Interplay between anisotropic spin texture and large gap topological insulating phases in functionalized MXenes*

Massive attention has been given to two-dimensional (2D) MXenes due to their predicted topological phase and promising diverse applications. Using first-principles with hybrid functional calculations, we systematically conducted an extensive study on M₂C (M = Mo, or W) in 1T and 2H structures with various surface terminations T₂ (T = H, O, OH, F, Cl, Br, or I). Pristine Mo₂C and W₂C compounds are in the 2H phase. Upon functionalization of H, O, OH and F, they retain the 2H structure, whereas Cl, Br, and I functionalization, the structures transform to 1T phase. Notably, M₂CO₂ are found to be 2D topological insulators (TIs) with sizable nontrivial bandgaps as large as 666 meV. Interestingly, 2H M₂CO₂ exhibits not only large-gapped Zeeman-type spin splitting at K-point but also anisotropic Rashba spin splitting at Γ-point. The successful synthesis of pristine Mo₂C and our theoretical study pave a path for future applications of MXene-based 2D TIs.