Ultralow lattice thermal conductivity of monolayer penta-silicene and penta-germanene
We study the lattice thermal conductivity of a two-dimensional (2D) pentagonal system, such as penta-silicene and penta-germanene. Penta-silicene has been recently reported, while the stable penta-germanene, belonging to the same group IV element, is revealed firstly by our ab initio calculations. We find that both penta-silicene and penta-germanene have ultralow lattice thermal conductivities κ of 1.29 W/mK and 0.30 W/mK at room temperature. To the best of our knowledge, penta-germanene may have the lowest κ in 2D crystal materials. We attribute both ultralow κ to the weak phonon harmonic interaction and strong anharmonic scattering. A small phonon group velocity, a small Debye frequency, a large Grüneisen parameter, and a large number of modes available for phonon-phonon interplay together lead to the ultralow κ of penta-silicene and penta-germanene. These discoveries provide new insight into the manipulation of ultralow κ in 2D materials and highlight the potential applications of designing silicon and germanium based high thermoelectric materials.