  We propose new 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 sizeable gaps under
achievable tensile strains. A unique mechanism is revealed to be responsible
for large topologically-nontrivial gap obtained: owing to the
functionalization, the $\sigma$ orbitals with stronger spin-orbit coupling
(SOC) dominate the states around the Fermi level, instead of original $\pi$
orbitals with weaker SOC; thereinto, the coupling of the $p_{xy}$ orbitals of
Ge and heavy halogens in forming the $\sigma$ 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.
