Probing Spatiotemporal Modulation of Strain in Van Der Waals Materials

Transmission electron microscopy presents a unique opportunity to investigate the modulation of strain in both space and time. In this talk, I will discuss our electron-microscopy based methods for studying the impact of atomic- to nanoscale strain and defects on the properties of van der Waals materials. We utilize ultrafast electron microscopy to spatially map the nanoscale softening of vibrational modes near crystal step edges in response to strain induced by in situ laser excitation. This effective bond-stiffness reduction arises from both the modified force field at the step edge and the ultrafast anisotropic expansion in the layer stacking direction, as verified by our finite-element model. Additionally, we report a novel, simple technique for strain patterning in 2D materials using amorphous, holey silicon nitride substrates with periodic surface topography. The formation of regularly undulating tessellation patterns manifests as diffraction-contrast patterns. We measure up to 2% isotropic, tensile strain in the freestanding regions in the 2D flakes. Atomistic simulations reveal that the 3D elastic deformation is driven by interactions between the 2D crystal and the underlying substrate. Our results highlight the remarkable sensitivity of electron-imaging methods in probing the effects of atomic- to nanoscale heterogeneous strain and defects that influence the overall material properties and functionalities.