Ultrafast Transmission Electron Microscopy
the black and white dotted lines); (b) nucleation and growth of larger grains at 20.00 μs with a transition to planar/columnar growth between 25.55 and 30.65 μs; and finally (c) fully planar crystal growth aſter 32.55 μs. Te identification of these complex dynamics during solidification in Al-Si alloys were previously unobserved and can be critical in modeling the rapid solidification process in Al-Si additive manufacturing materials. Another example of the unique capability exclusive to
the DTEM technique is the complexity of chemical processes when nanoparticle precursors are used, specifically, the phase changes required for the nanothermite reaction process [18] of Al/CuO nanoparticles when heated at 1011
+ Aly Cuz + Cu. Changes in the Al/ /Aly Cuz spheroids. K/s by a Nd:YAG laser
pulse (12 ns, 532 nm, < 1 μJ). In Figure 4, the high-resolution initial and final state images allow the lower-resolution movie mode images to capture the evolution of the nanoparticle reaction Al + CuO → AlOx
CuO aggregates are initiated within 45 ns of the laser pulse with the CuO nanostructures melting first (spheres in 330 ns image). Te Al nanoparticles coalesce (central structure, 520 ns image) and then finally form phase-separated AlOx
Tese observed morphology changes limited the reaction’s heat- transfer rather than the reaction being limited by mass-transfer as seen in macroscopic systems, due to the much stronger absorption of the 532 nm radiation by the CuO nanoparticles.
RF (Laser-Free) UTEM Until recently, laser-UTEM was the only method available
to achieve ps time resolution in TEM imaging. In the past few years, laser-free UTEM has been developed, alleviating the laser requirements by employing radiofrequency (RF) modulation of the TEM’s source beam. High-frequency (GHz) beam pulses have been demonstrated using a resonant deflecting cavity [19– 21] and a traveling wave stripline [6,22] design. Te resonant deflecting cavity is a fixed frequency RF device, thereby fixing the beam modulation rate. Typically, the cavity is filled with a dielectric material for compactness, and the unit is water- cooled to maintain the cavity’s resonant frequency. When RF is applied at the resonant frequency, the electron beam traveling through the structure is deflected from the beam axis and becomes blocked by a chopping aperture. Tus, only electron trajectories with near-zero deflection can pass the aperture; for a sinusoidal field this occurs at each zero-crossing point, that is, twice per RF period. Te traveling wave stripline uses the same type of aperture but has two phase-controlled RF inputs to create a well-defined transverse electric field. Te field propagates through a slow wave structure that is phase-matched to the electron beam velocity. Tus, each beam electron sees a virtually constant electrostatic field as it propagates, and therefore the native beam properties are minimally perturbed, and the deflection is maximized for the RF cavity size. Te RF is widely tunable, although the beam energy is fixed by the slow wave structure. Tese two approaches are compared in more detail in the Summary section, although they both require no modification to the electron gun and basically preserve the peak brightness and energy spread to the native source. Te traveling wave stripline design was incorporated in the commercially available [23–25] Ultrafast Pulser (UFPTM
) 50
Figure 4: DTEM movie mode representation of the formation of AlOx Cuz
+ Aly Cuz permission from the American Chemical Society.
and has demonstrated straightforward implementation with excellent long-term stability. Figure 5 illustrates the UFPTM
and Aly- nanoparticles from Al and CuO after being heated by a 532 nm laser pulse.
Times listed in lower images are elapsed time after laser pulse, showing the roughly 1 μs reaction of Al + CuO AlOx
+ Cu. Reprinted from [18], with
basic
design and integration just below the native TEM electron gun. As shown in the inset, two traveling wave stripline “kickers” (K1, K2) are used with a beam-limiting aperture between them. Te first kicker (K1) sweeps the beam across an aperture by imparting a small sinusoidal transverse momentum to the beam. Te second kicker (K2) operates to remove the transverse momentum imparted by K1 to maintain the original beam axis. Te RF driving signals to the kickers have variable phase and amplitude to optimize beam condition and enable custom beam configurations [22 – Figures 6 and 7]. An RF pump/probe arrangement [6,26] is also shown in Figure 5: since two electron probe pulses are created in each RF cycle (one for each zero- crossing point), a frequency doubler has been added to bring the pump rate equal to the probe rate. Te phase delay enables probing of the sample’s response throughout the RF cycle. As there is no laser driving the pulsed beam emission, the
RF design dictates the electron pulse repetition rate in the RF UTEM designs. For the UFPTM
, the native practical operating
frequency is 500 MHz up to 10 GHz with pulse picking techniques employed to reach down to sub-Hz frequencies. For the resonant deflection cavity design, the resonant frequency is fixed, typically in the 1–10 GHz range, but has been operated as low as 75 MHz using a dual-mode resonator [27]. With the introduction of the newer RF UTEM technology,
there has been significant work on the resultant beam charac- teristics and ultimate spatio-temporal resolution capabilities. Since these methods employ the original electron gun installed in the electron microscope, the pulsed beam is naturally closer to the native TEM beam character compared to a laser- driven beam with a modified cathode and gun. In practice, the UFPTM
capabilities have been shown to be nearly identical
www.microscopy-today.com • 2021 September
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