  Recently, two-dimensional (2D) transition metal carbides and nitrides,
namely, MXenes have attracted lots of attention for electronic and energy
storage applications. Due to a large spin-orbit coupling (SOC) 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 Z2 index calculations, we demonstrate that some of the MXene nitrides can
also be TIs. We find that Ti3N2F2 is a 2D TI, whereas Zr3N2F2 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 Hf3N2F2
semiconductor into a 2D TI. Since Ti is one of the mostly used transition metal
element in the synthesized MXenes, we expect that our prediction can advance
the future application of MXenes as TI devices.
