Uncovering a Stable Phase in Group V Transition-metal Dinitride (MN₂, M = Ta, Nb, V) Nanosheets and Their Electronic Properties via First-principles Investigations

Owing to the fast developments of transition-metal dichalcogenides (TMDs), their cousins, that is, layered transition-metal dinitrides (TMDNs), have also drawn appreciable research interests. Generally, these TMDNs nanosheets are expected to share similar geometries to the TMDs ones with trigonal prismatic (H-phase) or octahedral (T-phase) structures. However, in this work, through first-principle calculations, a different geometry is identified as the most stable structure of group V MN 2 (M = Ta, Nb, V) nanosheets so far. In this new phase (M-phase), the upper and lower N atoms are alternately bonding and staggering to each other, which significantly enhances the energetic stability and leads to robust mechanical, dynamical, and thermal stabilities. Unlike the metallic H- and T-phases, these M-MN 2 nanosheets are indirect-gap semiconductors, which account for the experimental observation on the TaN 2 system. Particularly, the band edges of M-TaN 2 can straddle the redox potentials of water, and the carrier mobilities of M-TaN 2 and M-NbN 2 nanosheets are much larger than those of the MoS 2 nanosheet, which endow these TMDNs with promising optoelectronic and green-energy applications. Our study demonstrates that new geometrical structures will emerge in the transition-metal dinitrides, which bring distinct electronic properties from the TMD systems.