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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-MoS2, 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 MoS2 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 MoS2 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.
Bibliographical noteFunding Information:
Y.Z. contributed formal analysis, investigation, methodology, software, validation, visualization, writing the original draft, reviewing, and editing. D.J.F. contributed conceptualization, formal analysis, funding acquisition, methodology, project administration, resources, supervision, visualization, writing the original draft, reviewing, and editing. This material is based upon work supported by the National Science Foundation under Grant DMR-1654318. This work was supported partially by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013. Part of this work was carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which has received capital equipment funding from the NSF through the UMN MRSEC program under Award Numbers DMR-0819885 and DMR-1420013. The authors declare no competing financial interest.
Copyright © 2019 American Chemical Society.
- Transition metal dichalcogenides
- in situ TEM
- photoinduced strain
- ultrafast electron microscopy
How much support was provided by MRSEC?
Reporting period for MRSEC
- Period 6
PubMed: MeSH publication types
- Journal Article
- Research Support, U.S. Gov't, Non-P.H.S.