4D imaging and diffraction dynamics of single-particle phase transition in heterogeneous ensembles.

In this Letter, we introduce conical-scanning dark-field imaging in four-dimensional (4D) ultrafast electron microscopy to visualize single-particle dynamics of a polycrystalline ensemble undergoing phase transitions. Specifically, the ultrafast metal-insulator phase transition of vanadium dioxide is induced using laser excitation and followed by taking electron-pulsed, time-resolved images and diffraction patterns. The single-particle selectivity is achieved by identifying the origin of all constituent Bragg spots on Debye-Scherrer rings from the ensemble. Orientation mapping and dynamic scattering simulation of the electron diffraction patterns in the monoclinic and tetragonal phase during the transition confirm the observed behavior of Bragg spots change with time. We found that the threshold temperature for phase recovery increases with increasing particle sizes and we quantified the observation through a theoretical model developed for single-particle phase transitions. The reported methodology of conical scanning, orientation mapping in 4D imaging promises to be powerful for heterogeneous ensemble, as it enables imaging and diffraction at a given time with a full archive of structural information for each particle, for example, size, morphology, and orientation while minimizing radiation damage to the specimen.