Four-Dimensional Ultrafast Electron Microscopy: Insights into an

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Four-Dimensional Ultrafast Electron Microscopy: Insights into an Emerging Technique Aniruddha Adhikari,†,§ Jeffrey K. Eliason,‡,§ Jingya Sun,† Riya Bose,† David J. Flannigan,*,‡ and Omar F. Mohammed*,† †

King Abdullah University of Science and Technology, KAUST Solar Center, Division of Physical Sciences and Engineering, Thuwal 23955-6900, Kingdom of Saudi Arabia ‡ Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States ABSTRACT: Four-dimensional ultrafast electron microscopy (4D-UEM) is a novel analytical technique that aims to fulfill the long-held dream of researchers to investigate materials at extremely short spatial and temporal resolutions by integrating the excellent spatial resolution of electron microscopes with the temporal resolution of ultrafast femtosecond laser-based spectroscopy. The ingenious use of pulsed photoelectrons to probe surfaces and volumes of materials enables time-resolved snapshots of the dynamics to be captured in a way hitherto impossible by other conventional techniques. The flexibility of 4D-UEM lies in the fact that it can be used in both the scanning (S-UEM) and transmission (UEM) modes depending upon the type of electron microscope involved. While UEM can be employed to monitor elementary structural changes and phase transitions in samples using real-space mapping, diffraction, electron energy-loss spectroscopy, and tomography, S-UEM is well suited to map ultrafast dynamical events on materials surfaces in space and time. This review provides an overview of the unique features that distinguish these techniques and also illustrates the applications of both S-UEM and UEM to a multitude of problems relevant to materials science and chemistry. KEYWORDS: 4D ultrafast electron microscopy, surface dynamics, energy loss mechanism, atomic resolution, electron impact dynamics, charge carrier dynamics

1. INTRODUCTION The paradigm of ascertaining the structure of a molecular entity to appreciate its function underwent a significant modification toward the end of the last century, when it became evident that the inclusion of dynamic information is an essential requisite to capture the whole picture in regard to understanding an observed phenomenon.1,2 Static images of molecular entities with atomic-scale spatial resolution, as obtained with electron microscopes, although a vital piece of information, fail to unravel the nonequilibrium electronic and structural dynamics of a complex transformation. To achieve the ultrafast time resolution of atomic motions, it is extremely important to obtain snapshots of the event with a time scale (femtoseconds, fs) on the order of the rate of the dynamic process under observation. Since the days of stroboscopic flash photography, the race toward higher temporal resolution has been an everevolving endeavor, and the advent of lasers with ultrashort pulse durations provided a crucial breakthrough in this direction. The giant strides made in the field of time-resolved spectroscopy, in particular following the development of ultrafast lasers, provided an expanding window of opportunity, wherein physicists, materials scientists, and chemists, among others, could probe the dynamics of molecular systems on increasingly smaller time scales, albeit not with the real-space © 2016 American Chemical Society

resolution of electron microscopes. Hence, the challenge resides in bringing together the atomic spatial resolution of the electron microscope with the ultrafast temporal resolution of time-resolved spectroscopy to yield a unique analytical tool that can provide dynamic information on molecular events with extremely fine detail, simultaneously in both space and time. Over the past decade, the most important step in this field (pioneered by Zewail et al. at Caltech) has been the development of the technique dubbed four-dimensional ultrafast electron microscopy (4D-UEM),3−8 which offers the unique advantage of retaining the excellent spatial resolution of conventional electron microscopes while also providing dynamic information along the fourth dimension (time) with the ultrafast (fs) temporal resolution of laser-based timeresolved measurements. In effect, it employs the idea of integrating a fs laser system with an electron microscope in order to generate pulsed electron packets from a photocathode source via photoelectric effect, rather than a continuous electron beam generated thermally, as in conventional thermionic electron microscopes. These pulsed electron Received: September 27, 2016 Accepted: December 15, 2016 Published: December 15, 2016 3

DOI: 10.1021/acsami.6b12301 ACS Appl. Mater. Interfaces 2017, 9, 3−16

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ACS Applied Materials & Interfaces

2. PRINCIPLE OF 4D-UEM The unique feature of 4D-UEM that sets it apart from conventional electron microscopes is the way in which the electrons are generated for probing the specimen under study. In conventional thermionic electron microscopes, continuous electron beams are generated through the current-driven thermal heating of cathode tips. The Coulombic repulsion among the large number of electrons generated, in addition to frame rates of current digital detectors, limits the temporal resolution to milliseconds. However, the electrons in 4D-UEM are generated through photoelectric effect, wherein ultrashort optical pulses incident on the cathode tip generate photoelectron packets. The controlled emission of electrons (e.g., one electron per pulse) enables control over pulse broadening and temporal resolution, as governed by the pulse width of the optical pump pulse, and is entirely independent of the response of detectors used for collecting signals. In principle, this configuration also preserves beam coherency and, therefore, spatial resolution intrinsic to the conventional instrument.5,9,18,33 The photoelectron probe pulse propagating down the instrument column is temporally overlapped with an excitation pulse via a mechanical delay line to provide snapshots of the dynamics being examined. The general experimental setup of 4D-UEM comprises a fs laser system integrated with an electron microscope (Scanning/ Transmission for S-UEM/UEM). Figure 2 shows the general experimental setup for the second-generation 4D S-UEM, where a fs fiber laser system (Clark-MXR) operating at a central wavelength of 1030 nm is integrated with a modified FEI Quanta 650 SEM. The experimental setup is detailed elsewhere.34 Briefly, the fundamental IR pulse output is split and guided to two separate harmonic generators (HGs) to create the second (515 nm) and third (343 nm) harmonic pulses. The 343 nm (or 515 nm) output is tightly focused onto the emitter tip within the electron microscope to generate the probe pulse, while the 515 nm output is incident onto the sample as a pump pulse to photoexcite the specimen under study. The relative timing between these two pulses is adjusted with precision through a computer-controlled optical delay. The emission of the secondary electrons (SEs) from the specimen is collected using a positively biased Everhart− Thornley detector. The spatial resolution of the S-UEM is determined to be approximately 5 nm by imaging gold nanoparticles using pulsed-generated photoelectrons, whereas kinetics plotted for the intensity change of the SEs from a CdSe single crystal yields a temporal resolution of 650 ± 100 fs.34 2.1. Scanning and Transmission Modes of 4D-UEM. As the pump−probe scheme for time-resolved measurements is identical for 4D S-UEM and UEM, the main difference between these two is the working principle of the electron microscope. TEM involves the use of electron beams with energy varying from 30 to 300 keV and relatively thin specimens. The transmitted electrons are subsequently recorded with a CCD camera, providing detailed insight into the material structure up to the atomic level, with additional information on the lattice structure and material composition obtained using Fourier space diffraction and electron energy-loss spectroscopy, respectively, thereby making the investigation of any structural change during chemical reaction or phase transition visible with UEM. In contrast, in the SEM, the accelerating voltage is in the range of 1−30 kV and hence involves electrons with much less energy than that used in TEM to scan the sample surface in a

packets are then used as the specimen probe and are synchronized with the optical excitation pulse used to initiate dynamics. Thus, in contrast to conventional ultrafast laser measurements, where both the pump and probe pulses are optical, the working principle of 4D-UEM relies on an optical pump-photoelectron probe configuration, thereby enabling retention of angstrom-scale spatial resolution but with extension of temporal resolution to the fs scale. Over the past few years, 4D-UEM has developed through the integration of different techniques of microscopy, including imaging and diffraction with scanning and transmission electron microscopy,9−12 spectroscopy,13−15 and tomography.16,17 This has prompted a growing body of studies that exploit its unique features to tackle several fundamental problems in physics, materials science, chemistry, and biology.9−45 The transmission mode of 4D-UEM (UEM) has been used to study elementary structural changes during photoinduced chemical reactions, structural phase transitions, and nanoscale elastic deformations with a combination of angstrom-scale spatial resolution (accessible in imaging and diffraction modes), access to energy space via spectroscopy, and fs time resolution. Complementary to UEM, the scanning mode (S-UEM) is well-suited to study the physical and chemical dynamics on (nano) material surfaces, which are of extreme interest for any optoelectronic application but remain inaccessible even to recently developed optical microscopic techniques, which collect information from the sample bulk and show no such spatial selectivity (Figure 1).

Figure 1. Principle of S-UEM imaging (left) and ultrafast pump− probe spectroscopic measurements (right) for an array of nanocrystals. For comparison, typical transient-absorption spectra (TA spectra) and S-UEM images (secondary electron, SE images), representative of data acquired with each technique, are shown on the left. The schematic on the right shows the main difference between the methodologies, which is the nature of the probing pulse: photoelectrons for S-UEM and photons for pump−probe spectroscopy. In general, the nature of the excitation pulse is the same for each.

In this review, we will discuss the unique characteristics of 4D-UEM that distinguish it from conventional electron microscopy by providing a short description of the experimental setup and illustrate the differences between its scanning and transmission modes. Additionally, a brief overview of the relevant literature will be provided, wherein specific cases are discussed to demonstrate the utility of the technique. 4


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Figure 2. Schematic of the S-UEM experimental setup. The left inset represents images of Au nanoparticles as obtained with and without photoelectron-generating laser irradiation. As clearly observed, the thermal electrons create a negligible background, confirming the involvement of only pulsed electrons for image construction. Right inset represents the instrument’s temporal resolution as obtained by fitting the time-dependent variation in the SE intensity from a CdSe single crystal. All optical components are labeled, and a key to the abbreviations is provided at the bottom of the figure. Reprinted with permission from ref 8. Copyright 2016 American Chemical Society.

Figure 3. Schematic for the probing regimes in S-UEM. SE images at selected time intervals are provided where the change in contrast can be observed with time (right panel). Bright contrast (left panel) is observed in the case of the energy gain, and the dark contrast (right panel) appears due to the energy loss of the SEs while transiting to the surface. Finally, the dashed ellipse indicates the laser footprint region (∼40 μm). Adapted with permission from ref 8. Copyright 2015 American Chemical Society.

of materials in terms of bulk properties (e.g., elasticity) breaks down or verifying if one can extrapolate from atomic-scale behavior (in space and time) to reproduce known macroscopic properties of a material.35 The richness of this information is evident by noting the immense scope of the technique. A few illustrative examples will be taken up for discussion in the next (sections 3 and 4) to provide deeper insights into how this is achieved. The unique capability of S-UEM to selectively monitor surface charge carrier dynamics has been utilized to investigate several dynamical processes in semiconductor materials, such as the surface dynamics in CdSe single crystals,39 multinary nanocrystals,40 and InGaN nanowire arrays.41,42 It has also enabled imaging of molecular solvation dynamics on various surface facets of the semiconductor,43 investigating the effect of doping on ultrafast carrier dynamics in single-crystal GaAs,44 as well as interfacial dynamics in p−n heterojunctions.45 Recently, this method was upgraded by establishing a second-generation S-UEM with longer probing time range and better spatial

raster pattern. The secondary electrons ejected from the top few nanometers on the sample surface are detected to record an image in 3D. This enables the study of photophysical processes typically on material surfaces and at the interfaces, which is pivotal for controlling their use for optoelectronic applications. Consequently, SEM experiments can be carried out on bulk samples, thereby ensuring better thermal dissipation and lesser chance of radiation-induced damage. Also, SEM involves easier sample handling protocols and allows sample imaging in the environmental mode without the constraint of high vacuum in the sample chamber. As explained, the two modes (scanning and transmission) of 4D-UEM aim at extracting different types of observables from the sample. They can play a critical role in characterization of transient structures generated during nonequilibrium phase transitions of materials at nanoscale, especially those triggered by ultrashort laser pulses. These studies address several fundamental issues relevant to nanoscale behavior of materials, such as determining the length scales at which the description 5


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ACS Applied Materials & Interfaces resolution.34 Additionally, features such as surface morphology, grain boundaries, and surface defects were observed to affect the surface charge carrier dynamics of semiconductor materials. It should be noted that transient absorption microscopy methods are also capable of providing a high spatiotemporal resolution for the investigation of charge carrier dynamics in semiconductors.46−52 However, the penetration depth of the laser beam being much larger, the dynamic information gleaned from these techniques mostly originates from the bulk of the specimens under study. Hence, they lack the unique surface specificity of S-UEM. In fact, S-UEM measurements offer flexibility in this regard through variation of the energy of the impinging primary pulsed electrons (by adjusting the accelerating voltage in the EM column). This can control their penetration depth into the sample and thereby the surface depth from which the secondary electrons are generated and subsequently detected. For example, by altering the working voltage from 30 to 1 kV, one can substantially reduce the probe depth of surface dynamics by almost one order of magnitude.

of SE emissions obtained from an array of InGaN NWs, as shown in Figure 4, clearly reveal the presence of these energy

3. S-UEM 3.1. Nature of the Signal in S-UEM. The unique capability of S-UEM to probe dynamic processes typically on the surface of the material in space and time arises from the pump−probe scheme used in the measurement. As shown in Figure 3, two different regimes of probing are used: electron− photon dynamic probing in negative time, where the electron pulse arrives before the excitation pulse and photon−electron probing, where the excitation pulse arrives early to initiate the dynamical process that is probed by the subsequent pulsed electrons. These two regimes of probing have a major effect on the type of observed image contrast evolved during the time-resolved measurements: either a bright (energy gain) or a dark (energy loss) contrast can be obtained depending on whether more or less SEs are detected with respect to a reference image (Figure 3). An interband carrier transition occurs when the pump pulse photoexcites the sample surface and promotes the valence-band electrons to the conduction band. As a result, in comparison to an unexcited sample, the conduction band electrons have a greater probability of emitting SEs above the vacuum level on being scattered by the primary photoelectron pulses. This leads to enhanced (SE energy gain) image contrast. In addition, the energy imparted to the specimen by the optical pulse may slightly increase the SE escape depth, thereby causing an overall bright contrast. On the other hand, dark contrast observed at positive delay times is an indicator for the suppression of the SE emission via scattering processes (SE energy loss). Such a scenario is possible if SEs in the conduction band suffer from energy loss while transiting to the sample surface during SE emission. In this case, scattering events with electron−hole pairs generated with the optical pulse remain a possibility for the energy loss. Since the effective scattering cross-section of SEs with electrons in the conduction band is significantly more than that with valence band electrons, it causes a drop in the SE signal (low contrast).8,34,44 3.2. Illustrative Examples. S-UEM has been employed to image the charge carrier dynamics and carrier diffusion on the surface of indium gallium nitride (InGaN) nanowires (NWs).41 InGaN NWs have been widely used as building blocks for several optoelectronic devices.53−57 However, the efficiency of such devices is limited by various energy loss mechanisms, the origins of which are still under debate. Time-resolved snapshots

Figure 4. SE snapshots of the InGaN NW array at different time delays. The dashed ellipses represent the laser footprint on the sample (∼40 μm). At very negative time, no significant change in contrast can be observed, which signifies that the system is fully recovered to the initial state after each pump−probe event. Inset of the top panel (left side) shows the SEM image of the NWs. The dynamics of the SE at the center and outside the laser footprint region have been provided in the inset. Adapted with permission from ref 41. Copyright 2016 WILEY-VCH Verlag GmbH & Co.

loss pathways, as the SE signals obtained in both the negative and positive time regimes remain dark. The diffusion of the charge carriers could also be visualized by the SE signal spreading beyond the laser-illuminated region. From the kinetics of the variation of the SE intensity, the plausible energy loss pathways for the SEs were identified. Temporal and spatial variations of SEs were also modeled numerically, providing an estimate of the carrier relaxation and diffusion process, in agreement with experimental results. Surface state related monomolecular Shockley−Read−Hall (SRH) recombination, rather than bimolecular electron−hole recombination or Auger processes, was identified as the leading mechanism of nonradiative energy loss in this case. To minimize the detrimental effect of such surface states, these NWs were passivated with octadecylthiol (ODT), and SE images were obtained to provide precise information on the effect of the passivation on the surface charge carrier dynamics of the NWs.42 The time-resolved SE images and kinetics of the SE intensity evolution clearly show that the carrier recombination is significantly reduced from 40% to 15% upon ODT treatment within the observed time window (Figure 5), providing direct evidence of the removal of surface states and hence nonradiative carrier recombination pathways in real space and time. This observation is directly correlated with the 6


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Figure 5. Time-resolved SE images of an InGaN NW array before and after surface passivation with ODT at the indicated time delays. Crosssectional SEM images of the NWs have been provided in the insets of the top panels. The extracted kinetics (fitted to exponentials) of the time-resolved SE intensities of the NWs before and after surface passivation have been provided in the middle inset. Adapted with permission from ref 8. Copyright 2016 American Chemical Society.

Figure 6. Time-resolved SE images from CIGSe and CIGSe-ZnS NC thin films at indicated time delays. The middle inset shows the dynamics (fitted to exponentials) of the evolution of the SE intensity with time at the center of the laser footprint region for the NCs before and after shelling with ZnS. Adapted with permission from ref 40. Copyright 2016 American Chemical Society.

enhanced performance of the light-emitting device based on such passivated InGaN/GaN quantum disks in NWs. Semiconductor nanocrystals, with their increased surface to volume ratio, also suffer from nonradiative energy loss via surface states, and different surface passivation techniques are used to minimize these undesirable deactivation channels for charge carriers. As S-UEM can provide precise knowledge about the surface charge carrier dynamics, which is a prerequisite to design efficient passivation strategies, it was employed to study the surface charge carrier dynamics of multinary copper indium gallium selenide (CIGSe) nanocrystals (NCs), which have many desirable attributes to be used as the active material in optoelectronic devices,58−62 but their efficiencies are limited by the multiple trap states present in the bandgap. It was observed that the recovery of the SE intensity, which signifies charge carrier combination, was almost complete for the CIGSe NCs within the experimental time frame, while it was only approximately 40% for the NCs shelled with the higher bandgap material ZnS (Figure 6), which clearly proves the efficient removal of the nonradiative surface states.40 This was also reflected in the improved device performance of a photodetector fabricated with CIGSe-ZnS NCs, with an almost three-fold increase in the photocurrent. Importantly, comparison of the charge carrier dynamics as obtained from transient ultrafast spectroscopic measurements revealed that the extent of the slowing down of the carrier dynamics after the ZnS shelling is more discernible in the SE kinetics obtained by 4D S-UEM because of the excellent surface selectivity afforded by the latter. S-UEM has also been used in determining the effect of surface morphology on the surface charge carrier dynamics.34 It is observed that a fast recovery of the SE signal is observed in the case of a CdSe powder film compared to the single crystal of the same (Figure 7). The reason behind this is that the

Figure 7. Time-resolved SE images of single crystal (left) and powder film (right) of CdSe at indicated time delays. The SEM images, which show distinguishable morphologies for both, are provided in the insets of the two topmost images. The dynamics of the temporal evolution of the SE intensity for both the samples have been provided in the middle inset. Reprinted with permission from ref 8. Copyright 2016 American Chemical Society.

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Figure 8. Top panels: SE difference images at indicated time delays for (A) n-type Si and (B) p-type Si. Bottom panel (C): SE difference images at indicated time delays for a Si p−n junction. Reprinted with permission from ref 45. Copyright 2015 American Association for the Advancement of Science.

surface defects in the powder film can serve as fast carrier quenching centers which reduce a significant number of excited carriers, resulting in bright contrast. This unambiguously proves that the surface morphology is a crucial component to determine the charge carrier dynamics on the surfaces of photoactive materials. In addition to its utility to study the charge carrier dynamics at the surface of nanocrystalline materials, S-UEM has also been employed to image the charge carrier generation, transport, and recombination at the silicon p−n junction, with a well-defined interface at the nanoscale.45 It was shown that the separation of carriers in the p−n junction is extended significantly beyond the depletion layer, in contrast to the range expected from the trusted drift-diffusion model, and the carrier density localizes across the junction over the time range of up to tens of nanoseconds, dictated by the laser fluence (Figure 8). These observations were attributed to the ballistic-type motion of the charge carriers, consistent with a model explaining the spatiotemporal density localization across the junction. It was also hinted that because the transient carrier density and electric field at the interfaces are governed by the optical excitation, it would pave the way toward the control of heterojunction properties on the nano-to-micrometer scale through the modulation of the applied optical excitation pulse characteristics. Apart from that, S-UEM in the environmental mode allowed the ultrafast imaging of solvation dynamics at “wet” material

surfaces beyond the diffraction limit of visible light.43 CdSe surfaces with atomically distinct surface structures and coated with polar and nonpolar molecules were studied as prototypes for mapping surface solvation in space and time (Figure 9). The recovery of transients suggested that although the early time dynamics are similar for different adsorbates, the long-term recovery components are strongly dependent on the solvent dipole moment. This was attributed to the scope of polar adsorbates acting as a “holding potential” for charge carriers, with the effect being more prominent for solvents of higher polarity.

4. UEM 4.1. Instrument and Concept. While the above discussion focused on scanning UEM, which is based on SEM technology, we now shift to transmission UEM, which is based on TEM instrumentation. While the techniques share several fundamental operating principles, the dynamics probed with each can be distinctly different though complementary in many ways. In this section, we briefly describe the general instrument configuration and operating concepts. Figure 10 contains an image of a typical UEM instrument and a schematic illustration of the integration of optical and TEM components. At the University of Minnesota, we employ a Yb:KGW diode-pumped solid-state fs laser (PHAROS, Light Conversion) with 1030 nm fundamental output coupled to a harmonics module (HIRO, Light Conversion) and integrated with an FEI Tecnai Femto 8


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Figure 10. Image and schematic illustration of a typical UEM instrument. (A) FEI Tecnai Femto in the UEM lab at the University of Minnesota. The instrument is a modified FEI Tecnai G2 20 200 kV thermionic TEM, with optical ports added to allow laser-light access to the electron gun and specimen regions. The location of the optical ports is highlighted by the colored arrows. (B) Simplified schematic of the integration between the laser system and the TEM. The basic components are indicated. Reprinted with permission from ref 7. Copyright 2015 American Chemical Society.

For nanosecond and slower experiments, the fs UV pulses are replaced with subnanosecond pulses (for example, WEDGE HF 1064-SB, Bright Solutions, quadrupled to 266 nm), while the pump can either also consist of subnanosecond pulses or remain unchanged as fs pulses, as is done at Minnesota. Universal use of fs pump pulses removes the need to switch laser lines when changing from fs to nanosecond experiments, and time delays can be generated electronically for nanosecond and slower experiments by triggering the subnanosecond laser with a transistor−transistor logic (TTL) signal from the fs laser.25,63 In this way, a time range spanning fs to milliseconds can be covered without any temporal gaps. The majority of UEM experiments can be categorized into two modes of operation: stroboscopic3,5,9,33,64 and singleshot.24,65−67 Generally, stroboscopic UEM is used for repeatable chemical or physical phenomena occurring on fs to millisecond time scales and is conducted with a train of photoelectron packets containing from one to ∼1000 electrons. Complementary to this, the single-shot mode is used for irreversible processes occurring on nanosecond and slower time scales and uses individual photoelectron packets containing >107 electrons. Here, we focus on the ultrafast stroboscopic approach. Discussions of the principles and applications of the single-shot mode can be found elsewhere.68,69 In addition, it is worth mentioning here that, for the purposes of this article, distinction is drawn between dedicated ultrafast electron diffraction instruments and those based on transmission and scanning electron microscopes, wherein multimodal approaches to comprehensive chemical and materials characterization can be conducted in real, reciprocal, and energy space.6,7,64,70,71 Indeed, the study of atomic-scale structural dynamics with ultrafast electron- and X-ray-based experiments has significantly increased over the past 20 years, and the interested reader is again referred to the many works in this area, a select few of which are cited here.72−86 In stroboscopic UEM, the pump− probe process is repeated with a chosen repetition rate (Hz to MHz, depending on the dynamics of interest),12,22 and images are formed by summing scattered electron intensity over many shots. This process is repeated for each time delay between the pump pulse and the probe packet, resulting in a series of images, diffraction patterns, or electronic spectra that capture the time-dependent behavior.

Figure 9. (a) Surface structures of CdSe (0001) and (101̅0) [Cd atom, yellow; Se atom, gray]. (b−d) Time-resolved SE images of CdSe (0001) (left) and CdSe (101̅0) (right) at different time delays in high vacuum condition (b); surfaces exposed to water vapor (c); and surfaces exposed to nonpolar ambient air (d). Bottom panel (e) shows the solvation dynamics of CdSe surfaces as a function of time in the presence of water vapor (left) and water and acetonitrile vapors (right). Adapted with permission from ref 43. Copyright 2016 WILEYVCH Verlag GmbH & Co.

UEM, generally operated at 200 kV (Figure 10A). The fundamental output of the laser is converted to 515 nm and is split into two beamlines. One beam is frequency doubled to 257.5 nm and directed to the gun region of the microscope, while the other is used to pump the specimen. The UV pulse is focused onto a room-temperature LaB6 cathode, which serves as the photoelectron source. The pump line is passed through a delay line and directed through an optical port on the microscope near the goniometer region where the specimen is held. The optical pump pulse (shown as red in Figure 10B) excites the specimen, and the photoelectron probe packet samples the response. It should be noted that the fundamental and harmonics can be used for specimen excitation, and implementation of optical parametric amplification further expands the pump wavelength parameter space. 9


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Figure 11. Phonon wavefront propagation in Ge and WSe2. (a,i) Bright-field UEM images of regions of interest from Ge and WSe2 specimens obtained at time delays of −50 and −5 ps, respectively. The colored lines correspond to areas from which the average image intensity was determined, the time-dependent response of which is plotted in (h) and (p). The phonon wavefronts propagated along a vector normal to the colored lines. The white rectangles outline the regions shown in the subsequent surface plots, and the colored arrows are to orient the reader. The scale bars represent 500 nm. (b−g) and (j−o) Time series of surface plots created from the white boxed region in (a) and (i). The select images were chosen to highlight the motion of a single phonon wavefront, with a pretime-zero plot shown for comparison. The white arrows are a guide to the eye for following the propagation of the phonon wavefront, depicted as a dark-red depression in the surface plot. The orange arrow indicates the orientation of the surface plots in relation to the bright-field images in (a) and (i). (h,p) Average image intensity determined from the colored lines in (a) and (i) plotted as a function of time. The data-plot colors match the colored bars from which they were generated. Reprinted with permission from ref 96 under the terms of the Creative Commons (CC-BY) license: https://creativecommons.org/licenses/by/3.0/.

thermal drift. To achieve the required resolution in space and time for each experiment, the photoelectron packet can be controlled through optimization of laser fluence, UEM gun region parameters, and in some cases synchronized radiofrequency (RF) cavities.90−92 4.2. Illustrative Examples. 4.2.1. Imaging. To highlight the versatility of UEM, we will discuss a few examples of the different modes of operation; namely, imaging, diffraction, and spectroscopy. Real-space UEM has a number of capabilities that make it a valuable tool for the study of structural dynamics.7,63 Because the diffraction phase problem is circumvented in imaging, one can identify and resolve spatial heterogeneities of the specimen and determine their localized effects on dynamics.

In order to reach fs time resolutions and angstrom-scale spatial resolutions with UEM, care must be taken to optimize the laser parameters for photoelectron generation. Space charge effects can contribute significantly to the spatiotemporal coherence of photoelectron packets.87−89 In essence, packets with a high density of electrons will broaden in space and time from Coulombic repulsion. Therefore, space-charge effects necessitate the use of photoelectron packets with a small number of electrons, but this in turn results in long acquisition times for generating images with sufficient signal-to-noise ratios. Thus, a balance must be struck between optimizing spatiotemporal resolutions while minimizing image collection times in order to mediate parasitic effects such as specimen 10


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Figure 12. Time-dependent structural response of Fe(pyrazine)Pt(CN)4 nanoparticles. (a) Bright-field image of two nanoparticles (labeled 1 and 2). The labels H and V correspond to the dimensions used for the time traces in (c) and (e). (b) Select difference images shown for three time delays and referenced to pretime-zero images. (c) Time-dependent real-space dimensional changes of particles 1 and 2 along dimensions H and V, as defined in (a). (d) Diffraction patterns and corresponding dark-field images of the particles. The diffracted beams used to generate the dark-field images are indicated by the dashed yellow circles in the diffraction patterns. Scale bars represent 2 nm−1. (e) Time-dependent radial peak positions of the dark-field Bragg spots in (d). Reprinted with permission from ref 94. Copyright 2013 Nature Publishing Group.

nanostructured materials.96,99,100 Indeed, phonon engineeringcontrolling coherent lattice vibrations by introducing nanostructured elementshas been the subject of considerable interest in recent years.101,102 Owing to the combined spatial and temporal resolutions, UEM enables direct probing of the ultrafast dynamics of particular phonon modes in the presence of nanoscale elements and confined within individual nanoparticles and few-particle clusters. Figure 11 displays two examples of UEM imaging of phonon generation and propagation in a thin Ge crystal and few-layer WSe2 flake. In this study, the photoinduced structural response was tracked by following dynamic contrast features as a function of time in the real-space image series. The resulting contrast variations were shown to have an oscillatory component that had phase velocities of 6.5 and 5.5 nm/ps and periods of 40 and 44 ps for Ge and WSe2, respectively. Further, the dynamics appeared to emerge from defects and interfaces, and the wavetrain traveled along a single in-plane direction. The phonon wavetrains consistently appear in the vicinity of defects and along existing contrast features, suggesting a strain nucleation mechanism resulting from photothermal elastic excitation. 4.2.2. Diffraction. Complementary to real-space imaging, ultrafast electron diffraction with UEM has been used to study a wide range of crystallographic and structural dynamics.18,19,22,23,25,63,94,103−106 As in dedicated ultrafast electron diffraction experiments, dynamics are studied by monitoring the time-dependent responses of coherently scattered beams, namely, intensity, magnitude of scattering vector, and range of scattering angles. As with conventional TEM, both paralleland convergent-beam modes can be used, and both elastic and

As a result, materials properties, such as thickness, morphology, and composition, can be correlated to specific regions of the real-space image.93 In addition, the ability to vary magnification in order to achieve fields of view of square micrometers to square nanometers allows one to study the local nucleation of energy at discrete structural features and to monitor the evolution in space and time.94,95 For crystalline specimens, sensitivity of diffraction contrast to reciprocal-lattice motion (relative to a fixed Ewald sphere) in UEM imaging has been exploited for isolation and quantification of photoinduced dynamics.19,22,96,97 As with conventional TEM, contrast can be enhanced through the use of an objective aperture to exclude electrons that scatter to relatively high angles (i.e., relative to the direct beam). In UEM, the optical excitation pulse causes an ultrafast electronic response and, subsequently, induces structural dynamics, which can be coherent. This response modulates scattering of the probe electron packets passing through the specimen, producing time-dependent contrast variations. By collecting the main beam and excluding diffracted beams, such bright-field UEM images reflect collective diffraction-contrast dynamics over an entire field of view. Complementary to this, dark-field UEM experiments can be performed by selecting a specific diffracted beam to form the images. This approach can be used to resolve dynamics within specific crystal grains or individual particles among an ensemble of randomly oriented regions.94,95,98 As an example, recent UEM work at Minnesota has focused on investigating the generation and propagation of acoustic phonons in metallic and semiconductor nanoscale and 11


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Figure 13. Visible and near-IR PINEM experiments from a Cu grid bar. (a) Bright-field image using an unfocused electron beam showing the interface between the vacuum (light region) and the Cu grid bar (dark region). The scale bar represents 500 nm, and the electron-beam diameter is 1.7 μm. (b) Bright-field image using a focused electron beam within the same field of view as (a). Here, the electron-beam diameter is 33 nm and is positioned at various points perpendicular to the grid bar−vacuum interface. The dashed circle in (a) and (b) represents the size of the unfocused beam. (c−f) Electron energy spectra in the low-loss region as a function of electron-beam size, excitation wavelength, and laser fluence. All spectra were collected at maximum temporal overlap. Reprinted with permission from ref 125.

particle-by-particle basis. This effect was corroborated by tracking the time-dependent diffraction peak positions, which were assigned to each respective nanoparticle through darkfield imaging. As can be seen in Figure 12, the expansion of particle 1 was anisotropic, which is consistent with a transition between the low-spin and high-spin states.107 In contrast, particle 2 underwent a contraction, and this observation, combined with the diffraction-peak shape, suggests rotational disorder in the a−b crystal planes. The contraction was explained by noting that the ligands of the nanoparticle undergo an irreversible chemical transformation upon laser excitation, and the resulting compound has a negative thermal expansion coefficient.28,110 The authors noted that about one in ten particles were shown to exhibit the negative expansion dynamics. 4.2.3. Spectroscopy. Inelastic scattering in TEM is useful for probing plasmon resonances and core−shell states and for spatially mapping chemical composition.111−113 With UEM, such processes are extended to fs time scales, thus enabling the dynamics of such processes to be resolved in energy space and time.13−15 Unique to UEM, as compared to conventional TEM, is the capability to precisely spatiotemporally overlap fs photon pulses and photoelectron packets. From this, it was found that the 200 keV free electrons could absorb and emit integer multiples of photon energy (e.g., 2.4 eV) owing to momentum conservation in the near field of the specimen. This crosscorrelation process can be detected and temporally resolved with an electron-energy spectrometer.114 Further, it was demonstrated with energy-filtered imaging that the spatial location in which absorption (emission) occurs could be visualized. This approach, called photon-induced near-field electron microscopy (PINEM), has resulted in numerous studies into a wide range of electron−plasmon interaction processes.26,27,29,115−123 For example, Ropers and co-workers were able to use the PINEM effect to study highly coherent Rabi oscillations of free electrons in the vicinity of a laserexcited metallic tip.38 Similarly, Carbone and co-workers have

inelastic scattering from single and polycrystalline specimens can be followed with fs resolution.63,103−105 In this way, one can access specific information on the ultrafast response of bond lengths, crystal symmetry, and structural phase to coherent optical excitation. Accordingly, real- and reciprocal-space ultrafast structural dynamics can be combined and correlated without the need for multiple instruments. Such flexibility provides a practically useful degree of experimental control for determining atomic to mesoscale specimen responses. As with conventional TEM and electron diffraction in general, interplanar spacings on the order of 1 Å can be resolved with UEM, and sensitivities to structural changes below 0.1 Å have been demonstrated.25,103,105 Further, identification and isolation of nanoscale specimen particles of interest, and the capability to probe coherently-excited nanoscale volumes within these particles, is an especially powerful feature of UEM. As an example here, Figure 12 summarizes UEM studies of photoinduced phase transitions in a single Fe(pyrazine)Pt(CN)4 nanoparticle.94 This compound exhibits the single-spin crossover phenomenon, a prototypical example of a cooperative phase transition.107−109 Such systems have been explored for applications in molecular switching, data storage, and optical displays. Initiation of the single spincrossover mechanism can be accomplished through a wide variety of processes, including changing temperature or pressure of the material or with optical excitation. The phase transition involves a change of the electronic structure from a low-spin ground state to a high-spin excited state, accompanied by a change in physical properties (e.g., magnetic or structural). The highlighted study used a combination of real- and reciprocal-space UEM to investigate the response of individual Fe(pyrazine)Pt(CN)4 nanoparticles to optical excitation.94 A series of images were collected on the nanosecond time scale, and difference images were generated at various time delays. In addition, time-resolved single-particle diffraction patterns were collected. From the real-space analysis, it was observed that both structural expansion and contraction occurred on a 12


Review

ACS Applied Materials & Interfaces Author Contributions

used PINEM to visualize the resonant spatial localization of surface plasmon polaritons and plasmon effects at metal− dielectric interfaces.116,124 A recent demonstration of UEM electron−photon coupling with visible and near-IR wavelengths is highlighted in Figure 13.125 In this study, the authors used a focused electron beam to probe the spatially varying energy-space response of the PINEM effect. An entrance aperture was used to isolate a diffracted beam in order to obtain a low-loss spectrum from a discrete region of the specimen. This resulted in the collection of electrons that had interacted with the optical near fields and gained (lost) photon quanta of energy as they were diffracted from the specimen. Figure 13 depicts the difference between unfocused and focused electron beams in relation to a Cu grid from which the PINEM signal was measured. A comparison of energy spectra generated with optical excitation at 515 and 1038 nm and using electron probe sizes of 130 and 33 nm is shown. Noteworthy observations include an increase in sensitivity relative to previous PINEM experiments using visible light. For example, comparable PINEM amplitudes were observed when the fluence of the 515 nm pulse was four times the fluence of the 1038 nm pulse. By reducing the probe size to 33 nm, the authors observed a similar increased sensitivity consistent with the weak interaction limit from PINEM theory.120,126,127 In addition, by scanning the nanometer-sized probe, the authors were able to measure the local evanescent field and determine the electric-field profile. It was found that the smaller probe size also increased the energy resolution of the technique to 0.63 eV, thus enabling analysis of the IR PINEM gain and loss peaks.

§

A. A and J. K. E, contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the King Abdullah University of Science and Technology. Funding for work conducted at the University of Minnesota was provided by the Arnold and Mabel Beckman Foundation in the form of a Beckman Young Investigator Award.

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(1) Zewail, A. H.; Thomas, J. M. 4D Electron Microscopy: Imaging in Space and Time; Imperial College Press: London, 2010. (2) Zewail, A. H. 4D Visualization of Matter; Imperial College Press: London, 2014. (3) Lobastov, V. A.; Srinivasan, R.; Zewail, A. H. Four-Dimensional Ultrafast Electron Microscopy. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 7069−7073. (4) Zewail, A. H. 4D Ultrafast Electron Diffraction, Crystallography, and Microscopy. Annu. Rev. Phys. Chem. 2006, 57, 65−103. (5) Zewail, A. H. Four-Dimensional Electron Microscopy. Science 2010, 328, 187−193. (6) Flannigan, D. J.; Zewail, A. H. 4D Electron Microscopy: Principles and Applications. Acc. Chem. Res. 2012, 45, 1828−1839. (7) Plemmons, D. A.; Suri, P. K.; Flannigan, D. J. Probing Structural and Electronic Dynamics with Ultrafast Electron Microscopy. Chem. Mater. 2015, 27, 3178−3192. (8) Sun, J.; Adhikari, A.; Shaheen, B. S.; Yang, H.; Mohammed, O. F. Mapping Carrier Dynamics on Material Surfaces in Space and Time using Scanning Ultrafast Electron Microscopy. J. Phys. Chem. Lett. 2016, 7, 985−994. (9) Park, H. S.; Baskin, J. S.; Kwon, O.-H.; Zewail, A. H. AtomicScale Imaging in Real and Energy Space Developed in Ultrafast Electron Microscopy. Nano Lett. 2007, 7, 2545−2551. (10) Yang, D.-S.; Mohammed, O. F.; Zewail, A. H. Scanning Ultrafast Electron Microscopy. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 14993− 14998. (11) Grinolds, M. S.; Lobastov, V. A.; Weissenrieder, J.; Zewail, A. H. Four-Dimensional Ultrafast Electron Microscopy of Phase Transitions. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 18427−18431. (12) Flannigan, D. J.; Samartzis, P. C.; Yurtsever, A.; Zewail, A. H. Nanomechanical Motions of Cantilevers: Direct Imaging in Real Space and Time with 4D Electron Microscopy. Nano Lett. 2009, 9, 875−881. (13) Carbone, F.; Kwon, O.-H.; Zewail, A. H. Dynamics of Chemical Bonding Mapped by Energy-Resolved 4D Electron Microscopy. Science 2009, 325, 181−184. (14) Carbone, F.; Barwick, B.; Kwon, O. H.; Park, H. S.; Baskin, J. S.; Zewail, A. H. EELS Femtosecond Resolved in 4D Ultrafast Electron Microscopy. Chem. Phys. Lett. 2009, 468, 107−111. (15) van der Veen, R. M.; Penfold, T. J.; Zewail, A. H. Ultrafast CoreLoss Spectroscopy in Four-Dimensional Electron Microscopy. Struct. Dyn. 2015, 2, 024302. (16) Kwon, O.-H.; Zewail, A. H. 4D Electron Tomography. Science 2010, 328, 1668−1673. (17) Kwon, O.-H.; Park, H. S.; Baskin, J. S.; Zewail, A. H. Nonchaotic Nonlinear Motion Visualized in Complex Nanostructures by Stereographic 4D Electron Microscopy. Nano Lett. 2010, 10, 3190−3198. (18) Barwick, B.; Park, H. S.; Kwon, O.-H.; Baskin, J. S.; Zewail, A. H. 4D Imaging of Transient Structures and Morphologies in Ultrafast Electron Microscopy. Science 2008, 322, 1227−1231.

5. CONCLUSION Since its initial development roughly a decade ago, 4D-UEM has emerged as an exciting analytical tool offering opportunities for scientists and engineers to investigate materials dynamics at unprecedentedly high combined spatial and temporal resolutions. These improved capabilities not only augment the existing corpus of knowledge regarding fundamental aspects of materials science and chemistry but also hold enormous scope in terms of potential applications. The insights obtained from SUEM measurements, for instance, with regards to surface morphology and defects on surfaces of photoactive semiconductor materials will aid in the design of more powerful and efficient optoelectronic devices through optimal control of trap states. Likewise, the demonstrated capabilities of UEM wherein complementary information regarding the crystallographic and morphological dynamics of materials can be obtainedhave expanded the temporal experimental parameter space of conventional TEM. Accordingly, it is the hope that such advances will prove useful for both practical applications but, more interestingly, for testing widely accepted, though (potentially) weakly supported, theories and well-known but otherwise poorly understood materials phenomena.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: fl[email protected]. *E-mail: [email protected]. ORCID

Omar F. Mohammed: 0000-0001-8500-1130 13


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