  Two-dimensional (2D) materials have attracted much recent attention because
they exhibit various distinct intrinsic properties/functionalities, which are,
however, usually not interchangeable. Interestingly, here we propose a generic
approach to convert 2D semiconductors, which are amply abundant, to 2D
topological insulators (TIs), which are less available, via selective atomic
adsorption and strain engineering. The approach is underlined by an orbital
design principle that involves introducing an extrinsic s-orbital state into
the intrinsic sp-bands of a 2D semiconductor, so as to induce s-p band
inversion for a TI phase, as demonstrated by tight-binding model analyses.
Remarkably, based on first-principles calculations, we apply this approach to
convert the semiconducting monolayer CuS and CuTe into a TI by adsorbing Na and
K respectively with a proper s-level energy, and CuSe into a TI by adsorbing a
mixture of Na and K with a tuned s-level energy or by adsorbing either Na or K
on a strained CuSe with a tuned p-level valence band edge. Our findings open a
new door to the discovery of TIs by a predictive materials design, beyond
finding a preexisting 2D TI.
