Novel 2D ferroelastic SnNX (X = Cl, Br) monolayers with anisotropic high carrier mobility and excellent thermoelectric transport properties

Abstract

In recent years, layered FeOCl-type materials have attracted considerable attention for their excellent thermoelectric properties. The electronic structure and thermoelectric properties of SnNX (X = Cl, Br) monolayers as novel FeOCl-type thermoelectric materials within 300–700 K are predicted using first-principles calculations. The findings demonstrate that the valence band valley effectively increases the Seebeck coefficient of p-type doping. The high power factors are caused by the extremely high carrier mobility and Seebeck coefficient of these monolayers. We also discovered that the two monolayers have high Grüneisen parameters (up to 75.6) and lower phonon relaxation time, which limits their phonon transport capacity and lowers their lattice thermal conductivity (10.25 and 9.26 W m⁻¹ K⁻¹). At 700 K, the optimal ZT of n-type doped SnNCl and SnNBr in the x direction reaches 2.55 and 3.23, respectively, which are superior to those of some previously reported FeOCl-type TE materials. In addition, it was found that the SnNX (X = Cl, Br) monolayers also have good spontaneous ferroelasticity, so that the direction control of anisotropic electron and phonon transport can be realized. In brief, SnNX (X = Cl, Br) monolayers may be potential high performance TE materials and controllable electronic device candidates.