Direct tuning of large-gap quantum spin Hall effect in mono transition metal carbide MXenes

Abstract

Some two-dimensional (2D) MXene topological insulators (TIs) show a large nontrivial bandgap, which strongly depends on the relativistic effect or electron correlation effects. However, the factors of orbitals are always neglected. Herein, focusing on 5d mono-transition metal MXenes, we demonstrate that the orbital effects of Ta₂CF₂ monolayers can directly enhance the nontrivial gaps through the changes of thickness (Δh) as a pivot. Compared to the 5d mono-transition metal MXene W₂CO₂, the surface-functionalized Ta₂CF₂ monolayers have a larger thickness (Δh) between F and F atoms. Meanwhile, the nontrivial bandgap (0.810 eV) at the Γ-point is significantly enhanced compared to that of W₂CO₂ (0.472 eV). To emphasize the important role of thickness in regulating the nontrivial bandgaps, we have studied a set of hypothetical Ta₂CF₂ monolayers, which have different Δh between F and F atoms on the surface of Ta₂CF₂ monolayers. The results show that the nontrivial gaps can be directly enhanced by increasing the Δh, which originates from the decrease of the orbital overlap between the d_z² and d_x²−y²,xy orbitals. In other words, the orbital effects are available for increasing bandgaps in MXene TIs. Our study provides an alternative view of designing large nontrivial bandgaps in 2D MXene TIs.