Substrate-Induced Bonding Asymmetry Leading to Strong Phonon Anharmonicity and Largely Reduced Thermal Conductivity of Monolayer Xs2 (X = Mo and W) from First-Principles Calculations

Abstract

Thermoelectric materials have properties such as direct conversion of waste heat into electricity and environmental friendliness, which make them promising for renewable energy utilization, however, the lower conversion efficiency limits the large-scale application of thermoelectric materials. Monolayer transition metal dichalcogenides (TMDs) are candidates for thermoelectric materials due to their high power factor, but their higher thermal conductivity (TC) limits the further improvement of the figure of merit (ZT). Here, we employ first-principles calculations and solve the Boltzmann transport equation (BTE) to compare the lattice TC of free-standing monolayer XS2 (X = Mo and W) and monolayer XS2 supported on SiC (0001) and ZnO (0001) substrates. We find that the lattice TC of monolayer XS2 supported on SiC (0001) and ZnO (0001) substrates are sharply reduced. Especially supported on the ZnO (0001) substrate, the lattice TC of XS2 reduce by 65%-70%, which is extremely important to improve the thermoelectric properties of monolayer XS2. In addition, we report the underlying cause of this significant reduction in TC. Firstly, the substrate drives softening of the acoustic branch, which leads to a decrease of phonon group velocity in the low-frequency and an enhancement of the three-phonon scattering. Then, the asymmetric bonding of XS2 is induced by the substrates, which enhance phonon anharmonicity and reduce the phonon lifetime. Our results demonstrate that the lattice TC of monolayer XS2 can be tuned by adjusting the substrate, which has certain reference values for the thermoelectric design of XS2.

Keywords: monolayer XS2 (X = Mo, W), thermal conductivity, substrate effects, asymmetric bonding, first-principles calculations