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Step edges are an important and prevalent topological feature that influence catalytic, electronic, vibrational, and structural properties arising from modulation of atomic-scale force fields due to edge-atom relaxation. Direct probing of ultrafast atomic-to-nanoscale lattice dynamics at individual steps poses a particularly significant challenge owing to demanding spatiotemporal resolution requirements. Here, we achieve such resolutions with femtosecond 4D ultrafast electron microscopy and directly image nanometer-variant softening of photoexcited phonons at individual surface steps. We find large degrees of softening precisely at the step position, with a thickness-dependent, strain-induced frequency modulation extending tens of nanometers laterally from the atomic-scale discontinuity. The effect originates from anisotropic bond dilation and photoinduced incoherent atomic displacements delineated by abrupt molecular-layer cessation. The magnitude and spatiotemporal extent of softening is quantitatively described with a finite-element transient-deformation model. The high spatiotemporal resolutions demonstrated here enable uncovering of new insights into atomic-scale structure-function relationships of highly defect-sensitive, functional materials.
Bibliographical noteFunding Information:
This material is based on work supported by the National Science Foundation under Grant No. DMR-1654318. This work was supported partially by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-2011401. Y.Z. acknowledges support from the Louise T. Dosdall Fellowship.
© 2021 American Chemical Society.
- coherent acoustic phonons
- femtosecond photoexcitation
- in situ TEM
- structural dynamics
- transition metal dichalcogenides
How much support was provided by MRSEC?
Reporting period for MRSEC
- Period 2
PubMed: MeSH publication types
- Research Support, Non-U.S. Gov't
- Research Support, U.S. Gov't, Non-P.H.S.
- Journal Article
FingerprintDive into the research topics of 'Imaging Nanometer Phonon Softening at Crystal Surface Steps with 4D Ultrafast Electron Microscopy'. Together they form a unique fingerprint.
- 1 Active
9/1/20 → 8/31/26
Project: Research project
UEM data supporting "Imaging Nanometer Phonon Softening at Crystal Surface Steps with 4D Ultrafast Electron Microscopy"
Zhang, Y. & Flannigan, D. J., Data Repository for the University of Minnesota, Aug 26 2021