Titanium nitride halides monolayers: promising 2D anisotropic thermoelectric materials

Abstract 2D graphene-like thermoelectric materials have been extensively explored, however, the studies have mostly focused on the binary compounds and most of them exhibit isotropic electron and phonon transport properties. Here, we investigate the thermoelectric transport properties of ternary TiNX (X = F, Cl, Br) monolayers by using first-principles combined with the Boltzmann transport theory. Both electron and phonon anisotropic transport properties are found. The large p -type power factor and the low lattice thermal conductivity along the y direction give rise to better thermoelectric performance along the y direction than the x direction, and the highest ZT values at 500 K reach 1.00, 0.89 and 1.17 along the y direction in p -type doping for TiNF, TiNCl, and TiNBr monolayer, respectively. The anisotropy and the difference of lattice thermal conductivities among TiNX monolayers are discussed in terms of the group velocities, the phonon relaxation time and the three-phonon scattering phase space. These results indicate that TiNX monolayers are promising candidates for 2D anisotropic thermoelectric materials.