Ultra-low lattice thermal conductivity of monolayer penta-silicene and penta-germanene

We study the lattice thermal conductivity of two-dimensional (2D) pentagonal systems, such as penta-silicene and penta-germanene. Penta-silicene has been recently reported, while the stable penta-germanene, made up of another group IV element, is first revealed by our ab initio calculations. We find that both penta-silicene and penta-germanene at room temperature have ultra-low lattice thermal conductivities, kappa, of 1.29 W m(-1) K-1 and 0.30 W m(-1) K-1, respectively. To the best of our knowledge, penta-germanene may have the lowest kappa in 2D crystal materials. We attribute the ultra-low kappa to the weak phonon harmonic interaction and strong anharmonic scattering. A small phonon group velocity, a small Debye frequency, a large Gruneisen parameter, and a large number of modes available for phonon-phonon interplay together lead to the ultra-low kappa of penta-silicene and penta-germanene. These discoveries provide new insight into the manipulation of ultra-low kappa in 2D materials and highlight the potential applications of silicon and germanium based high thermoelectric materials.