Designing Flexible Quantum Spin Hall Insulators through 2D Ordered Hybrid Transition-Metal Carbides

Quantum spin Hall (QSH) insulators have attracted much attention due to their potential applications ranging from electronic devices to quantum computing. In general, a large band gap is regarded as a critical descriptor in the design of QSH insulators; however, it faces challenges when additional factors such as strain and surface oxidation are involved in practical applications. In this work, taking M″₂M′C₂O₂ (M′ = Ti, Zr, Hf; M″ = Mo, W) as a representative, results reveal that 2D ordered transition-metal carbides (MXenes) are promising candidates for flexible spintronic devices, which is ascribed to the mechanical flexibility and robust QSH states under strain. Although a large bulk band gap is shown in M″₂HfC₂O₂, a strain-induced topological phase transition may limit its flexible application. On the contrary, M″₂TiC₂O₂ has a smaller gap, and its topological nontrivial state survives under strain. When n changes from 0 to 4 in M″₂Ti_nC_n+1O₂, a topologically nontrivial–trivial phase transition is observed in W₂Hf_nC_n+1O₂, whereas a topologically nontrivial state remains in Mo₂Ti_nC_n+1O₂. After further screening a variety of promising coatings, it is found that fluorographene may effectively preserve the topologically nontrivial nature of M″₂M′C₂O₂ with surface oxidation resistance, even under strain, providing a feasible application of M″₂M′C₂O₂ as flexible QSH insulators.