Phonon transport in the ground state of two-dimensional silicon and germanium

Abstract

Large honeycomb dumbbell (LHD) silicene/germanene was recently found to be the ground state of two-dimensional (2D) silicon and germanium, which was much more stable than the well-known low buckled (LB) silicene/germanene (Matusalem et al., Phys. Rev. B, 92, 045436 (2015)). The existence of an intrinsic band gap of LHD silicene/germanene makes them prospective in future thermoelectric devices. In this work, lattice thermal conductivity (κ) of the LHD silicene/germanene is investigated systematically by solving the phonon Boltzmann transport equation with interatomic force constants extracted from first-principles calculations, and compared with that of low buckled (LB) silicene/germanene. It is intriguing to find that, as compared with LB silicene, although the much flatter structure of the LHD silicene/germanene leads to a significantly larger portion of the flexural modes to the overall thermal transport, the κ of the LHD silicene/germanene is only 5.9/1.6 W m⁻¹ K⁻¹, which is substantially lower than that of the LB silicene/germanene. We found that the volumetric specific heat, group velocity square and the phonon lifetime of the LB silicene are all larger than that of the LHD silicene/germanene, with group velocity playing the dominant role, which is further linked with the higher Young's modulus of LB silicene. Besides, the difference in phonon lifetime is further explained in terms of the potential energy change. The trend of the root mean-square displacement values and the phonon anharmonicity is opposite to that of the normalized mean lifetime, which is physically justified, as larger displacements will lead to stronger anharmonicity and reduced lifetimes. The intrinsic ultralow κ of the LHD silicene/germanene makes it prospective for thermoelectrics in the future.