Ultrafast electron microscopy (UEM): four-dimensional imaging and diffraction of nanostructures during phase transitions.

Four-dimensional (4D) imaging during structural changes are reported here using ultrafast electron microscopy (UEM). For nanostructures, the phase transition in the strongly correlated material vanadium dioxide is our case study. The transition is initiated and probed in situ, in the microscope, by a femtosecond near-infrared and electron pulses (at 120 keV). Real-space imaging and Fourier-space diffraction patterns show that the transition from the monoclinic (P21/c) to tetragonal (P42/mnm) structure is induced in 3 +/- 1 ps, but there exists a nonequilibrium (metastable) structure whose nature is determined by electronic, carrier-induced, structural changes. For the particles studied, the subsequent recovery occurs in about 1 ns. Because of the selectivity of excitation from the 3d parallel-band, and the relatively low fluence used, these results show the critical role of carriers in weakening the V4+-V4+ bonding in the monoclinic phase and the origin of the nonequilibrium phase. A theoretical two-dimensional (2D) diffusion model for nanoscale materials is presented, and its results account for the observed behavior.