Spatio‐Temporal Probing of Lattice Dynamics in Graphite by Ultrafast TEM

Abstract

Over the last decades, electron microscopy was tremendously successful in unravelling material structures and compositions, resolved on the atomic scale, but only with limited temporal resolution. Optical pump‐probe techniques are now applied routinely for the study of ultrafast dynamics. Nevertheless, we still lack tools for accessing nanoscale dynamics on a femtosecond timescale. Such a capability can be provided by ultrafast transmission electron microscopy (UTEM), which employs a pulsed electron beam with sub‐picosecond pulse duration to stroboscopically probe ultrafast laser‐driven dynamics with the imaging and diffraction capabilities of electron microscopy [1,2]. So far, the potential of this approach is limited by the availability of a high brightness laser‐driven electron source within a transmission electron microscope. Here, we apply UTEM for the study of ultrafast local lattice dynamics in single crystalline graphite, enabled by the generation of highly coherent electron bunches from a point‐like photoelectron source [3]. The Göttingen UTEM instrument is based on the custom modification of a JEOL 2100F Schottky field emission TEM, allowing for optical sample excitation and the generation of optically triggered ultrashort electron pulses (Fig. 1a) [4]. The laser‐triggered nanoscopic electron source [5–7] employs localized single‐photon photoemission from the front facet of a tip‐shaped ZrO/W(100) emitter (Fig. 1b). Highly coherent ultrashort electron pulses with a normalized emittance of 3 nm·mrad are generated, enabling ultrafast electron imaging with phase‐contrast and time‐resolved local probing (Fig. 2). Specifically, at the sample position, we obtain electron focal spot sizes down to 1 nm with a temporal pulse width of 300 fs (full‐width‐at‐half‐maximum) and a spectral bandwidth of 0.6 eV (cf. Fig. 1c–e) [3]. We demonstrate ultrafast nanoscale diffractive probing, by studying the local light‐induced structural dynamics close to the edge of a single‐crystalline graphite thin film (Fig. 2a) [8]. Local convergent beam electron diffraction (CBED) patterns from nanoscale sample areas are recorded using tightly focused electron pulses (diameter of about 10 nm). The complex local distortion of the crystal structure is retrieved by utilizing the broad angular range of the incident electron beam (convergence angle of about 48 mrad) to probe several Bragg scattering conditions simultaneously in reciprocal space (cf. Fig. 2b,c). For the case of graphite, we observe strongly pronounced lattice vibrations at the crystalline edge (Fig. 2d,e), corresponding to out‐of‐plane breathing modes, as well as in‐plane shearing modes mapped with 10‐nm spatial resolution. Considering the time‐dependent relative line shifts, the individual contributions of mechanical deformation modes are disentangled. Furthermore, raster‐scanning the electron focal spot across the sample allows for a comprehensive spatio‐temporal reconstruction of the involved dynamics. In conclusion, we have developed a novel UTEM instrument, relying on highly coherent electron pulses generated from a nanoscale photoemitter. Additionally, we presented first results on its capability for the investigation of ultrafast nanoscale dynamics in graphite.