Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125, USA.
Ultramicroscopy. 2009 Dec;110(1):7-19. doi: 10.1016/j.ultramic.2009.08.005. Epub 2009 Aug 29.
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 (c(33)) 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.
在四维（4D）超快速电子显微镜（UEM）中，使用定时脉冲电子成象和选区衍射来研究具有空间和时间分辨率的结构动力学，这些分辨率允许直接观察影响材料基本性质的转变。直到最近，UEM 研究才开始揭示纳米级试样对材料激发的各种动态响应，其超快时间尺度可达微秒。在这里，我们给出了其中一些结果的说明，包括金和石墨单晶薄膜的成象和衍射动力学，揭示了前者中的原子运动和形态变化，以及后者中的两种声学共振形式。我们还首次报告了石墨中晶格激发引起的莫尔条纹的动态变化，这些变化是在明场和暗场图像中记录的。在莫尔条纹间距和其他选区图像特性中观察到的振荡与来自同一试样区域的衍射图案中布拉格斑点变化观察到的振荡具有相同的时间周期。该周期被证明与试样局部厚度呈线性变化，从而证明振荡是由于对膜厚度的共振弹性调制的激发引起的，并允许得出膜厚度的杨氏模量（c(33)）值为 36 GPa。在石墨中观察到的第二种共振动力学形式的时间尺度要长得多，它对应于膜的面外击鼓振动，与平面内（a 轴）拉伸的 0.94 TPa 弹性模量一致。对于后者，纳米级膜的运动在早期显得复杂（“混沌”），并在较长时间内建立起共振。最后，UEM 中的电子能量损失光谱（EELS）为在变化时间尺度（飞秒）内研究化学键提供了一个独特的领域，并且讨论了其在石墨中的应用。
