Article

# Functionalized Germanene as a Prototype of Large-Gap Two-Dimensional Topological Insulators

Physical Review B (Impact Factor: 3.74). 01/2014; 89(11). DOI: 10.1103/PhysRevB.89.115429

Source: arXiv

**ABSTRACT**

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.

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.

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**ABSTRACT:**The adsorption of common gas molecules (N2, CO, CO2, H2O, NH3, NO, NO2, and O2) on germanene is studied with density functional theory. The results show that N2, CO, CO2, and H2O are physisorbed on germanene via van der Waals interactions, while NH3, NO, NO2, and O2 are chemisorbed on germanene via strong covalent (Ge-N or Ge-O) bonds. The chemisorption of gas molecules on germanene opens a band gap at the Dirac point of germanene. NO2 chemisorption on germanene shows strong hole doping in germanene. O2 is easily dissociated on germanene at room temperature. Different adsorption behaviors of common gas molecules on germanene provide a feasible way to exploit chemically modified germanene.