Direct View of Phonon Dynamics in Atomically Thin\nMoS₂

Abstract

Transition-metal\ndichalcogenide monolayers and heterostructures\nare highly tunable material systems that provide excellent models\nfor physical phenomena at the two-dimensional (2D) limit. While most\nstudies to date have focused on electrons and electron–hole\npairs, phonons also play essential roles. Here, we apply ultrafast\nelectron diffraction and diffuse scattering to directly quantify,\nwith time and momentum resolution, electron–phonon coupling\n(EPC) in monolayer molybdenum disulfide and phonon transport from\nthe monolayer to a silicon nitride substrate. Optically generated\nhot carriers result in a profoundly anisotropic distribution of phonons\nin the monolayer within ∼5 ps. A quantitative comparison with <i>ab initio</i> ultrafast dynamics simulations reveals the essential\nrole of dielectric screening in weakening EPC. Thermal transport from\nthe monolayer to the substrate occurs with the phonon system far from\nequilibrium. While screening in 2D is known to strongly affect equilibrium\nproperties, our findings extend this understanding to the dynamic\nregime.