Understanding the origins of low lattice thermal conductivity in a novel two-dimensional monolayer NaCuS for achieving medium-temperature thermoelectric applications

Abstract

Understanding the lattice dynamics from the perspective of chemical bonds and phonon transport is essential for finding and designing high-efficiency thermoelectric (TE) materials and achieving applications. The coexistence of weak chemical bonding and strong phonon anharmonicity is elucidated as the origin of the intrinsically low lattice thermal conductivity by combining the first principles with the Boltzmann transport theory. By utilizing the crystal orbital Hamilton population (COHP) analysis, the weaker Cu-S chemical bonding originating from the filled antibonding orbitals gives rise to the low average phonon group velocity (va). Here, this research also investigates the scattering processes (the out-of-plan acoustic mode (ZA) + optical mode (O) → O (ZA + O → O), the in-plane transverse acoustic mode (TA) + O → O (TA + O → O), and the in-plane longitudinal acoustic mode (LA) + O → O (LA + O → O)), from which we find that 2D NaCuS possesses strong phonon anharmonicity. The low va and strong phonon scattering enable 2D NaCuS to exhibit a low room temperature lattice thermal conductivity (klat) of 1.95 W/mK. Furthermore, our studies demonstrate 2D NaCuS presents a high ZT of 1.44 at 500 K due to the low klat and excellent electronic transport performance. This study demonstrates that 2D NaCuS is identified as a new TE material for promising medium-temperature applications.