2D NbSiTeand NbGeTe: promising thermoelectric figure of merit and gate-tunable thermoelectric performance.

Recently, the experimentally synthesized NbSiTewas found to be a stable layered narrow-gap semiconductor, and the fabricated field-effect transistors (FETs) based on few-layers NbSiTeare good candidates for ambipolar devices and mid-infrared detection (Zhao201910705-10). 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 NbXTe(X = Si, Ge) and the gate voltage effect on the thermoelectric performance of the FET based on monolayer NbSiTe. It is found that both monolayers have large-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 NbSiTe(NbGeTe). The gate voltage can effectively increase the thermoelectric performance for the NbSiTe-based FET. The high thermoelectric figure of merit can be maintained in a wide temperature range under a negative gate voltage. Furthermore, the FET exhibits a good gate-tunable Seebeck diode effect. The present work suggests that NbXTemonolayers are promising candidates for 2D thermoelectric materials and thermoelectric devices.