Dimensionality reduction induced synergetic optimization of the thermoelectric properties in Bi 2 Si 2 X 6 (X = Se, Te) monolayers.

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 Bi 2 Si 2 X 6 (X = Se, Te) through theoretical calculations using density functional theory based on first-principles and Boltzmann transport theory. Monolayer Bi 2 Si 2 X 6 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 Bi 2 Si 2 Se 6 (Bi 2 Si 2 Te 6 ) is as low as 0.72 (0.95) W m -1 K -1 at 700 K. Moreover, monolayer Bi 2 Si 2 X 6 exhibit a higher Seebeck coefficient compared with bulk Bi 2 Si 2 X 6 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 Bi 2 Si 2 Se 6 and Bi 2 Si 2 Te 6 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 Bi 2 Si 2 X 6 for promising thermoelectric conversion applications.