Ultrafast Transmission Electron Microscopy: Techniques and Applications
Eric Montgomery,1 Darrin Leonhardt,2, * and John Roehling3
1Euclid Techlabs, LLC, 365 Remington Blvd., Bolingbrook, IL 60440 2Euclid Techlabs, LLC, 6 Montgomery Village Ave., Gaithersburg, MD 20879
3Materials Science Division, Lawrence Livermore National Laboratory, Livermore, CA 94550 *
d.leonhardt@euclidtechlabs.com
Abstract: With the growing applications of temporally resolved elec- tron microscopy for probing basic phenomena and reducing beam- induced damage, a multifaceted introduction to the field of ultrafast transmission electron microscopy is provided. This primer includes techniques and equipment as well as implementation perspectives. Historical developments and recent technical advances will provide insight into ultrafast capabilities for research as well as educate electron microscopists on the general techniques. This technology review also includes applications enabled by ultrafast using various sample stimuli from multidisciplinary fields.
techniques
Keywords: ultrafast electron microscopy, stroboscopic, time- resolved imaging, high-resolution transmission electron microscopy
Introduction Originally a basic research tool for materials science, transmission electron microscopes (TEMs)
have seen a
renaissance, as they have been applied in nearly every technology-based field. Transmission electron microscopy (TEM) has become the gold standard of high-spatial-resolution techniques and an ever-increasing list of applications, from quantum dots to cellular 3D tomography and holography, and a wider range of capabilities. TEMs are used to connect photonics, nanodevice architecture, and biophysics, each with their individual intrinsic response times on the nanoscale. Te continued evolution of applications and maturation of basic TEM instruments have created additional sectors in the TEM industry (life sciences, nanotechnology, and semiconductor) and have fostered sufficient growth in these areas that the new market sectors are comparable in size to the once-dominant materials science market [1]. To interrogate time-resolved responses to optical stimuli,
ultrafast TEM (UTEM) was developed using lasers and photocathodes in the mid-2000s [2]. While ultrafast lasers were a natural enabler for early research in UTEM, the explosive growth of new applications based on large molecules (proteins, cells) and new 2D/3D architectures (NEMS/MEMS, nanosheets, spintronics) requires broader temporal capabilities due to their widely varying response times. Tis article focuses on the challenges for growing ultrafast techniques and compares these complementary methods to laser-UTEM techniques for general electron microscope users to consider when expanding their research capabilities [3,4].
An Ultrafast Overview Te adoption of electron microscopy into more cross-
disciplinary fields of research has expanded the need for tem- porally resolved measurements at the nanoscale. “Ultrafast” timescales have been driven by the available laser pulse lengths
46 doi:10.1017/S1551929521001140
for fundamental atom-photon interactions. Initially “ultra- fast” referred to picosecond (10-12
been extended through the femtosecond (10-15 into the attosecond (10-18 of
s; ps) time scales but has s; fs) and then
s; as) regime with the development laser technologies for materials processing and military
applications. In electron microscopy, the ps regime is common for interrogating basic material phenomena, then longer time scales are generally necessary as material systems get larger physically. Figure 1 shows scales of interest for time-dependent areas of study in materials science (red), life sciences (blue), semiconductor (gray), and nanotechnology (green). Superim- posed on the areas of study in Figure 1 are the typical tempo- ral ranges for UTEM techniques being discussed here. While significant temporal overlap between the UTEM techniques is clear from this representation, these technologies are highly complementary to one another. Tese complementary features are the primary focus of this article.
Reversible or Irreversible Te most significant complementary capability of
the
UTEM techniques is perhaps the type of process being studied. Most UTEM techniques are performed on reversible processes, where the sample reverts to the same initial condition aſter the stimulus, or pump, has been completed. Tis allows the repeated pump-probe of the sample to accumulate sufficient electrons for a satisfactory image (typically >106
electrons).
Tus, stroboscopic imaging [5,6] is used in concert with a series of pump-probe delay times to generate a time-resolved series of images showing the transition from the initial to the final state due to the pump stimulation. For a given pump- probe measurement, a sample undergoes many reversible cycles as the probe electron beam builds up sufficient detected electrons to form the image. Next, the probe beam delay is increased by a user-selected time step, and the process is repeated. (Tis is analogous to boxcar averaging in laser spectroscopy techniques.) Note that
the sample’s intrinsic
process time to return to the initial state determines the maximum stroboscopic cycle frequency. Tus, aſter some time (hours or even days, depending on the process and UTEM technique) a series of high-spatial-resolution images depicting the temporally resolved sample process is generated. Te spatial resolution of the TEM can be maintained depending on the details of the UTEM technique; such details will be discussed in the following UTEM section. In contrast to reversible processes where stroboscopic
imaging can integrate over many pump-probe cycles, imaging irreversible processes requires enough electrons in one pulse to
www.microscopy-today.com • 2021 September
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