Large band gap quantum spin hall insulators of fluorinated Pb-X (X= C, Si, Ge, Sn)

The Quantum Spin Hall Insulating (QSHI) phase was first observed in the HgTe/CdTe quantum well structure. However, the observed band gap of 5 meV is too small for practical applications. Other materials have also been proposed for the observation of the QSHI phase, such as silicene, germanene, stanene, and its halogenated phases. The spin-orbit interaction is a key feature in topological insulators, raising the interest in heavy elements, such as Bismuth. In fact, Bi is responsible for the high spin-orbit coupling that drives the band inversion in Bi$_{2}Se$_{3}$ and Bi$_{2}Te$_{3}$. Another element that also has a large spin-orbit interaction is Lead (Pb). Here we present a set of 2D QSH insulators with a very large band gap based on fluorinated Pb-X (X= C, Si, Ge, Sn). First-principles phonon dispersion calculations indicate that these systems are structurally and mechanically stable. By performing DFT-based electronic structure calculations we show that 2D Pb-X functionalized with fluorine are topological insulators with very large band gaps (over 0.7 eV). Addtional calculations, for nanoribbons structures, show the presence of a Dirac cone at the center of the Brillouin zone. These results can establish a new route to the observation of QSHI phase at room temperature.