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 MN2 (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 Hand T-phases, these M-MN2 nanosheets are indirect-gap semiconductors, which account for the experimental observation on the TaN2 system. Particularly, the band edges of M-TaN2 can straddle the redox potentials of water, and the carrier mobilities of M-TaN2 and M-NbN2 nanosheets are much larger than those of the MoS2 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.