Halogenated two-dimensional germanium: candidate materials for being of Quantum Spin Hall state

The electronic band structure of halogenated germanene in the presence of spin-orbit coupling is investigated using first-principles calculations. Our results demonstrate that, compared with pure germanene, the pi and pi* bands of germanene adsorbed with Cl, Br and I remain crossed at the Fermi level - despite the crossing point shifting from K to Gamma points. Moreover, we find that appreciable gaps in halogenated germanene can be opened at Dirac-like points, several orders of magnitude larger than that in pure germanene due to the robust spin-orbit coupling; for example, Cl, Br and I yield a SOC-induced gap of 86 meV, 237 meV and 162 meV at Gamma points, respectively. In addition, since the germanene would be unstable at ambient conditions due to the dangling unsaturated Ge bonds, the manufacture of fully halogenated germanene with a robust spin-orbit coupling effect is more feasible than that of germanene experimentally. Therefore, our work may provide a potential avenue to observe the Quantum Spin Hall Effect at room temperature.