Four-phonon scattering of so-As and improvement of the thermoelectric properties by increasing the buckling height.

In recent years, more and more thermoelectric (TE) materials have been discovered as the research boom of TE materials advances. However, due to the low conversion efficiency, most of the current TE materials cannot meet the commercial demand. The low-dimensional nanomaterials are promising to break the current status quo of low conversion efficiency of TE materials. Here, we predicted a stable two-dimensional TE material, namely so-As, based on density functional theory. The so-As has an ultra-low lattice thermal conductivity, κl=1.829 W/m/K at 300 K, and when the temperature rises to 700 K the κl is only 0.788 W/m/K. This might be caused by the strong anharmonic interaction among the so-As phonon and the out-of-plane vibration of the low-frequency acoustic modes. Moreover, the maximum ZT value of the p-type so-As is 0.18 at room temperature (0.45 at 700 K), while that of the n-type can even reach 0.75 at 700 K. In addition, we have also studied the difference between the four- and three-phonon scattering rates. The increase of scattering channels leads to the ultra-low κl, which is only 3.33×10-4 W/m/K at room temperature, showing an almost adiabatic property. Finally, we adjust the thermoelectric properties of so-As by changing the buckling height. With the buckling height is increased by 2%, the scattering rate of so-As is extremely high. When T is 700 K, the maximum ZT of the n-type is 0.94 (p-type can also reach 0.7), which is 25% higher than the pristine one. Our work reveals the impact of buckling height on the thermoelectric figure of merit, which provides a direction for future search and regulation of the high ZT thermoelectric materials.