Ultralow lattice thermal conductivity at room temperature in 2D KCuSe from first-principles calculations

Ultralow lattice thermal conductivity is crucial for achieving a high thermoelectric figure of merit for thermoelectric applications. In this work, using first-principles calculations and the phonon Boltzmann transport theory, we investigate the phonon thermal transport properties of 2D KCuSe. Our calculations indicate that the strong acoustic-optical coupling, the low-lying acoustic phonon modes and the strong lattice anharmonic effect with a large Grüneisen parameter and phase space volume result in an ultralow lattice thermal conductivity of 0.021 W m^-1 K^-1 at 300 K for monolayer KCuSe, which is lower than those of recently reported KAgSe (0.26 W m^-1 K^-1 at 300 K) and TlCuSe (0.44 W m^-1 K^-1 at 300 K). Importantly, although the Coulomb interactions and the tensile biaxial strain lead to the increase of lattice thermal conductivity due to the increasing relaxation time (0.056 and 0.28 W m^-1 K^-1 at 300 K without and with 6% tensile strain, respectively), it is still lower than those of most 2D thermoelectric materials. The advantages of being cheap, environmentally friendly and having low lattice thermal conductivity compared to the KAgSe and TlCuSe derivatives make KCuSe a promising candidate for thermoelectric applications, which will stimulate more efforts toward theoretical and experimental studies on this class of 2D ternary semiconductors.