Theoretical prediction of two-dimensional functionalized MXene nitrides as topological insulators

Recently, two-dimensional (2D) transition-metal carbides and nitrides, namely, MXenes have attracted a lot of attention for electronic and energy storage applications. Due to a large spin-orbit coupling and the existence of a Dirac-like band at the Fermi energy, it has been theoretically proposed that some of the MXenes will be topological insulators (TIs). Up to now, all of the predicted TI MXenes belong to transition-metal carbides, whose transition-metal atom is W, Mo, or Cr. Here, on the basis of first-principles and ${\mathbb{Z}}_{2}$ index calculations, we demonstrate that some of the MXene nitrides can also be TIs. We find that ${\mathrm{Ti}}_{3}{\mathrm{N}}_{2}{\mathrm{F}}_{2}$ is a 2D TI, whereas ${\mathrm{Zr}}_{3}{\mathrm{N}}_{2}{\mathrm{F}}_{2}$ is a semimetal with nontrivial band topology and can be turned into a 2D TI when the lattice is stretched. We also find that the tensile strain can convert ${\mathrm{Hf}}_{3}{\mathrm{N}}_{2}{\mathrm{F}}_{2}$ semiconductor into a 2D TI. Since Ti is one of the most used transition-metal elements in the synthesized MXenes, we expect that our prediction can advance the future application of MXenes as TI devices.