Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published September 2007 | public
Journal Article

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 (P2_1/c) to tetragonal (P4_2/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_∥-band, and the relatively low fluence used, these results show the critical role of carriers in weakening the V^(4+)−V^(4+) 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.

Additional Information

© 2007 American Chemical Society. Received June 5, 2007. Publication Date (Web): July 10, 2007. This research was supported by the Gordon and Betty Moore Foundation, the National Science Foundation, and the Air Force Office of Scientific Research. J.W. gratefully acknowledges partial support from the Knut and Alice Wallenberg foundation.

Additional details

August 19, 2023
October 20, 2023