4D ultrafast electron microscopy: Imaging of atomic motions, acoustic resonances, and moiré fringe dynamics
Introduction
Research on the structural, morphological and mechanical properties of materials with different scales of length and time has attracted much attention due to its prime role in fundamentals and applications for nanoscience and nanotechnology [1], [2], [3], [4], [5], [6]. Four-dimensional (4D) ultrafast electron microscopy (UEM) [2], [7], [8] permits visualization of these dynamics with atomic-scale spatial resolution and time resolution down to the femtosecond (fs) domain. With the additional time resolution, novel variants of imaging methods [9], [10] can be exploited. The time scales prior to the UEM development were either milliseconds (video camera response) with high spatial resolution, in environmental construct [11], or nanoseconds (ns) with relatively low spatial resolution because of electron repulsion [12], [13]. In UEM, single electron imaging was introduced to nip a primary constraint on space–time resolution, the problem of space charge [8].
Initial progress in the realization of this potential has been provided by recent UEM studies of single-crystal thin-film specimens of gold and graphite, utilizing both selected-area imaging and diffraction. In these, a variety of dynamic responses to heat/laser pulse excitation were observed, on time scales ranging from pico- to microseconds [14], [15]. The results provide real-time visualization of atomic motion and morphology changes, and, in graphite, of two distinct forms of acoustic resonance. In other studies on these time scales, mechanical motions of cantilevers were directly visualized in the UEM images of the moving objects, rather than indirectly through image contrast effects [16].
Another capability of transmission electron microscopy (TEM) not previously exploited in UEM is the in situ observation of crystal defects such as dislocations and stacking faults [17], which is key to characterization and improvement of material properties and development of novel advanced materials. Observing moiré fringes is a classic method to visualize dislocations and stacking faults [17], [18], [19], [20]. For nanocrystals of graphite, three types of diffraction contrast are typically observed in TEM: (1) bend contours [21]; (2) contrast fringes due to the physical buckling of the graphene layers [22]; and (3) moiré fringes as a result of the misorientation between overlapping crystal layers [17], [18], [19], [20], [21]. Recently, moiré fringes due to rotational stacking faults (ca. 0.4°–30°) in few-layer graphene have been studied at atomic-scale resolution by scanning tunneling microscopy (STM) as well as TEM [23], [24], [25]. However, no direct observation of moiré fringe dynamics has been reported.
In this contribution, we give a detailed account of the UEM studies of gold and graphite single crystals, building on preliminary reports, and also report our first in situ observation of moiré fringe dynamics in graphite. Observations of coherent resonance modulations in graphite diffraction and images, including oscillation of moiré fringe spacing, firmly establish the linear relation between oscillation period and film thickness. These measurements demonstrate that the large amplitude, non-thermal changes in diffraction that follow non-equilibrium pulsed excitation of the graphite specimen are due to a resonant elastic modulation of its thickness and allow a determination of the Young's elastic modulus of the graphite c-axis.
Section snippets
UEM: time, space, and energy resolutions
Ultrafast electron imaging and diffraction were carried out with the second-generation ultrafast electron microscope (UEM-2) [14], [15], [26] built at the California Institute of Technology (Caltech), as shown in Fig. 1. Briefly, the instrument consists of a hybrid 200 kV TEM designed for pulsed dynamic measurements by directing of either fs or ns laser pulses to the cathode (LaB6), and specimen. The ultraviolet pulses (fs: 346 nm; ns: 355 nm) directed at the cathode create packets of at most a
Atomic motions
When a crystal is excited by absorbing energy from a heating light pulse, the lattice must undergo changes to accommodate the deposited energy. In the absence of a phase change in the material, the lattice will typically expand. The primary influence of this expansion on the level of the individual lattice unit cell, and the effect on diffraction, are illustrated in Fig. 2, middle. Unconstrained thermal expansion of a high symmetry lattice, such as a face-centered cubic (fcc) gold crystal, is
Coherent motion in diffraction: longitudinal elastic properties
The visualization of resonances caused by coherent atomic motions is illustrated by the observations made in a series of fs UEM diffraction experiments carried out on graphite. Fig. 5 shows the graphite unit cell, the hexagonal graphite lattice structure, and an electron diffraction pattern captured at negative time, i.e., before arrival of an excitation pulse, in the UEM mode. The crystal structure corresponds to the space group P63/mmc, having a hexagonal unit cell with lattice dimension of a=
Moiré fringes dynamics
With the UEM resolutions of time and space, it was possible to visualize the diffraction contrast phenomenon of moiré fringes, and its dynamics. At the top of Fig. 10, two representative moiré fringe images of graphite are shown, which were taken by using the DF imaging technique for better contrast [18] (see below). From these and other measurements made at various points of the specimen, fringes with spacings from 2.6 to 107.5 nm were found. It is known that rotational moiré fringes result
Femtosecond EELS and bonding
Time-resolved EELS was recently demonstrated [27], [28] in the mapping of chemical bonding dynamics, which require nearly ten orders of magnitude increase in time resolution from the detector-limited millisecond response [54]. By following the evolution of the energy spectra with fs resolution, it was possible to resolve in graphite the dynamical changes at the millielectronvolt (sub-pm motion) scale. In this way, the influence of surface and bulk atoms motion was examined (Fig. 11). Among the
Conclusion
The works described here provide a sample of studies undertaken with the second-generation UEM at Caltech, designed for high spatial resolution and fs temporal resolution. Dynamic phenomena reflecting atomic motions, morphology changes, and acoustic resonances are observed in both the image and diffraction modes, and the time scales range from picoseconds to hundreds of microseconds. We also report picosecond dynamic changes upon pulsed, non-equilibrium excitation of moiré fringes in
Acknowledgements
This work was supported by the National Science Foundation and Air Force Office of Scientific Research in the Gordon and Betty Moore Center for Physical Biology at Caltech. We thank F. Carbone for preparation of the graphite specimen.
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