Imaging Nanometer Phonon Softening at Crystal Surface Steps with 4D Ultrafast Electron Microscopy

Yichao Zhang, David J. Flannigan

Research output: Contribution to journalArticlepeer-review

12 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)7332-7338
Number of pages7
JournalNano letters
Issue number17
StatePublished - Sep 8 2021

Bibliographical note

Funding 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.

Publisher Copyright:
© 2021 American Chemical Society.


  • coherent acoustic phonons
  • femtosecond photoexcitation
  • in situ TEM
  • MoS
  • structural dynamics
  • transition metal dichalcogenides

MRSEC Support

  • Partial

PubMed: MeSH publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Journal Article


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