Filming Photoinduced Ultrafast Phase Transformation of Vanadium Dioxide at The Single Nanoparticle Level

Visualizing structural rearrangements at the atomic/molecular level is essential in understanding the functions of matter. Once isolated, a molecule's intermediate and transitional structures during a chemical reaction may be routinely determined via ultrafast spectroscopy with femtosecond temporal precision. When the structural degrees of freedom increase, e.g., condensed matter comprising countless atoms or organized molecular units, local nanoscale structural defects become prevalent, and thus, the physical processes of each singularity do not proceed as those in bulk and even diverge. If controlled, these structural defects may yield functional benefits and be exploited to overcome the current technological challenges ranging from catalysis to quantum computing. Because the ensemble natures of spectroscopic measurements do not reveal the characteristic transition of each nanoscopic structure, simultaneous spatiotemporal imaging is required to resolve complex processes, hierarchically spanning small-amplitude, ultrafast atomic displacements to collective structural rearrangements at expanded spatiotemporal scales. As an energy filter in transmission electron microscopy has improved the precision of structural determination by filtering out inelastic imaging electrons, introducing the energy filter to ultrafast electron microscopy (UEM) can advance the time resolution to the domain of atomic motion. Imaging transient structures with femtosecond temporal precision was made possible by gating imaging electrons of narrow energy distribution from dense chirped photoelectron packets, thus typically posing picosecond duration. Presented are the concept and proof-of-principle demonstration of the energy-filtered UEM achieving the temporal resolution limited by the briefness of an optical excitation pulse, i.e., 500 fs in this study, filming ultrafast insulator-to-metal phase transition of vanadium dioxide (VO₂), a representative strongly correlated system. In this study, the heterogeneous phase transformations of VO₂ nanoparticles were revealed and attributed to the emergence of a transient, low-symmetry metallic phase caused by different local strains. Our approach leads the access of electron microscopy to the timescale of elementary nuclear motions, visualizing the onset of structural dynamics of matter at nanoscales.