XTlO (X = K, Rb, Cs): Novel 2D semiconductors with high electron mobilities, ultra-low lattice thermal conductivities and high thermoelectric figures of merit at room temperature

• The monolayers are indirect band-gap semiconductors with energy gaps of 2.31 eV, 2.27 eV and 2.48 eV, respectively. • The monolayers shown strong anharmonic phonon behavior, resulting in lattice intrinsic thermal conductivity as low as 0.32 ∼ 0.51 Wm -1 K −1 . • They exhibited high ZT values of 1.22, 1.41 and 1.24 at 300 K, respectively. Thermoelectric materials have attracted much attention in recent years because of their potential application prospects in clean energy and waste heat utilization. In this work, we proposed three novel two-dimensional materials, namely KTlO, RbTlO and CsTlO. Based on the state-of-the-art first principles calculation methods, we found that the three monolayers shown high stability, and with indirect band-gaps of 2.31 eV, 2.27 eV and 2.48 eV, respectively. Their carrier mobilities showed obvious anisotropy, with electron and hole mobilities reaching ∼10 3 and ∼10 2 cm 2 ·V −1 ·s −1 , respectively. In addition, the monolayers shown large Seebeck coefficient (1.75 ∼ 2.09 mV·K −1 ), high conductivities (10 6 ∼ 10 8 Ω -1 m −1 ), as well as power factor (3.19 ∼ 52.19 mW·K -2 ·m −1 ). Also, the monolayers possess ultra-low lattice thermal conductivities of 0.32 ∼ 0.51 W·m −1 ·K −1 . As results, their ZT values are up to 1.22, 1.41 and 1.24 at 300 K, respectively. All these properties indicate that monolayer XTlO (X = K, Rb, Cs) have promising applications in the fields of microelectronic devices and thermoelectric materials.