Hydrostatic Pressure Induced Anomalous Enhancement in the Thermoelectric Performance of Monolayer MoS$_{2}$

TL;DR

Applying hydrostatic pressure to monolayer MoS$_2$ significantly enhances its thermoelectric performance by increasing valley degeneracy, thermopower, and power factor while reducing thermal conductivity, leading to higher zT values at elevated temperatures.

Contribution

This study demonstrates that hydrostatic pressure induces a remarkable enhancement in the thermoelectric properties of monolayer MoS$_2$, a novel approach for improving 2D material-based thermoelectrics.

Findings

Thermopower (S) increases by up to 140% at 300 K under pressure.

Power factor (S^2σ/τ) increases by up to 310% at 300 K.

zT values reach 1.63 at 900 K with 25 GPa pressure for electron doping.

Abstract

The hydrostatic pressure induced changes in the transport properties of monolayer (ML) MoS$_2$ have been investigated using first-principles density functional theory based calculations. The application of pressure induces shift in the conduction band minimum (CBM) from K to $\Lambda$, while retaining the band extrema at K in around the same energy at a pressure of 10 GPa. This increase in valley degeneracy is found to have a significant impact on the electronic transport properties of ML-MoS$_2$ via enhancement of the thermopower (S) by up to 140\% and power factor (S$^{2}$$\sigma$/$\tau$) by up to 310\% at 300 K. Besides, the very low deformation potential (E$_\text{DP}$) associated with the CB-$\Lambda$ valley results in a remarkably high electronic mobility ($\mu$) and relaxation time ($\tau$). Additionally, the application of pressure reduces the room temperature lattice thermal conductivity ($\kappa_\text{L}$) by 20\% of its unstrained value, owing to the increased anharmonicity and resulting increase in the intrinsic phonon scattering rates. The hydrostatic pressure induced increase in power factor (S$^{2}$$\sigma$) and the decrease in $\kappa_\text{L}$ act in unison to result in a substantial improvement in the overall thermoelectric performance (zT) of ML-MoS$_2$. At 900 K with an external pressure of 25 GPa, zT values of 1.63 and 1.21 are obtained for electron and hole doping, respectively, which are significantly higher compared to the zT values at zero pressure. For the implementation in a thermoelectric module where both n-type and p-type legs should be preferably made of the same material, the concomitant increase in zT of ML-MoS$_2$ for both types of doping with hydrostatic pressure can be highly beneficial.