Direct Imaging of Step-induced Phonon Softening with 4D Ultrafast Electron Microscopy*

The abrupt lattice termination at surface steps leads to modulation of local electronic, vibrational, and structural properties via edge atom relaxation. [1-3] However, relevant spatiotemporal scales are such that resolving ultrafast structural dynamics at individual steps is challenging. Here, we report direct imaging of the step-induced softening of photoexcited phonons in MoS₂ using ultrafast electron microscopy (UEM). [4] The sensitivity of UEM to diffraction contrast dynamics allows us to resolve few-percent reductions in phonon frequency spanning tens of nanometers laterally away the step, displaying an upward exponential behavior. This arises from a combination of anisotropic bonding inherent to layered materials and incoherent photoinduced atomic displacements at the discontinuity. The behavior is in quantitative agreement with a finite element transient deformation model. The results provide new insights into the structure-function relationships of defect-sensitive materials on the atomic scale.

[1] Niu, L., et al. Science 1995_, 268, 847

[2] Ibach, H., et al. Phys. Rev. Lett. 1978, 41, 958

[3] Durukanoglu, S., et al. Phys. Rev. B 1997, 55, 13894

[4] Zhang, Y., et al. Nano Lett. 2021, 21, 7332