Direct View of Phonon Dynamics in Atomically Thin MoS₂

Abstract

Transition-metal dichalcogenide monolayers and heterostructures are highly tunable material systems that provide excellent models for physical phenomena at the two-dimensional (2D) limit. While most studies to date have focused on electrons and electron–hole pairs, phonons also play essential roles. Here, we apply ultrafast electron diffraction and diffuse scattering to directly quantify, with time and momentum resolution, electron–phonon coupling (EPC) in monolayer molybdenum disulfide and phonon transport from the monolayer to a silicon nitride substrate. Optically generated hot carriers result in a profoundly anisotropic distribution of phonons in the monolayer within ∼5 ps. A quantitative comparison with ab initio ultrafast dynamics simulations reveals the essential role of dielectric screening in weakening EPC. Thermal transport from the monolayer to the substrate occurs with the phonon system far from equilibrium. While screening in 2D is known to strongly affect equilibrium properties, our findings extend this understanding to the dynamic regime.

Real-Time GW-Ehrenfest-Fan-Migdal Method for Nonequilibrium 2D Materials

“…In fact, phonons lose coherence and eventually thermalize on a much longer time scale than electrons; see the undamped oscillations of the nuclear displacements in the first 400 fs reported in Figure d,e. We have singled out the emission of optical phonons around the Γ point as a mechanism for intravalley scattering and the emission of zone-edge acoustic and optical phonons as a mechanism for intervalley scattering. − It follows that lattice heating is confined around a few high-symmetry points (Figure b), in agreement with recent experiments in MoS 2 . This is a general feature of materials hosting several valleys.…”

“…We have singled out the emission of optical phonons around the Γ point as a mechanism for intravalley scattering and the emission of zone-edge acoustic and optical phonons as a mechanism for intervalley scattering. 60−63 It follows that lattice heating is confined around a few high-symmetry points 49 (Figure 5b), in 66 This is a general feature of materials hosting several valleys. In Figure 5c we show the branch-resolved temperature map for the longitudinal acoustic (LA), transverse acoustic (TA), LO, and TO phonons at times t = 20 and 50 fs.…”

A spectroscopic overview of the differences between the absorbing states and the emitting states in semiconductor perovskite nanocrystals

“…However, scattering based experiments, although they have less time resolution, work on the principle of interactions between electrons and nuclei and hence can provide information about structural dynamics-like unit cell deformation. Owing to the advancement of laser sources for electrons and X-rays, ultrafast electron diffraction (UED) [104][105][106][107][108][109] and ultrafast X-ray diffraction (UXRD) 110 techniques enable direct probing of structural evolution in perovskite materials. Since then, other experiments have also contributed to the advancement in understanding the underlying concepts.…”

Time- and angle-resolved photoemission spectroscopy (TR-ARPES) of TMDC monolayers and bilayers

“…88,[105][106][107][108] The large monolayers can be used to construct bilayer and multilayer stacks, with desired twist angles that are uniform over the mm-sized sample. 88,109 The obtained single crystal monolayer and heterostructures displayed well-resolved reciprocal-space band structure and crystal structures in photoemission 88 and electron diffraction 110 and can be an excellent material platform for TR-ARPES experiments.…”