Observation of Anisotropic Strain-Wave Dynamics and Few-Layer Dephasing in MoS₂ with Ultrafast Electron Microscopy

Abstract

The large elastic strains that can be sustained by transition metal dichalcogenides (TMDs), and the sensitivity of electronic properties to that strain, make these materials attractive targets for tunable optoelectronic devices. Defects have also been shown to influence the optical and electronic properties, characteristics that are especially important to understand for applications requiring high precision and sensitivity. Importantly, photoexcitation of TMDs is known to generate transient strain effects but the associated intralayer and interlayer low-frequency (tens of GHz) acoustic-phonon modes are largely unexplored, especially in relation to defects common to such materials. Here, with femtosecond electron imaging in an ultrafast electron microscope (UEM), we directly observe distinct photoexcited strain-wave dynamics specific to both the ab basal planes and the principal c-axis crystallographic stacking direction in multilayer 2H-MoS 2 , and we elucidate the microscopic interconnectedness of these modes to one another and to discrete defects, such as few-layer crystal step edges. By probing 3D structural information within a nanometer−picosecond 2D projected UEM image series, we were able to observe the excitation and evolution of both modes simultaneously. In this way, we found evidence of a delay between mode excitations; initiation of the interlayer (c-axis) strain-wave mode precedes the intralayer (ab plane) mode by 2.4 ps. Further, the intralayer mode is preferentially excited at free basal-plane edges, thus suggesting the initial impulsive structural changes along the c-axis direction and the increased freedom of motion of the MoS 2 layer edges at terraces and step edges combine to launch in-plane strain waves at the longitudinal speed of sound (here observed to be 7.8 nm/ps). Sensitivity of the c-axis mode to layer number is observed through direct imaging of a picosecond spatiotemporal dephasing of the lattice oscillation in discrete crystal regions separated by a step edge consisting of four MoS 2 layers. These results uncover new insights into the fundamental nanoscale structural responses of layered materials to ultrafast photoexcitation and illustrate the influence defects common to these materials have on behaviors that may impact the emergent optoelectronic properties.

Time-Domain Investigations of Coherent Phonons in van der Waals Thin Films

“…Coherent in-plane phonon propagation can be directly followed thanks to the nanoscale spatial resolution in bright field imaging. Applied to thin slabs of TMD such as WSe 2 [255], TaS 2 [256] and MoS 2 [130], in-plane modes are observed at frequencies in the range of a few GHz for antisymmetric flexural modes and a few tens of GHz for symmetric dilatation modes. Corresponding extracted in-plane phonon velocities are in good agreement with known bulk values.…”

“…Corresponding extracted in-plane phonon velocities are in good agreement with known bulk values. Interestingly, the high resolution allows to identify the discrete phonon-nucleation sites in space and time [130,255,256], which correspond to atomic-scale defects and mostly step edges at the layered crystal surface. The stress required to emit the in-plane propagative phonons appears to be provided at nucleation sites by priorly excited uniform out-of-plane oscillations, as evidenced by the picosecond delay before their launching.…”

“…When no independent quantitative characterization of the crystal thickness is available, the extracted RCP frequency is used for thickness evaluation by assuming the material phonon velocity from literature [253,256]. In other works, thickness at the probed area is evaluated in situ from the X-ray diffraction Bragg peak satellite distance [131] or from the relative electron energy loss intensity scaled by the scattering mean free path [124,130]. Consequently, phonon velocities for MoS 2 and graphene can be extracted.…”

Coherent Phonon Disruption and Lock-In During a Photoinduced Charge-Density-Wave Phase Transition

“…Further, rapid suppression of the NC phase should pin CAP nucleation and wave vectors (kCAP) to static structures, such as linear defects, owing to correlated initial symmetry-raising and fewpicosecond c-axis coupling to basal-plane modes. [40][41][42][43] Accordingly, we establish connections between linear defects, kCAP and symmetries, and CDW PLD wave vectors during the photoinduced NC-to-incommensurate (IC) phase transition in 1T-TaS2 with 4D UEM imaging and diffraction. [44][45][46] We find that in situ femtosecond photoexcitation of an ultrathin, freestanding crystal produces a hybridized CAP mode launched from a linear defect within a select nanoscale region of interest (ROI).…”

Femtosecond TEM: Imaging Nanoscale Materials Dynamics at Temporal Extremes

“…Here, the goal is twofold: to provide an overview of the current state-of-the-art in stroboscopic fs TEM, supplemented with select recent examples, and to also provide an assessment of challenges yet to be overcome and the approaches currently being pursued to advance UEM spatiotemporal resolutions and to expand the application space for femtosecond-picosecond studies. Following a basic description of stroboscopic fs TEM operating principles and hardware, two recent studies of the direct imaging of photoexcited coherent acoustic phonons in metallic nanocrystals (plasmonic Au nanorods) and layered semiconducting materials (MoS2) will be described [11,12]. Imaging of novel behaviors will be emphasized, including nanoscale anisotropic phonon dynamics and energy transfer and sequential excitation of lattice oscillations nucleated at individual defect structures, in addition to sensitivities to variations in dynamics driven by step-edges that are one unit-cell in height.…”