2, Boise State University, Boise, Idaho, United States
The vapor phase growth of atomically sharp lateral heterojunctions between MoS2 and WS2 monolayers has been recently demonstrated in experiments , and these two-dimensional (2D) heterostructures have potential applications in p-n junctions and other optoelectronic devices based on atomically thin semiconducting transition metal dichalcogenides (TMDs). However, the abrupt change in crystallographic structure at these heterojunctions leads to phonon scattering and thermal boundary resistance which can limit the effective thermal management of localized Joule heating. To understand the phonon dynamics underlying the thermal boundary resistance of the MoS2/WS2 interface, we study the transmission and reflection of individual phonon modes, using a refined version of the extended Atomistic Green’s Function (AGF) method  combined with force constant inputs derived from ab initio calculations, and perform a high-resolution analysis of the individual transmission channels in MoS2 and WS2, analyzing their dependence on frequency, momentum and polarization.
We compute the Brillouin zone distribution of the transmitted and reflected phonon modes for each phonon band. Our analysis reveals that the transmission and reflection spectra of the acoustic phonon bands exhibit polarization-dependent ‘critical angles’, analogous to the Brewster angle, because of differences in phonon group velocity. The overall results also show that the room-temperature interfacial heat flux is strongly dominated by acoustic phonons, with the flexural acoustic phonons constituting the largest group of heat carriers in both MoS2 and WS2 in spite of their low phonon group velocities. Our study highlights the role of phonon polarization in the thermal boundary conductance which can potentially be exploited for the control of lateral heat diffusion in 2D heterostructures.
1. Y. Gong et al., “Vertical and in-plane heterostructures from WS2/MoS2 monolayers,” Nature Mater. 13, 1135-1142 (2014).
2. Z.-Y. Ong and G. Zhang, “Efficient approach for modeling phonon transmission probability in nanoscale interfacial thermal transport,” Phys. Rev. B 91, 174302 (2015).