Dimensionality reduction induced synergetic optimization of the thermoelectric properties in Bi₂Si₂X₆ (X = Se, Te)...

Different from three-dimensional bulk compounds, two-dimensional monolayer compounds exhibit much better thermoelectric performance on account of the quantum confinement and interface effect. Here, we present a systematic study on the electronic and thermal transport properties of bulk and monolayer Bi₂Si₂X₆ (X = Se, Te) through theoretical calculations using density functional theory based on first-principles and Boltzmann transport theory. Monolayer Bi₂Si₂X₆ are chemically, mechanically and thermodynamically stable semiconductors with suitable band gaps, and they have lower lattice thermal conductivity (κ_L) in the a/b direction than their bulk counterparts. The calculated κ_L of monolayer Bi₂Si₂Se₆ (Bi₂Si₂Te₆) is as low as 0.72 (0.95) W m^-1 K^-1 at 700 K. Moreover, monolayer Bi₂Si₂X₆ exhibit a higher Seebeck coefficient compared with bulk Bi₂Si₂X₆ owing to the sharper peaks in the electronic density of states (DOS). This results in a significant increase in power factor by dimensionality reduction. Combined with the synergetically suppressed thermal conductivity, the maximum ZT values for monolayer Bi₂Si₂Se₆ and Bi₂Si₂Te₆ are significantly enhanced up to 5.03 and 2.87 with p-type doping at 700 K, which are more than 2 times that of the corresponding bulk compounds. These results demonstrate the superb thermoelectric performance of monolayer Bi₂Si₂X₆ for promising thermoelectric conversion applications.