Abstract: Investigations into the interactions between light and matter at the ultrafast time scale are essential for the design and optimization of highly efficient nanomaterials. For example, 2-D layered materials, topological insulators, ferromagnets, plasmonic structures, and strongly correlated electronic materials have been extensively studied due to their wide range of applications. Despite the remarkable progress achieved, various structural rearrangements and phase transitions that occur in nanomaterials remain poorly characterized. Understanding these processes and their underlying mechanisms, at the relevant dynamical time scales, is essential for the design and optimization of the next generation of high-performance materials.
Conventional ultrafast methods are predominantly sensitive to electronic relaxation dynamics. However, since the wavelength of light far exceeds the interatomic distances involved, these techniques cannot directly sense any structural changes or lattice rearrangements. Introducing the temporal dimension to electron imaging is the basis of ultrafast electron techniques, which allows for studies of transient nonequilibrium structures with unprecedented spatiotemporal resolution. Here, we report the ultrafast electron diffraction results on an atomically thin TiSe2 and WSe2 monolayer. Complex room-temperature lattice dynamics observed in TiSe2 are attributed to strong electron–phonon coupling and electron–lattice equilibration processes, which highlight the importance of these dynamics in the charge-density wave mechanism. On the other hand, anisotropic lattice expansion leading to symmetry breaking is observed in WSe2 monolayer, following laser excitation.