Correlating Atomic-Scale Lattice and Nanoscale Structural Dynamics with Ultrafast Electron Microscopy

Ultrafast photoexcitation of transition metal dichalcogenides (TMDs) has been shown with ultrafast electron microscopy (UEM) imaging to produce coherent intra and interlayer low-frequency acoustic phonons that propagate along preferential wavevectors at the in-plane and c-axis speeds of sound In defect-laden crystals, coherent acoustic phonons were found to preferentially emerge from abrupt and extended crystal-crystal and crystal-vacuum interfaces. For certain morphologies and structural configurations, wave interference and phonon beating manifests as bend-contour motion that rapidly transitions from coherent to incoherent behaviors Nanosecond bright-field imaging over longer relaxation times revealed further relaxation of localized incoherent motion to lower-frequency, wholecrystal, coherent mechanical oscillations consisting of multiple modes having microsecond lifetimes. Owing to the highly anisotropic structures arising from in-plane covalent and c-axis van der Waals bonding, the elastic tensors and speeds of sound are significantly different along these crystallographic directions. With UEM, it was shown that the coherent c-axis acoustic-phonon dynamics could be monitored by tracking oscillatory moir-fringe dynamics. Finite element modeling indicated fs photoexcitation initially produces an impulsive expansion along the layer stacking direction that rapidly couples to the in-plane propagating modes at discrete structural discontinuities, though direct evidence of this is currently lacking. Importantly, the nanoscale motions characteristic of these coherent structural dynamics must originate from electron-phonon coupling and atomic-scale variations in vibration amplitudes. Thus, reciprocal-space studies with UEM correlated with femtosecond brightfield imaging has the potential to provide a fuller picture of the overall structurally-and morphologically-dependent dynamics in these materials.