Thermoelectric performance of Bi-based novel Janus monolayer structures†

This work systematically investigates the stability and electronic and thermoelectric characteristics of newly discovered 2D Janus monolayers BiYZ (Y ≠ Z = Te, Se and S) according to the first-principles theory. Janus BiYZ monolayers are stable based on the AIMD simulations, positive phonon spectra plots and the evaluation of elastic strain tensor. These monolayers show a high carrier mobility (∼10³ cm² V⁻¹ s⁻¹) and an indirect bandgap nature. The Janus monolayers BiTeSe, BiTeS and BiSeS show an ultralow lattice thermal conductivity of 0.04 W m⁻¹ K⁻¹, 0.20 W m⁻¹ K⁻¹ and 0.02 W m⁻¹ K⁻¹, respectively, at room temperature. Low lattice thermal conductivity is obtained due to a small phonon group velocity, high Grüneisen parameter, small phonon relaxation time and significantly reduced phonon transport. The maximum ZT values at 500 K reach up to 0.97, 0.60 and 1.78 for BiTeSe, BiSeS, and BiTeS monolayers, respectively. Our results suggest Janus BiYZ monolayers to be promising thermoelectric candidates due to their superior thermal and electrical transport characteristics and subsequent strong thermoelectric performance.