  Research on two-dimensional (2D) topological insulators (TIs) is obstructed
due to the lack of feasible approaches to growing 2D TIs in experiments.
Through systematic first-principles calculations and tight-binding simulations,
we propose that alloying Os in 2D MoX2 (X = S, Se, Te) monolayers is an
effective approach to inducing semiconductor-to-TI transitions, with sizable
nontrivial gaps of 25-37 meV. Analysis of the electronic structures reveals
that the topological property mainly originates from the 5d orbitals of the Os
atom. Furthermore, the TI gaps can be modulated by external biaxial strain.
