Janus 2D titanium nitride halide TiNX_0.5Y_0.5 (X, Y = F, Cl, or Br, and X ≠ Y) monolayers with giant out-of-plane piezoelectricity and high carrier mobility

Due to their broken out-of-plane inversion symmetry, Janus two-dimensional (2D) materials exhibit some exceptional and interesting physical properties and have recently attracted increasing attention. Herein, based on first-principles calculations, we propose a series of Janus 2D titanium nitride halide TiNX0.5Y0.5 (X, Y = F, Cl, or Br, and X not equal Y) monolayers constructed from 2D ternary compounds TiNX (X = F, Cl, or Br), where the halogen atoms X or Y are located on each side of the monolayer, respectively. Our calculations confirm that the Janus monolayers are both dynamically and thermally stable. As compared with those of perfect TiNX monolayers, the band-structure changes of Janus TiNX0.5Y0.5 monolayers are very limited and the corresponding bandgaps only increase by about 0.1-0.2 eV. Meanwhile, the Janus TiNX0.5Y0.5 monolayers show remarkable out-of-plane piezoelectricity by virtue of their broken centrosymmetry. The calculated out-of-plane piezoelectric coefficient d(31) is as high as 0.34 pm V-1, which is larger than those of most 2D piezoelectric materials reported previously. In addition, it is found that the formation of Janus structures could effectively improve the carrier mobility. The hole mobilities along the x-direction (y-direction) of Janus TiNF0.5Cl0.5 and TiNF0.5Br0.5 monolayers reach as high as 5402 (5118) and 5538 (4135) cm(2) V-1 s(-1) at 300 K, respectively, which is almost twice as large as those of perfect TiNX monolayers. The giant out-of-plane piezoelectricity and high carrier mobility of Janus TiNX0.5Y0.5 monolayers suggest that these novel 2D materials could be promising for applications in electronic and piezoelectric devices.