Using first principle calculations, we propose functionalized germanene (GeX, X = H, F, Cl, Br, I, OH, CH3) as emerging nanomaterials. Although germanene has no band gap, complete functionalization with H induces band gap of ~1.80 eV. A 50% H functionalization shows a dangling band at the Fermi level. Germanene I (GeI) is a 2D Topological Insulators (TI). GeH, GeF, GeCl, and GeBr can be transformed into TI by applying strain.The methyl-functionalized two-dimensional germanium monolayer sheets have been synthesized with a facile, one-step metathesis approach from CaGe2 crystals. We find that tensile strain can induce topological phase transition with band inversion at Gamma point. The band gap opened by spin-orbit coupling in this quantum spin Hall insulator can be as large as 0.1 eV ample for practical applications at room temperature
