The stability, mechanical, electronic and thermoelectric properties of unexplored monolayer Ca4Sb2O and Ca4Bi2O

Thermoelectric materials can directly convert heat gradient into electricity, thus have attracted much attention in the field of clean energy and ecological protection. In this work, we proposed two novel two-dimensional materials, Ca4Sb2O and Ca4Bi2O, and investigated their stability, mechanical, electronic and thermoelectric properties. We revealed that these two monolayers are direct semiconductors with bandgaps of 1.57 and 1.08 eV, respectively. Due to the lower electron effective mass, the electron mobilities (252.90-799.97 cm2/Vs) are significantly higher than those of hole (58.16-59.21 cm2/Vs), leading to high n-type conductivities (106-107 S/ m), as well as high thermoelectric power factors of 28.80 and 10.83 mW K-2 m- 1 at 300 K. Also, owing to the low phonon group velocities and high phonon-phonon scattering rates, the monolayers also possess low lattice thermal conductivities of 1.86 and 1.01 W/mK at room temperature, which are dominated by acoustic modes. As a result, Ca4Sb2O and Ca4Bi2O deliver high n-type thermoelectric figure of merit of 0.27 and 0.29 at 300 K, and increase even further to 1.50 and 1.67 at 700 K, suggesting that they are candidates for low -medium tem-perature thermoelectric appliances.