Dynamic stabilization and heat transport characteristics of monolayer SnSe at finite temperature: A study by phonon quasiparticle approach

The structural stability of monolayer SnSe at finite temperature is investigated by the phonon quasiparticle approach that combines first-principles molecular dynamics and lattice dynamics. At 300 K, we witness the dynamic stability of monolayer Pmmn SnSe that is originated from the Cmcm bulk phase. The pair correlation and atomic displacement distribution functions of Sn and Se atoms confirm the lattice stability at high temperature. The hardening of Raman activated ${\mathrm{A}}_{g}^{2}$ and ${\mathrm{B}}_{2g}$ modes at the $\mathrm{\ensuremath{\Gamma}}(0,0,0)$ point and normal modes at the boundary $M(0,\frac{1}{2},\frac{1}{2})$ point of the Brillouin zone is observed with giant frequency shifts, revealing the heavy anharmonicity in monolayer SnSe. The calculated anharmonic phonon dispersions and density of states show significant temperature dependence and some medium-frequency phonon modes adopt giant frequency shifts. Due to the lattice anharmonicity, the predicted lattice thermal conductivity within the framework of a phonon gas model is very low for monolayer SnSe, close to the value of the bulk phase along the out-of-plane direction. As temperature increases, the characteristic of covalent bonds between Sn and Se atoms in plane and the normal direction of monolayer SnSe becomes isotropic. This results in a high degeneracy of the electronic band structure, beneficial to the electronic transport properties. A thermoelectric $ZT$ of $\ensuremath{\sim}0.52$ for $n$ doping is predicted at 300 K, hinting that the monolayer SnSe is a fairly good thermoelectric material suitable for application at room temperature.