Lone-pair electron-induced low lattice thermal conductivity and excellent thermoelectric performance of AuX (X = S, Se, Te) monolayers

We present a detailed study on the phonon thermal transport and thermoelectric (TE) properties of gold monochalcogenide AuX (X = S, Se, Te) monolayers via first-principles calculations and the Boltzmann transport theory. At room temperature, the calculated lattice thermal conductivity (xl) values in the x-direction (y-direction) were determined to be 12.66 (3.59), 6.06 (1.23), and 3.02 (0.40) W/(m & sdot;K) for the AuS, AuSe, and AuTe monolayers, respectively. The analysis of scattering matrix elements and anharmonic scattering rates suggested that the presence of strong bond anharmonicity leads to low xl values. This anharmonicity was induced by the nonlinear electrostatic repulsive force between lone-pair electrons (LPEs) surrounding the chalcogen atom and adjacent bonding electron. Furthermore, the AuS monolayer exhibited the highest bond anharmonicity among the three monolayers. This can be attributed to the reduced strength of the electrostatic repulsive force when electronegativity decreased from S to Se and Te. Although the AuS monolayer exhibited the strongest bond anharmonicity, its xl was still the largest, demonstrating that among the factors that influence xl, harmonic factors have a greater effect than anharmonic parameters. When the atomic mass was increased, the phonon frequencies, phonon group velocities, and xl decreased. Benefiting from low xl induced by LPEs, the maximum ZT values of p-type doped AuSe and AuTe monolayers exceeded 1.0 and 2.0 at T = 800 K, respectively. These results not only confirmed that the introduction of LPEs can reduce the lattice thermal conductivity and enhance the TE performance of 2D materials, but also demonstrated the promising potential of the 2D AuX family for applications in high-temperature TE devices.