The impact of electron-phonon coupling on the figure of merit of NbSiTe and NbGeTe ternary monolayers.

We have comprehensively demonstrated the thermal transport properties of Nb2SiTe4 and Nb2GeTe4 ternary monolayers by employing first-principles calculations and the semi-classical Boltzmann transport theory, including the electron-phonon coupling. The appealing features uncovered here for the monolayers are their colossal Seebeck coefficient and power factor at a higher carrier concentration. For example, the room temperature Seebeck coefficient lasts as high as 200 μV K-1 even at an increased hole (electron) concentration 6.32 × 1020 cm-3 (5.17 × 1019 cm-3) and 1.47 × 1020 cm-3 (5.18 × 1019 cm-3) for Nb2SiTe4 and Nb2GeTe4 monolayers, respectively. Our findings disclose similar band structures and moderate indirect bandgaps of 0.55 eV and 0.41 eV for Nb2SiTe4 and Nb2GeTe4 monolayers. The absence of imaginary frequencies in phonon band dispersion confirms the dynamic stability of both monolayers. The lowest value of lattice thermal conductivity turns out to be 14.30 W m-1 K-1 (12.30 W m-1 K-1) for Nb2SiTe4 and 11.70 W m-1 K-1 (8.34 W m-1 K-1) for Nb2GeTe4 in the x(y) direction. Besides, both monolayers express tremendous potential to further reduced lattice thermal conductivity by nano-structuring without requiring a diminished sample size that is technically challenging to synthesize.