A Novel Two-Dimensional Thermoelectric Material Silicene with High ZT for both N-Type and P-Type Doping at Low Carrier Concentration

Two-dimensional nanostructures shed new light on the enhancement of the thermoelectric figure of merit due to the potential decoupling of electronic and phononic transport coefficients. In contrast to the gapless character of graphene-like silicene, a recently reported silicon allotropy with a honeycomb-kagome lattice is a semiconductor. Here, based on first-principles calculations, we set out to investigate the thermoelectric transport performance of this semiconducting silicene. Since the mean free path of a large number of phonons in this structure is less than the Ioffe-Regel limit, we employ the quantum Boltzmann transport equation (BTE) method to obtain an accurate prediction of lattice thermal conductivity. Importantly, we unexpectedly find much lower lattice thermal conductivity compared to that of graphene-like silicene, i.e., about 1.73 W\cdotm^-1\cdotK^-1 at room temperature. Meanwhile, the electronic transport coefficient is calculated within the strictly electron-phonon coupling calculation and a full solution of the electron BTE. The optimal thermoelectric figure of merit ZT reaches 3.2 in N-doped silicene at 700 K with an optimized low carrier concentration of 8\times10^10 cm^-2, which is a recorded value among two-dimensional materials. Our work paved the way for applications of silicon-based two-dimensional materials in on-chip thermoelectric cooling and clean energy.