Predicting two-dimensional topological phases in Janus materials by substitutional doping in transition metal dichalcogenide monolayers*

Ultrathin Janus two-dimensional (2D) materials are attracting intense interest currently. Substitutional doping of 2D transition metal dichalcogenides (TMDs) is of importance for tuning and possible enhancement of their electronic, physical and chemical properties toward industrial applications. Using systematic first-principles computations, we propose a class of Janus 2D materials based on the monolayers MX₂ (M=V, Nb, Ta, Tc, or Re; X=S, Se, or Te) with halogen (F, Cl, Br, or I) or pnictogen (N, P, As, Sb, or Bi) substitution. Nontrivial phases are obtained on pnictogen substitution of group VB (V, Nb, or Ta), whereas for group VIIB (Tc or Re), the nontrivial phases are obtained for halogen substitution. Orbital analysis shows that the non-trivial phase is driven by the splitting of M-d_yz and M-d_xz orbitals. Our study demonstrates that the Janus 2D materials have the tunability and suitability for synthesis under various conditions.