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

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, M 2 C (M = V, Nb, or Ta), using first-principles calculations. Our study shows that all the three bulk M 2 C are semi-metallic and host topological phases. Interestingly, our study showed pristine monolayers V 2 C and Nb 2 C exhibit non-trivial topological phases, while Ta 2 C exhibits trivial phase. Unlike other MXenes, monolayer V 2 C and Nb 2 C 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 V 2 C 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 V 2 C is driven by quantum size effects as the Z 2 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. • The bulk M 2 C (M=V, Nb, or Ta) are found to be weak topological insulators (0; 1, 1, 1) with semi-metallic characteristics. • A Lifshitz electronic transition from semi-metallic bilayer to the monolayer semiconductor has been observed in V 2 C with a large bandgap of 0.32 eV. • The pristine monolayer V 2 C and Nb 2 C are found to be topologically non-trivial while pristine monolayer Ta 2 C is a trivial semimetal. • Topological phase transition between topologically non-trivial and trivial states occurred in these compounds by varying the thickness.