Electronic and topological band evolution of VB-group transitionmetal monocarbides M2C (M=V, Nb, or Ta) bulk and monolayer

Abstract

Two-dimensional transition metal monocarbides (2D MXenes) have attracted intense interest due to their versatility, predicted topological phase, and immense applications. In this study, we investigated the electronic and topological band evolutions of both the bulk and monolayer MXenes, M2C (M = V, Nb, or Ta), using first-principles calculations. Our study shows that all the three bulk M2C are semi-metallic and host topological phases. Interestingly, our study showed pristine monolayers V2C and Nb2C exhibit non-trivial topological phases, while Ta2C exhibits trivial phase. Unlike other MXenes, monolayer V2C and Nb2C host the quantum spin Hall effect without functionalizations. Furthermore, thickness-dependent calculations intriguingly show Lifshitz electronic transition from semi-metallic to topological insulating phase in V2C from bilayer to monolayer with a sufficiently large bandgap of 0.32 eV. Moreover, the topological phase transition between the TI state and the trivial state in V2C is driven by quantum size effects as the Z2 topological invariant notably oscillates between 1 and 0 with varying thickness. Finally, our study demonstrated that VB MXenes could be promising topological materials for spintronics applications.