  Interfaces between normal and topological insulators are bound to host
metallic states that are protected by time-reversal symmetry and are therefore
robust against disorder and interface reconstruction. Two-dimensional
topological insulators (quantum spin Hall insulators) offer a unique
opportunity to change the local topology by adsorption of atoms or molecules
and thus comprise an ideal platform for designing topological heterostructures.
Here, we apply first principles calculations to show that the quantum spin Hall
insulator 1T'-MoS2 exhibits a phase transition to a trivial insulator upon
adsorption of various atoms. It is then demonstrated that one-dimensional
metallic boundary states indeed arise in a ribbon geometry of alternating
regions with and without adsorbed oxygen and that these boundary states
generically constitute simple linear connections between valence and conduction
bands. This is in sharp contrast to topological edge states that typically
exhibit strong dispersion that are sensitively to a particular edge
termination. The heterostructure is also suggestive of a simple design of
one-dimensional metallic networks in sheets of 1T'-MoS2.
