Monolayer Cu<sub>2</sub>X (X=S, Se): excellent thermoelectric material with low lattice thermal conductivity

Abstract

Two-dimensional (2D) materials with lower lattice thermal conductivity and high figure of merit are useful for applications in thermoelectric (TE) devices.In this work,the thermoelectric properties of monolayer Cu<sub>2</sub>X (X=S,Se) have been systematically researched through first-principles and Boltzmann transport theory.We have verified the dynamic stability of monolayer Cu<sub>2</sub>X (X=S,Se) through elastic constants and phonon dispersion.The results show that monolayer Cu<sub>2</sub>X (X=S,Se) together with small lattice constants,resulting in lower phonon vibration modes.Phonon transport calculations confirm that monolayer Cu<sub>2</sub>Se has lower lattice thermal conductivity (1.93W/mK) than Cu<sub>2</sub>S (3.25W/mK) at room temperature,which is due to its small Debye temperature and stronger anharmonicity.Moreover,the heavier atomic mass of Se atoms effectively reduces the phonon frequency,resulting in a ultra narrow phonon band gap (0.08THz) and lower lattice thermal conductivity for monolayer Cu<sub>2</sub>Se.The band degeneracy effect at the VBM of monolayer Cu<sub>2</sub>X (X=S,Se) significantly increases its carrier effective mass,resulting in higher Seebeck coefficient and lower conductivity under p-type doping.The electric transport calculation at room temperature shows that the conductivity of monolayer Cu<sub>2</sub>S (Cu<sub>2</sub>Se) under n-type doping about 10<sup>11</sup>cm<sup>-2</sup> is 2.8×10<sup>4</sup>S/m (4.5×10<sup>4</sup>S/m),obviously superior to their conductivity about 2.6×10<sup>2</sup>S/m (1.6×10<sup>3</sup>S/m) under p-type doping.At the optimum doping concentration for monolayer Cu<sub>2</sub>S (Cu<sub>2</sub>Se),the n-type power factor is 16.5mW/mK<sup>2</sup>(25.9mW/mK<sup>2</sup>),which is far higher than p-type doping 1.1mW/mK<sup>2</sup>(6.6mW/mK<sup>2</sup>).Through the above results,the excellent figure of merit of monolayer Cu<sub>2</sub>S and Cu<sub>2</sub>Se under optimal n-type doping at 700 K can approach 1.85 and 2.82,which are higher than 0.38 and 1.7 under optimal p-type doping.The excellent thermoelectric properties of monolayer Cu<sub>2</sub>S and Cu<sub>2</sub>Se are comparable to those of many promising thermoelectric materials reported recently.Especially,the figure of merit of monolayer Cu<sub>2</sub>Se is larger than the well-known high-efficient thermoelectric monolayer SnSe (2.32).Therefore,monolayer Cu<sub>2</sub>X (X=S,Se) are potential thermoelectric material with excellent performance and good application prospects.Such results provide theoretical basis for the follow-up experiments exploring the practical applications of 2D thermoelectric semi-conductors materials and offer an in-depth insight into the effect of phonon thermal transport on improvement of TE transport properties.