2D Nb2SiTe4 and Nb2GeTe4: promising thermoelectric figure of merit and gate-tunable thermoelectric performance.

Recently, the experimentally synthesized Nb2SiTe4 was found to be a stable layered narrow-gap semiconductor, and the fabricated field-effect transistors based on few-layers Nb2SiTe4 are good candidates for ambipolar devices and mid-infrared detection (Zhao et al., 2019 ACS Nano 13 10705-10). Here, we use first-principles combined with Boltzmann transport theory and non-equilibrium Green's function method to investigate the thermoelectric transport coefficients of monolayer Nb2XTe4 (X=Si, Ge) and the gate voltage effect on the thermoelectric performance of the field-effect transistor based on monolayer Nb2SiTe4. It is found that both monolayers have large p-type Seebeck coefficients due to the "pudding-mold-type" valence band structure, and they both exhibit anisotropic thermoelectric behavior with optimal thermoelectric figure of merit of 1.4 (2.2) at 300 K and 2.8 (2.5) at 500 K for Nb2SiTe4 (Nb2GeTe4). The gate voltage can effectively increase the thermoelectric performance for the Nb2SiTe4-based field-effect transistor. The high thermoelectric figure of merit can be maintained in a wide temperature range under a negative gate voltage. Furthermore, the field-effect transistor exhibits a good gate-tunable Seebeck diode effect. The present work suggests that Nb2XTe4 monolayers are promising candidates for thermoelectric materials and thermoelectric devices.