Stability of symmetry required 2D topological insulators: ab-inito calculations of point defects in monolayer WTe₂

The prediction of a 2D TI state in monolayer WTe₂ has stimulated large excitement and an immense experimental effort. In what we call symmetry required TIs, such as graphene or WTe₂, defects can act as trivial mass terms that can destroy the 2D TI state at a low critical disorder strength or defect concentration. However little is known about the types and energetics of defects that can occur in monolayer WTe₂ and their effects on the electronic structure.We therefore calculate the formation energies of point defects of monolayer WTe₂ in the 1T' structure. Three defects, all of them adatoms, have negative formation energies, while all other defects have large positive formation energies. The small difference in electronegativity between W and Te leads to a covalent network of bonds with strong contributions of both W-d and Te-p states around the Fermi energy. We show that the adatoms with negative formation energies are weak perturbations that do not influence the 2D TI state, while the other defects can induce a phase transition to a trivial, insulating state. Additionally, we present STM images of Te-adatoms on mono-, bi- and trilayers of WTe₂ that agree well with STM images reported in the literature.