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 σ orbitals with stronger spin-orbit coupling (SOC) dominate the states around the Fermi level, instead of original π orbitals with weaker SOC; thereinto, the coupling of the pxy orbitals of Ge and heavy halogens in forming the σ 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.
@article{arxiv.1401.4100,
title = {Functionalized Germanene as a Prototype of Large-Gap Two-Dimensional Topological Insulators},
author = {Chen Si and Junwei Liu and Yong Xu and Jian Wu and Bing-Lin Gu and Wenhui Duan},
journal= {arXiv preprint arXiv:1401.4100},
year = {2014}
}