New family of room temperature quantum spin Hall insulators in two-dimensional germanene films

Searching for two-dimensional (2D) group IV films with high structural stability and large-gaps is crucial for the realization of a dissipationless transport edge state using the quantum spin Hall effect (QSHE). Based on first-principles calculations, we predict that 2D germanene decorated with ethynyl-derivatives (GeC₂X; X = H, F, Cl, Br, I) can be a topological insulator (TI) with a large band-gap for room-temperature applications. Both GeC₂I and GeC₂Br films are intrinsic TIs with a gap reaching up to 180 meV over a wide range, while GeC₂H, GeC₂F, and GeC₂Cl transform from trivial to nontrivial phases under tensile strain. This topological characteristic can be confirmed by s-p_xy band inversion, topological invariant Z₂, and time-reversal symmetry protected helical edge states. Notably, the characteristic properties of edge states, such as the Fermi velocity and edge shape, can be tuned by edge modifications. Furthermore, we demonstrate that the h-BN sheet is an ideal substrate for the experimental realization of GeC₂X, maintaining their nontrivial topology. Considering their higher thermo-stability, these GeC₂X films may be good QSHE platforms for topological electronic device design and fabrication in spintronics.