alignments, including types I, II, and V, with the H-MoS2 nanosheet. Our study unveils the robust structural stability and versatile electronic properties of 4d/5d MSi2N4 nanosheets, which enable their potential applications in electronics, spintronics, and valleytronics.