Journal
NANO LETTERS
Volume 21, Issue 17, Pages 7332-7338Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.nanolett.1c02524
Keywords
transition metal dichalcogenides; MoS2; structural dynamics; coherent acoustic phonons; femtosecond photoexcitation; in situ TEM
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Funding
- National Science Foundation [DMR-1654318]
- National Science Foundation through the University of Minnesota MRSEC [DMR-2011401]
- Louise T. Dosdall Fellowship
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This study demonstrates the direct observation of ultrafast atomic-to-nanoscale lattice dynamics at individual surface steps using femtosecond 4D ultrafast electron microscopy, revealing nanometer-variant softening of photoexcited phonons. The softening effect is attributed to anisotropic bond dilation and photoinduced incoherent atomic displacements, extending laterally tens of nanometers from the atomic-scale discontinuity. The high spatiotemporal resolutions achieved in this study provide new insights into atomic-scale structure-function relationships of highly defect-sensitive, functional materials.
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, straininduced 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.
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