Layer-dependent electronic structure, dynamic stability, and phonon properties of few-layer SnSe

There is still a lack of fundamental understanding of the structural properties and thermal conductivity of few-layer SnSe, although it is the current recording material for thermoelectric conversion efficiency. From first-principles molecular dynamics and lattice dynamics, the electronic structure, structural properties, and lattice vibrational properties of few-layer SnSe are thus investigated in this study. The results show that SnSe exhibits semiconducting characteristics with the optical absorption mainly concentrated in the visible and near-ultraviolet regions. The band gap varies from 0.90 to 1.35 eV as the number of atomic layers decreases from bulk to single layer (1L). Four-layer (4L) SnSe shows typical bulk characteristics in terms of phase stability and electronic and vibrational properties. On further reducing the number of atomic layers, the quantum confinement effect becomes significant. At room temperature, the Pnma structural symmetry of 4L SnSe is dominated by the Jahn-Teller effect, consistent with the bulk phase. The second-order phase transition from Pnma to Cmcm driven by the phonon anharmonic effect occurs in 1L SnSe. A competitive picture of these two effects can be observed in 3L and 2L SnSe. At the enhancement temperature, the distribution peak of the phonon density of states between 3.5 and 4.5 THz shows a significant redshift relative to the harmonic case, exhibiting strong phonon anharmonicity. Correspondingly, the Raman-active normal modes ${B}_{3g}^{1}$ and ${A}_{g}^{2}$, which contribute to the peak, present significant frequency softening, consistent with the experimental observations. The reduction in the number of atomic layers will aggravate the frequency softening of these modes. Due to the strong phonon anharmonicity and dynamic instability of the Pnma phase, the lattice thermal conductivity of few-layer SnSe at room temperature is significantly lower than that of the bulk phase.