4D ultrafast electron microscopy: Imaging of atomic motions, acoustic resonances, and moiré fringe dynamics
In four-dimensional (4D) ultrafast electron microscopy (UEM), timed-pulse electron imaging and selected-area diffraction are used to study structural dynamics with space- and time-resolutions that allow direct observation of transformations affecting the fundamental properties of materials. Only recently, the UEM studies have begun to reveal a variety of dynamic responses of nanoscale specimens to material excitation, on ultrafast time scales and up to microseconds. Here, we give an account of some of these results, including imaging and diffraction dynamics of gold and graphite single crystal films, revealing atomic motions and morphology change in the former and two forms of acoustic resonance in the latter. We also report, for the first time, dynamic changes upon lattice excitation of moiré fringes in graphite, recorded in bright- and dark-field images. Oscillations that are seen in moiré fringe spacing and other selected-area image properties have the same temporal period as observed in Bragg spot changes in diffraction patterns from the same specimen areas. This period is shown to vary linearly with the local thickness of the specimen, thus establishing that the oscillations are due to excitation of a resonant elastic modulation of the film thickness and allowing derivation of a value of the Young's modulus (c33) of 36 GPa for the c-axis strain. The second form of resonance dynamics observed in graphite, on much longer time scales, corresponds to an out-of-plane drumming vibration of the film consistent with a 0.94 TPa elastic modulus for in-plane (a-axis) stretching. For the latter, the nanoscale membrane motion appears complicated ("chaotic") at early time and builds up to a resonance at longer times. Finally, electron energy loss spectroscopy (EELS) in the UEM provides a unique domain of study of chemical bonding on the time scale of change (femtoseconds), and its application to graphite is discussed.