Thermoelectric performance in ultra-thin transition metal dichalcogenides

The thermoelectric figure of merit, ZT, is calculated for one to four monolayers of MoS$_{2}$, MoSe$_{2}$, WS$_{2}$ and WSe$_{2}$. The maximum ZT in this family of materials occurs in bilayer MoSe$_{2}$. Its ZT value of 2.39 is a factor of 8 increase compared to that of the bulk at room temperature. The values for the power factors and ZT change non-monotonically as the film thicknesses are increased from a single monolayer up to four layers. In contrast to Bi$_{2}$Te$_{3}$, the peak value of ZT occurs at a thickness greater than a single monolayer for all 4 materials. The shape of the distribution of the valence band and the conduction band density of modes explains the enhanced thermoelectric performance that occurs for film thicknesses above a single monolayer. Ab-initio electronic structure calculations are used in a Landauer approach to calculate the thermoelectric transport coefficients.