Functionalized germanene as a prototype of large-gap two-dimensional topological insulators

We propose two-dimensional (2D) topological insulators (TIs) in functionalized germanenes (GeX, X= H, F, Cl, Br, or I) using first-principles calculations. We find GeI is a 2D TI with a bulk gap of about 0.3 eV, while GeH, GeF, GeCl, and GeBr can be transformed into TIs with sizable gaps under achievable tensile strains. A unique mechanism is revealed to be responsible for the large topologically nontrivial gap obtained: due to the functionalization, the sigma orbitals with stronger spin-orbit coupling (SOC) dominate the states around the Fermi level, instead of original p orbitals with weaker SOC. Thereinto, the coupling of the p(xy) orbitals of Ge and heavy halogens in forming the s orbitals also plays a key role in the further enlargement of the gaps in halogenated germanenes. Our results suggest a realistic possibility for the utilization of topological effects at room temperature.