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

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.