Proc. Natl Acad. Sci. USA 111, 5491–5496 (2014)

A transmission electron microscope is usually thought of as a tool for investigating a material's crystal structure, not for probing its elastic properties. However, a series of electron-microscopy images, recorded sufficiently rapidly one after another, should provide information on structural dynamics.

Wenxi Liang and colleagues have done exactly that, using a method called convergent-beam electron diffraction (CBED). Whereas most electron-microscopy techniques require thin samples so that electrons travelling through the sample scatter only once, CBED works well with thicker samples. The diffraction pattern then contains lines, known as Kikuchi lines, resulting from many electrons being incoherently scattered.

The team applied femtosecond laser pulses, used to excite lattice vibrations, to a graphite sample and recorded the time evolution of the Kikuchi lines. They collected data spanning a total time interval of 1 millisecond. The results (involving Fourier transforms of Kikuchi line evolutions, as pictured) revealed strong vibrational features at 9.1 and 75.5 GHz, attributed to motion of the graphene planes along the stacking direction and an in-plane 'breather' mode, respectively.