Probing Structural and Electronic Dynamics with Ultrafast Electron

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Probing Structural and Electronic Dynamics with Ultrafast Electron Microscopy† Dayne A. Plemmons,‡ Pranav K. Suri,‡ and David J. Flannigan* Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States ABSTRACT: In this Perspective, we provide an overview of the field of ultrafast electron microscopy (UEM). We begin by briefly discussing the emergence of methods for probing ultrafast structural dynamics and the information that can be obtained. Distinctions are drawn between the two main types of probes for femtosecond (fs) dynamicsfast electrons and X-ray photonsand emphasis is placed on how the nature of charged particles is exploited in ultrafast electron-based experiments. Following this, we describe the versatility enabled by the ease with which electron trajectories and velocities can be manipulated with transmission electron microscopy (TEM) hardware configurations, and we emphasize how this is translated to the ability to measure scattering intensities in real, reciprocal, and energy space from presurveyed and selected nanoscale volumes. Owing to decades of ongoing research and development into TEM instrumentation combined with advances in specimen holder technology, comprehensive experiments can be conducted on a wide range of materials in various phases via in situ methods. Next, we describe the basic operating concepts of UEM, and we emphasize that its development has led to extension of several of the formidable capabilities of TEM into the fs domain, thus increasing the accessible temporal parameter space by several orders of magnitude. We then divide UEM studies into those conducted in real (imaging), reciprocal (diffraction), and energy (spectroscopy) space. We begin each of these sections by providing a brief description of the basic operating principles and the types of information that can be gathered followed by descriptions of how these approaches are applied in UEM, the type of specimen parameter space that can be probed, and an example of the types of dynamics that can be resolved. We conclude with an Outlook section, wherein we share our perspective on some future directions of the field pertaining to continued instrument development and application of the technique to solving seemingly intractable materials problems in addition to discovery-based research. Our goal with this Perspective is to bring the capabilities of UEM to the attention of materials scientists, chemists, physicists, and engineers in hopes that new avenues of research emerge and to make clear the large parameter space that is opened by extending TEM, and the ability to readily manipulate electron trajectories and energies, into the ultrafast domain.

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INTRODUCTION It has been roughly 100 years since the field of X-ray crystallography emerged via the ideas put forth by von Laue and the Braggs and diffraction from CuSO4, NaCl, diamond, and Cu was observed.1−5 In this relatively short period of time, the complexity of structures that can be determined with atomic-scale precision has dramatically increased from simple binary salts and elemental solids to macromolecular assemblies containing large numbers of proteins and spanning several nanometers.6−10 Similarly, soon following de Broglie’s hypothesis formulated in 1924 that matter exhibits wave-like behavior, it was observed that interference patterns are generated by directing a beam of electrons onto a single crystal of nickel.11 Unlike X-ray photons, however, electrons have charge, and thus their trajectories and velocities can be readily manipulated with electric and magnetic fields, and they are scattered by both the electron clouds and nuclei of atoms. These factors, coupled

with the inverse relationship of momentum and wavelength, form the foundation of electron microscopy. The dual wave-particle behavior of electrons together with their charge, when generated and manipulated in a modern transmission electron microscope (TEM), enables comprehensive atomic-scale characterization of the structure, chemical composition, and electronic properties of materials.12,13 Manipulation of the imaging system (i.e., the postspecimen electron optics) allows for rapid switching between real and momentum (reciprocal) space,14 and incorporation of spectrometers enables determination of composition, oxidation state, bond order, density of states, and plasmonic properties.15,16 Further, the large scattering cross section of fast electrons relative to X-ray photons produces significant signal (e.g., X-ray emission, elastically and inelastically scattered Received: February 3, 2015 Revised: April 14, 2015

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This Perspective is part of the Up-and-Coming series. © XXXX American Chemical Society

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DOI: 10.1021/acs.chemmater.5b00433 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

the frequency. Generally, this explains why micrometer-sized objects have mechanical resonances on the order of 109 Hz (MHz), while molecular vibrations and phonons in crystals occur in the tens of THz (∼1013 Hz) range. Indeed, fundamental phenomena such as electron−phonon coupling, phonon propagation and scattering, bond rotation, first-order phase transitions, plasmon dynamics, and spin/charge/orbital ordering and meltingthe macroscopic manifestations of which originate at the atomic leveloccur on time scales well below 1 nanosecond (ns). In order to elucidate the atomic positions of transient structures displaying dynamics occurring faster than the millisecond range of current digital detectors, approaches based on concepts of stroboscopic, time-resolved pump−probe experiments employing fast electrons and X-ray photons have been developed.41−71 This now rapidly growing and evolving area shares many similarities with the field of ultrafast optical spectroscopy, which enables access to the transient electronic, vibrational, and rotational dynamics of molecules in the gaseous and condensed phases.72−74 These ultrafast spectroscopic experiments generally entail coherent optical excitation (pumping) of a specimen followed by measuring (probing) the resulting transient states. The moment at which the specimen is probed after pumping is controlled via variation in the relative path lengths such that probing can occur within as little as a few femtoseconds (fs) after the pump. Further, the pump and probe pulses are typically of fs duration, and thus dynamics that are slow relative to this time scale can be resolved. In this Up-and-Coming series Perspective, we will discuss the emergence and advancement of ultrafast electron microscopy (UEM) as a means to directly probe the transient structures and electronic dynamics of materials at the micrometer, nanometer, and atomic scales with up to fs temporal resolution.75−89 We begin by introducing the basic concepts of UEM, briefly describing the fundamental basis of two distinct operating modes(i) fs stroboscopic and (ii) ns single-shot. We will then divide the three main regimes accessible with TEMsreal (imaging), reciprocal (diffraction), and energy (spectroscopy) spaceand discuss the extension of UEM approaches and studies to ultrafast phenomena that can be categorized into these areas. We will emphasize the breadth of capabilities of modern TEMs enabled by decades of instrument engineering and development, and we will illustratevia descriptions of several different experimentshow, by extending these into the fs domain, new details about materials dynamics can be discovered. At present, fs studies have yet to be extended to all TEM techniques (e.g., imaging of atomic columns), and it is not our intention to give the impression that such extensions would be feasible. Rather, our goal is to illustrate the progress and promise of current and future work already taking shape through a few select examples and to emphasize that the ability to perform comprehensive materials characterization on selected specimen regions prior to UEM studieswithin a single instrumentis an appealing feature of the technology. Finally, we will close with our outlook for the field and describe some possibilities for future research directions related to fundamental advances in experimentation and probing of ultrafast structural and electronic phenomena.

primary electrons, secondary electron emission, etc.) in a short amount of time and from nanoscale volumes of material.17,18 Specific regions of interest can be initially surveyed and selected for study, and the amount of material over which signal is accumulated can be carefully controlled. Strong excitation of the objective lens and manipulation of the illumination system (i.e., the prespecimen electron optics) enables formation of nanometer-sized probes in most standard TEMs, while instruments equipped with aberration correctors can reach below 1 Å.19−23 In addition, ancillary equipment such as in situ holders as well as specimen-based methods can be used to observe atomic-scale processes occurring under conditions that are otherwise vacuum incompatible.24−26 Such conditions include controlling temperatures and pressures,27−30 immersion in liquid phases,31−34 and electrical biasing and mechanical deformation.35−37 Static structures determined via crystallography and highresolution TEM mainly represent equilibrium ground states. In order to unambiguously uncover function, however, direct determination of motions and rearrangements of atoms and molecules during dynamic processes is critical. This is one of the main driving forces behind the development of in situ methods, and advances in digital-detector technology have pushed TEM detector-dependent temporal resolutions into the millisecond range. Current state-of-the-art technologies and approaches combine in situ studies with the high spatial resolutions achievable with modern TEMs and the millisecond temporal resolutions and increased sensitivities of directelectron detectors (Figure 1).33,38−40

Figure 1. In situ time-resolved facet-dependent growth of a Pt nanoparticle. (A) Select frames from a high-resolution TEM movie showing cluster attachment at the (100) facet (blue circle in center frame; t = 0.325 s) and subsequent growth (blue arrows in the frame to the right; t = 0.5625 s). (B) Select frames showing attachment of Pt atoms on the flat (100) facet. Dashed blue boxes highlight the corresponding magnified (100) facet shown below each frame. Red dots highlight newly attached Pt atoms (t = 0.7 and 6.3 s). Reprinted with permission from ref 33. Copyright 2014 AAAS.

While millisecond temporal resolutions are suitable for studying a host of atomic-scale phenomena, many more require much faster capabilities in order to be directly probed. This need can be readily appreciated by observing scaling laws. For example, the frequency of motion of a simple harmonic oscillator increases with increasing spring constant (i.e., bond strength or stiffness of the material) and, importantly, decreasing mass. Further, the velocity of motion scales with

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ULTRAFAST ELECTRON MICROSCOPY To facilitate understanding of the UEM approach, we provide a brief description of the hardware followed by a discussion of the B


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Figure 2. (A) Ultrafast electron microscope in our lab at the University of Minnesota. The UEM is an FEI Tecnai Femto and is based on a Tecnai G2 20 200 kV thermionic TEM modified such that optical access to both the electron gun and specimen regions is possible. The optical periscopes are highlighted by the colored arrows indicating the points of entry of the laser pulses. (B) Simplified schematic of the UEM experimental layout. Several critical pieces of equipment and points of interest are labeled. Panel (B) is adapted with permission from ref 95. Copyright 2014 Elsevier.

through the photoelectric effecthas been pursued and refined over several decades.76,89,104−108 The accessible time scales of each mode are dictated mainly by the number of photoelectrons in each probe packet, which again can be precisely controlled with laser fluence. Because electrons are charged, for fs durations in stroboscopic UEM mode, the number per packet must be limited in order to preserve spatial and temporal coherency. This could be overcome if modifications involving packet compression schemes, such as synchronized RF cavities, or high-brightness photocathode RF guns were incorporated into the TEM column.102,103,109−121 Nevertheless, because ∼108 electrons impinging on the specimen and ultimately reaching the detector are required to form an image having a sufficient signal-to-noise ratio, signal is accumulated over many laser cycles at a single fixed time delay. A balance must then be struck between several UEM experimental parameters, including laser pulse frequency, specimen reversibility, signal acquisition time, thermal drift, etc. The single-shot mode is named as such because a single ns laser pulse, the photons of which are spread in time by several orders of magnitude relative to fs durations, can be used to generate a single electron packet populated with >109 electrons and having a duration of 0), where the incoming electron wavevectors are now nonparallel, can be used to probe even smaller distances with greater sensitivity due to higherorder coherent scattering (i.e., larger diffraction vectors).21 This approach is called convergent-beam electron diffraction (CBED). In TEMs, one can quickly vary the convergence

to view areas ranging from square micrometers to square nanometers provides a means to move beyond ensemble averaging to potentially study the effects of individual structural features on fast and ultrafast dynamic processes.123,124 Darkfield imaging allows one to select diffracted beams to form images such that individual crystal grains can be studied.125 Energy filtering can be used to select electrons having a specific and narrow range of energies to form images and to improve image quality, especially from relatively thick specimens within which plural scattering is likely. Additionally, scanning TEM (STEM), wherein a small electron probe is formed and quickly scanned over the specimen, can be used to study atomic-scale regions via image formation from electrons scattered at relatively high angles.17,18 Several UEM imaging studies have been reported on a variety of materials, both with the stroboscopic and the single-shot modes, and these studies illustrate the unique information that can be obtained.101,126−142 While individual TEM images are 2D projections, 3D realspace structural maps can be obtained via a method called electron tomography.143 The technique is especially useful for complex, nonperiodic structures, including cellular ultrastructure and chemically inhomogeneous polycrystalline alloys. Generally, 3D images are generated by obtaining a series of 2D projections at different specimen tilt angles or by varying the wavevector of the incident beam and using algorithms to reconstruct the volume information on the object from the different views. Electron tomography was first extended to UEM via demonstration on a 55 nm diameter coiled multiwalled carbon nanotube (MWCNT; Figure 3A).144 In UEM imaging experiments where 2D projections are obtained, the specimen is optically pumped and subsequently probed at a fixed tilt angle for all time delays (Figure 3B);127 the angle formed between the incoming photoelectron wavevector and specimen plane is held fixed for the entirety of the scan, though transient morphological changes induced during laser excitation can cause slight variations in this angle, as observed via ultrafast modulation of diffraction contrast.96,126,129 In the electron tomography study, a series of UEM experiments were conducted at different specimen tilt angles, and the corresponding images obtained at specific time delays were subsequently combined in order to reconstruct the 3D structure. By combining the images into a temporal sequence, the photoinduced 3D dynamics of the object were revealed. In addition, analysis of the motion along different viewing directions uncovered resonant oscillations with frequencies in the tens of MHz range (i.e., periodicities < 100 ns). With UEM, visualizing 2D projection dynamics can also reveal resonant oscillatory behaviors and effects of photoinduced charge transfer on single crystals and nanoscale features.123,124,127,145 Figure 3B illustrates this for a single crystal of copper 7,7,8,8-tetracyanoquinodimethane [Cu(TCNQ)] grown on a thin silicon nitride membrane by reacting a film of copper with a solution of TCNQ in deaerated acetonitrile.127,145 The synthetic approach involved depositing a 10 nm thick copper film via thermal evaporation directly onto a plasma-cleaned silicon nitride membrane followed by reacting with a droplet of TCNQ solution and quenching after approximately 1 min. This approach produces the kinetic product, which has a rod-like crystal habit with dimensions on the order of 100 nm to a few micrometers in width and several micrometers in length, often with a significant amount of disorder in the molecular arrangement, as observed with both E


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Figure 4. Ultrafast structural dynamics of elemental materials in UEM diffraction studies. (A) Parallel-beam electron diffraction (PBED) of randomly oriented MWCNTs (average diameter = 12 nm) (left panel). The right panel shows the radial average (red) with fit (blue), diffuse background correction (dashed), and indexed reflections. Reflections corresponding to radial and axial real-space directions for a single nanotube are labeled. (B) UEM PBED patterns acquired before (−60 ps, upper half) and after (+60 ps, lower half) optical excitation (left panel). A difference diffractogram of the two is displayed in the right panel showing a contraction of the (002) Debye−Scherrer ring indicating a ps radial expansion of the MWCNTs in real space. Select reflections are labeled. Adapted with permission from ref 150. Copyright 2012 American Chemical Society. (C) UEM convergentbeam electron diffraction (CBED) of single-crystal silicon. Second-order Laue zone (SOLZ) rings and Kikuchi bands can be seen, especially in the reference frame in the upper left-hand panel (the white circle marks the position of the direct beam). UEM CBED patterns obtained at t = +5.2 ps and +38.2 ps after optical excitation show an intensity drop in the SOLZ ring relative to the reference frame acquired −14.8 ps before excitation. Adapted with permission from ref 147. Copyright 2009 AAAS.

one acquired 60 ps after (+60 ps). Inspection of the rings after determining the center of each pattern with subpixel precision, radial averaging, background subtraction, and peak fitting showed the MWCNTs expand radially on time scales commensurate with the instrument response for the configuration used, while no statistically significant response was observed in the axial direction. Note that the observed ps contraction of the (002) ring in reciprocal space corresponds to a real-space change of 0.05 Å. The second example is a CBED study conducted on a wedge-polished silicon single crystal (Figure 4C).147 In CBED experiments, disks rather than Bragg spots are observed across the zero-order Laue zone of the Ewald sphere.12,13,21 These disks contain contrast patterns (provided plural scattering occurs) that depend upon the crystal structure, specimen thickness, and particular zone axis along which the experiment is being conducted. Plural-scattering effects also generate features in the CBED intensity distribution due to incoherent scattering, including Kikuchi patterns (which can also appear for divergent beams) and rings corresponding to intersection of the Ewald sphere at relatively high angles (e.g., second-order Laue zones, SOLZ). Such scattering intensity distributions are useful in static and dynamic structural studies because they are sensitive to minor perturbations of the lattice due to low-energy excitations (e.g., phonons) and contain 3D crystallographic information. With UEM, analyzing the response of the intensities and positions of these features following coherent optical excitation of the silicon wedge revealed the ultrafast thermal response of a nanoscale volume and ps shear-wave motion of the lattice. It was found that, for both processes, the

semiangle of the incoming beam by changing the degree of excitation of the objective lens, thus continuously moving from uniform to nonuniform illumination. Further, probes on the order of a few nanometers can be formed on the specimen in most modern TEMs, with sizes reaching well below 1 Å in probe-corrected instruments.19,20,22 Both PBED and CBED have been extended into the ultrafast temporal domain with UEM, and Figure 4 illustrates two examples. The first, summarized in Figure 4A,B, is a UEM PBED study of a freestanding mat of randomly oriented multiwalled carbon nanotubes (MWCNTs) with average diameters of 12 nm.150 Specimens were prepared by drop casting a dilute solution of MWCNTs dispersed in ethanol via sonication and isolated via centrifugation onto copper mesh TEM grids and then dried in air. Imaging was used prior to UEM diffraction studies in order to identify a suitable electrontransparent region. The diffraction pattern from such a region is shown in Figure 4A; the random orientation of the MWCNTs combined with the relatively large field of view produced continuous Debye−Scherrer rings. A radial average of such a pattern produces discernible peaks that can be indexed to dspacings corresponding to both radial (along which van der Waals bonding occurs) and axial (along which covalent bonding occurs) directions within the MWCNTs. With UEM, the properties of each of the observed rings (e.g., intensity, position, and width) were followed over time scales ranging from picoseconds (ps) to milliseconds in order to study both the initial ultrafast and subsequent slower relaxation dynamics. Figure 4B shows a comparison of two UEM PBED patterns, one acquired 60 ps prior to excitation (−60 ps) and F


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Figure 5. Ultrafast electronic dynamics of single-crystal PrSr0.2Ca1.8Mn2O7 in UEM energy space. (A) Static electron-energy loss spectrum (experiment = blue, DFT calculation = red). The x-axis corresponds to energy lost due to inelastic scattering of the incoming 200 keV electrons via excitation of the various processes and transitions labeled in the spectrum (e.g., plasmons, Pr O2,3, Sr N2,3, etc.). (B) Transient energy spectra in the low-loss region. The time−energy−intensity plot was generated by taking the difference of a reference spectrum obtained prior to optical excitation and all subsequent spectra. Vertical lines extending from the static spectrum mark specific features of interest in the 3D plot. (C) Intensity as a function of time of specific features of interest extracted from the 3D plot in (B). The vertical line cutting through each profile marks time zero (i.e., precise overlap of the electron packet and photon pulse at the specimen).87,95 Reprinted from ref 155 under the terms of the Creative Commons (CC-BY) license: https://creativecommons.org/licenses/by/3.0/.

initial response occurred in less than 10 ps, and the subsequent structural dynamics could be followed with better than 0.1 Å precision. Indeed, the UEM intensity distributions obtained via CBED can be used to elucidate weaker excitations (and thus smaller atomic displacements) versus PBED due to increased sensitivity of the Debye−Waller effect at large diffraction vectors and motion induced in the Kossel cones (and thus Kikuchi lines) during shear wave motion of the lattice. Energy−Space UEM. The large scattering cross sections of fast electrons (relative to X-ray photons) enables spectroscopic measurements to be conducted on small volumes of material over acquisition times that are experimentally practical.12,16 Electron energy-loss spectrometry (EELS) is one such measurement technique wherein inelastically scattered electrons are passed through a magnetic field that behaves as a prism before being dispersed onto a detector.15 The information displayed shows the intensity distribution of the detected electrons as a function of energy lost to the specimen. Particular energy ranges of interest can be chosen by using slits and varying the strength of the magnetic field in the spectrometer. This technique allows one to determine the elements present in a material, the bonding type, plasmon energies, and oxidation states, in addition to other practical information such as specimen thickness. In addition, fine structure in the ionization edges [energy-loss near-edge structure (ELNES) and extended energy-loss fine structure

(EXELFS)] arising from bond type, coordination number, and density of states for the element of interest can be resolved. Typical energy resolutions are on the order of 1 eV for TEMs with thermionic emission sources or a few hundred meV for cold field-emission guns. Recent advances in this area include incorporation of monochromators into aberration-corrected instruments such that energy resolutions on the order of 100 meV can be reached, with the state-of-the-art now extending below 10 meV, comparable to phonon energies and other lowenergy excitations.152 As with real and reciprocal space, several UEM EELS studies have been conducted.153−155 One recent study, which also included photoinduced structural dynamics experiments in reciprocal space, was performed on the layered manganite PrSr0.2Ca1.8Mn2O7 (Figure 5).155 The specimen consisted of micrometer-sized single crystals dispersed onto a lacey carbon TEM grid. An initial real-space imaging and spectroscopic survey of the specimen was conducted in order to identify suitable crystallites for the UEM EELS studies and generate ground-state spectra for comparison to fs measurements (Figure 5A). Several peaks corresponding to shallow core-loss excitations of the constituent elements were observed in the initial survey beginning at 20 eV (Pr O2,3) and extending beyond 600 eV (Mn L2,3). In addition, plasmon excitations at approximately 3 and 13 eV were observed in the low-loss region. For the ultrafast EELS studies, the specimen was G


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Figure 6. Photon-induced near-field electron microscopy (PINEM). (A) Electron-energy spectra in the low-loss/gain region. Shown are spectra obtained with the electron packet and photon pulse offset in time by 2 ps (black, t = −2 ps; i.e., the photon pulse arrives at the specimen 2 ps before the electron packet) and precisely at maximum temporal overlap (red, t = 0 fs). The inset shows a 5× magnified view of the t = 0 fs spectrum to the gain side of the zero-loss peak (ZLP) illustrating the observation of discrete bands occurring at energies of up to eight photon quanta. (B) Magnified view of the t = 0 fs electron-energy spectrum. The peak labels correspond to integer multiples of photon energy (2.4 eV) gained or lost (e.g., +3ℏω = energy gain equivalent to 3 photons). (C) False-color PINEM images of a freestanding MWCNT. Electrons with energy greater than 200 keV [i.e., to the gain side of the ZLP in (B)] were used to form the images. The time stamps in the upper left-hand corner of each frame correspond to the delay of the photon pulse relative to the photoelectron packet at the specimen. Adapted with permission from ref 157. Copyright 2009 Macmillan Publishers Ltd.

pumped with 400 nm laser light with a full-width at halfmaximum pulse duration of 80 fs and a fluence of tens to hundreds of μJ/cm2 and subsequently probed with low-density photoelectron packets containing one electron on average. A repetition rate of 1 MHz was used for the experiments, with calculations indicating the specimen temperature initially increased before leveling off at a new, elevated equilibrium value. Before describing the results summarized in Figure 5, we mention an additional factor dictating the selected experimental parameters. In UEM, beam currents can be low relative to standard thermionic emission depending upon the parameter space being explored. In such instances, operation at laser repetition rates that are hundreds of kHz to a few MHz are required in order to keep acquisition times to a reasonable duration. In UEM EELS, such experimental challenges can be compounded by the typical exponential drop in signal intensity with increasing energy loss, thus rendering studies of the ultrafast response of core−shell ionization edges to valenceelectron excitation difficult. The fs studies summarized in Figure 5 were conducted on several shallow core levels below 60 eV (e.g., Pr O2,3, Sr N2,3, Ca M2,3, and Mn M2,3). Thus, the ultrafast response of these spectral features during photoinduced structural dynamics could be studied, as sufficient signal was generated over this low-loss region for 10 min acquisition times. Figure 5B shows a series of spectra ranging from zero to 70 eV, and Figure 5C displays the transient response of the intensities arising from specific excitations [highlighted by the black vertical lines cutting across panels (A)

and (B)]. Ultrafast modulation of these peak intensities is attributed to effects induced by coherent photoexcited lattice oscillations on the orbitals participating in the observed transitions. Thus, this study ties together UEM structural and electronic dynamics in one set of experiments, illustrating the comprehensive information that can be gathered on such strongly correlated materials. Combined Real/Energy−Space UEM. Another TEM capability that is enabled due to the nature of electron scattering is energy filtering (EFTEM). In EFTEM, electrons can be selected (i.e., filtered) based upon their energy and used to form real-space images and diffraction patterns.12,15 There are several applications of EFTEM that lead to improvement in image quality as well as chemically specific, spatially resolved information. For core−shell ionization (i.e., the high-loss regime), imaging can be done by selecting electrons that have lost energy corresponding to core−shell states of specific elements. In this way, the images formed are effectively spatial distributions of the element-specific ionization process and thus are chemical maps of the specimen. One can also form a spectrum image, wherein the specific energy of interest is selected after acquisition. With probe-corrected instruments, chemical mapping can be done on discrete atomic columns, which is useful for determining the precise composition of angstrom-scale and epitaxial interfaces.17,18,156 In the low-loss regime, inelastically scattered electrons arising from plasmon excitations can be filtered such that images are generated from a narrow energy distribution about the zero-loss peak (ZLP). This is especially useful for relatively thick specimens that suffer H


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ment, and short-pulsed laser systems. An initial goal of the development of UEM was to combine the capabilities of each piece of equipment (e.g., a TEM and a fs laser) such that the temporal resolution accessible with TEM could be extended by several orders of magnitude to fs time scales. As in TEM and short-pulsed laser research, advances in understanding of the limiting fundamental processes in UEM, and clearly defining them within the currently accessible parameter space, will lead to improvements in resolution and new approaches to both stroboscopic and single-shot experiments. One challenge that is currently receiving a great deal of attention is overcoming the limitations imposed by spacecharge effects (i.e., Coulomb repulsion of the electrons in the packet). Currently, a balance must be struck between number of electrons per packet and temporal resolution; single- or fewshot UEM experiments, wherein images and diffraction patterns are obtained with as few as one laser pulse, are currently limited to ns temporal resolutions. To achieve fs durations, the number of electrons must be limited such that an image is built-up by summing many pulses at a fixed time delay. This approach then requires a highly reversible process in order to preserve spatial resolution; at high magnifications, spatial resolution can become limited by relatively low-precision specimen reversibility. Further, if the number of electrons per packet must be limited to ∼103 or less, then many pulses must be used in order to generate images with sufficient signal-to-noise ratios over an acquisition time that avoids deleterious effects of thermal specimen drift, an ever-present potential limit to resolving power. This necessitates a high repetition rate which thus limits time for specimen heat dissipation following laser excitation, potentially leading to irreversible effects that slowly accumulate with time. One approach that has been successfully applied in dedicated table-top ultrafast electron diffraction experiments is incorporation of an RF cavity synchronized to the laser system.111,113,115,116,118 This technology is being explored in UEM, as are MeV electron guns, to potentially extend the temporal resolution of single-shot experiments by 3 orders of magnitude to ∼10 ps without compromising spatial resolution.102,103 Indeed, owing to the relative infancy of the field (or, perhaps more appropriately, its re-emergence after the ns work initiated several decades ago), there is a great deal of effort being devoted to instrument development and advancement, especially new emission sources and pulse-compression methods that could find use in UEM.109−118,170−186 Technology advancement as well as elucidation and definition of the accessible parameter space, operating principles, and resolution limits of UEM will be critical to expanding the field beyond experts in instrumentation development and will likely be an active area of research for some time. Growth of the field will also come about via identification of important and interesting yet challenging fundamental questions in chemistry, physics, and materials science that can best be answered with UEM. One such area that is beginning to receive attention, mainly due to advances in instrumentation and commensurate improvements in resolutions, is fs crystallography. Currently, fs X-ray crystallography is an active area of research, wherein use of ultrashort X-ray probe pulses are used to conduct both diffract-before-destroy experiments as well as structural dynamics studies on molecular crystals.53,58,187,188 In such studies, emphasis is placed on determining atom positions via standard phase-recovery methods and, especially, for the small-molecule studies, mapping bond dynamics and atomic motion as a function of

from plural and potentially multiple scattering and thus have a significant fraction of incoming electrons losing energy to plasmon excitations. Energy filtering such that only electrons in the ZLP are used to form images and diffraction patterns leads to an improvement in contrast and image quality. As with EELS, EFTEM is used to study electronic excitations. The dynamics associated with these processes including electron−phonon coupling, plasmon oscillations and lifetimes, and relaxation of discrete electronic excitations occur on ultrafast time scales. An example of extending the EFTEM technique to UEM is shown in Figure 6 and also illustrates an ancillary method called photon-induced near-field electron microscopy (PINEM).157,158 The method uses energy filtering coupled with the effect shown in Figure 6A,B, wherein photon absorption by incoming electrons comprising the direct beam can occur when the laser pump pulse and photoelectron probe packet are overlapped precisely in space and time at the specimen.95,159−163 When this occurs, peaks are observed at integer multiples of the incident photon energy to both the loss and what corresponds to the gain side of the ZLP. The effect is greatest when the pulse and packet are precisely overlapped and diminishes as one is temporally translated relative to the other. At a time delay that corresponds to zero overlap, the effect is not observed, and the usual low-loss spectrum expected for the specimen is seen [e.g., t = −2 ps in Figure 6A]. Depending upon a variety of parameters, the effect shown in Figure 6A,B can be such that a large fraction of the incoming electrons can be partitioned into the discrete side bands, thus significantly depleting the ZLP. By using energy filtering, electrons comprising side bands to the gain side of the ZLP can then be used to form real-space images, thus revealing the spatial location and distribution of the photon-absorption events at the specimen (Figure 6C). Further, modulation of the intensity distribution arises via temporal translation of, for example, the photon pulse relative to the photoelectron packet, and, depending upon their durations, contrast variations can be seen at time delays below 100 fs. In addition to temporal translation, the intensity distribution arising from photon absorption can be spatially modulated by varying the orientation of laser polarization relative to the specimen. For example, the PINEM images in Figure 6C were obtained with the laser polarization oriented perpendicular to the long axis of the MWCNT. Thus, absorption occurred mainly along the length of the tube owing to the polarization dependence of electric-field enhancement arising from light scattering and the increased probability of coupling of electrons and photons at the specimen. A number of studies have been conducted using PINEM on a variety of materials, including metallic particles of different geometries (spheres, triangles, rods),164−167 few-layer graphene,168 and biological structures such as protein spheres and whole E. coli cells.169

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OUTLOOK Current rapid development of techniques for directly probing structural and electronic dynamics, wherein concepts of fs optical spectroscopy are combined with methods for determining atomic arrangements, is occurring, appropriately enough, when the International Year of Crystallography (2014) and the International Year of Light (2015) are also taking place. The technologies and instrumentation used in static-structural and all-optical studies have been under continuous development and advancement for decades, and improvements continue to be made on TEMs, dedicated diffraction equipI


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3M Nontenured Faculty Award under Award Number 13673369. Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research under Award Number 53116DNI7.

time. While large scattering cross sections pose increased challenges, electron crystallography on the ultrafast time scalewhere atom positions and dynamics are determined and followedcan be done and represents a table-top, singlePI lab-scale method, as is being pursued for hard X-ray sources.189 Indeed, advances in this area are currently being made, an example of which is the recent fs electron diffraction work conducted on the structural dynamics associated with the photoinduced insulator-to-metal transition of ethylenedioxytetrathiafulvalene hexafluorophosphate [(EDO-TTF)2PF6].190 With UEM, one could conduct comprehensive fs studies, wherein crystallography is combined with electron-energy spectroscopy and nanoscale real-space imaging to fully elucidate dynamics and follow the emergence of mesoscale phenomena from initial atomic-scale excitations and motions. Indeed, a large body of work on electron crystallography exists,191,192 and it is now a matter of extending it into the fs domain. There are many other research directions currently being explored and emerging as instrumentation evolves and improves. We selected the ones described here only as representative examples of two thrusts in current UEM researchinstrument development and discovery-based and applied experiments. It is our hope that this Perspective is successful in illustrating the opportunities present in this field and in catalyzing ideas that lead to new avenues of research and subsequent discoveries in chemistry, physics, and materials science.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: fl[email protected]. Author Contributions ‡

(D.A.P. and P.K.S.) Equal contributions.

Notes

The authors declare no competing financial interest. Biography David J. Flannigan earned his B.S. in chemistry with a minor in mathematics at the University of Minnesota in 2001. Under the mentorship of Prof. Wayne L. Gladfelter, he conducted research on the synthesis of anhydrous metal nitrates as volatile single-source precursors for the CVD of carbon-free thin metal oxide films. He earned his Ph.D. in chemistry at the University of Illinois at Urbana− Champaign in 2006 with Prof. Kenneth S. Suslick, where he studied the physical conditions and chemical processes occurring during single-bubble acoustic cavitation in aqueous mineral acid solutions. From 2007 to 2012, he was a Postdoctoral Scholar and then a Senior Postdoctoral Scholar in the laboratories of Prof. Ahmed H. Zewail at Caltech. There, his research focused on the development and application of ultrafast electron microscopy. He is currently a McKnight Land-Grant Professor and the Ray D. and Mary T. Johnson/Mayon Plastics Assistant Professor of Chemical Engineering and Materials Science at the University of Minnesota. His research interests are in the development and advancement of ultrafast electron microscopy and its application to a wide range of chemical and materials problems and phenomena.

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ACKNOWLEDGMENTS This work was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013. Additional support was provided by the DOE Advanced Research Projects AgencyEnergy (ARPA-E) under Contract Number 0472-1595 and by a J


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