ultrafast electron microscopy

S5MicroscopyandAnalysis | November 2013 Nanotechnology Issue

4D ultrafast electron microscopy sheds light on dynamic processes from the micrometer to the atomic scale David J. Flannigan1 and Oleg Lourie2

1. Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, USA

2. Materials Science Business Unit, FEI, Eindhoven, The Netherlands

IntroductionOver the last decade or so, researchers and manufacturers have made significant improvements in the spatial and temporal resolution of transmission electron microscopes (TEM). In the case of spatial resolution, the availability of correctors that reduce the degrading effects of aberrations caused by lens imperfections has made imaging with atomic scale resolution almost routine.

Similarly, the development of ultrafast electron microscopy (UEM) and variant techniques now allows scientists to record dynamic processes with temporal resolutions down to the femtosecond (10-15 s) level. The combination of sub-angstrom spatial and femtosecond temporal resolutions opens to investigation myriad fundamental, atomic-scale processes in fields that range from materials science to biology.

In this article, we first describe the operating concepts that enable UEM, and then offer several examples that illustrate its power to explore ultrafast dynamic processes at a range of spatial scales from micrometer to atomic.

Principles of ultrafast electron microscopyConventional TEMs are limited in temporal resolution by the millisecond shutter speeds of current compatible detectors as well as the current per unit time impinging on the CCD chip. With UEM, the same detectors are used, but the exposure time is effectively shortened by illuminating the sample with a very brief packet of electrons.

The electrons in this probe pulse are photoelectrons that are generated when the emitter in the TEM source is exposed to a laser pulse and subsequently accelerated in the conventional manner by an applied electric field into the optical column of the TEM. The dynamic event to be investigated is typically also initiated by a laser pulse

(i.e. pump pulse) directed at the specimen. The delay between the pump and probe pulses must be precisely controlled. For shorter delays in the femtosecond to picosecond range, this is accomplished by dividing a single laser beam in two and directing one of the pulses over a variably extended path. Delays in the nanosecond to millisecond range (up to video rate where current detectors operate) are achieved with a digital delay generator.

One of the advantages of UEM is that it requires no significant modifications to the basic TEM design beyond providing access to the electron emitter and sample chamber for the probe and pump laser pulses, respectively.

The UEM can be operated in either single-pulse or repetitive stroboscopic modes. The single-pulse mode is employed when studying processes that are non-reversible on the experimental time scale. However, Coulombic forces

limit the number of electrons that can be packed into the single probe pulse, which currently places the upper bound on spatial and temporal resolutions in the nanometer (imaging) and nanosecond ranges, respectively.

For reversible processes, the stroboscopic mode can be used to achieve relatively high spatial and, especially, temporal resolutions. In this mode, the CCD shutter is left open, and the exposure is repeated many times to allow sufficient signal to accumulate on the detector. Each probe pulse can be made to contain only one electron on average, thus completely eliminating detrimental Coulombic interactions. Repeated exposures are separated by enough time to ensure the sample returns to its initial state between pulses. The image is recorded when sufficient signal has accumulated at a discrete delay value. The delay is then incrementally changed to a new value,

Figure 1 Schematic representation of the ultrafast transmission electron microscope (UEM). Critical components of the UEM are labeled in the figure and include the appropriately modified TEM, the laser systems (here, femtosecond and nanosecond), the motorized optical delay line, and the digital delay generator. Here, the UEM is shown with a standard CCD camera, but any TEM-compatible detector can be employed (e.g. post-column energy filter).

ultrafast electron microscopy

S6 Nanotechnology Issue November 2013 | MicroscopyandAnalysis

and the acquisition of the next image is initiated. The final result is a sequence of images, often viewed as a movie, with a spatiotemporal resolution that can reach the angstrom-femtosecond range.

Single-pulse techniques that employ packets containing millions of electrons to characterize irreversible processes mainly in the nanosecond regime are sometimes referred to as “fast” EM. The moniker “ultrafast EM” is generally reserved for multi-pulse stroboscopic

Figure 2 UEM images of the initial photoinduced motion of a single-crystal cantilever of Cu(TCNQ). The time at which the images were acquired (relative to excitation with a nanosecond laser pulse) is shown in the upper left corner of each image. The dotted horizontal line is included to highlight the crystal motion. The magnification was 21,000x, and the repetition rate of the laser and electron pulses was 100 Hz.

techniques in the femtosecond regime. All of the work described below employed stroboscopic UEM techniques and was performed on an appropriately modified FEI Tecnai TEM [1, 2].

ApplicationsStructural Properties of Nanomaterials [3]Many materials are polymorphic (allotropic for elemental solids, e.g. carbon). That is, they can have more than one crystal structure for the same composition. Further, some of these materials will follow Ostwald’s step rule, which states that, in general, if a material can form into more than one crystal structure, the kinetic (i.e. least stable) phase will form first followed in time by the thermodynamic (stable) phase. As a consequence, kinetic phases of materials can go unnoticed for decades due to their fleeting nature and small size.

One such material is the metal-organic hybrid, copper 7,7,8,8-tetracyanoquinodimethane [Cu(TCNQ)]. It was first synthesized in the 1960s, but its polymorphic nature was not discovered for nearly four decades. Further, upon discovery of polymorphism, it was found to follow Ostwald’s step rule; the material initially forms crystals having a rod-like morphology (Phase I) that rapidly transform into thermodynamically stable plates (Phase II) upon continued reaction in solution. Due to significant structural differences at the unit-cell level, these two polymorphs display dramatically different electrical and thermal properties, as measured on bulk specimens. However, quantifying the structural properties of the Phase I polymorph (including the crystal structure) proved to be challenging due to the small size of the single crystals; the Phase I crystals quickly redissolve and form Phase II in solution, thus limiting the overall size while introducing a non-trivial amount of disorder in the lattice.

Single crystals of the Phase I polymorph were studied with UEM in the elastic regime. Fixed-free beams (i.e. cantilevers) of Phase I Cu(TCNQ) were excited in situ with a nanosecond laser pulse, and the resulting structural oscillations were imaged in real-space and time (Figure 2). The crystal motion was quantified from the UEM images and plotted as a function of time (Figure 3). The presence of multiple oscillatory

modes was observed in the UEM images, with higher-order modes having spatial displacements of one nanometer after damping had occurred over a few microseconds. By modeling the crystal as a fixed-free beam and converting the observed spatial displacement into frequency space via Fourier transform (Figure 4), the elastic properties of the material (e.g. Young’s modulus, speed of sound) were determined.

Photoactuation and Dynamic Diffraction Contrast [4]Materials, whether single component or composite, that display photoactuation properties (i.e. mechanical response upon exposure to light) are being explored for a variety of applications, including remote modulation and energy conversion technologies. An understanding of the fundamental mechanisms of operation is needed to design new materials and further optimize existing systems. To this end, a model system consisting of a crystalline silicon nitride (a-Si3N4) cantilever and an adhering mat of multiwalled carbon nanotubes (MWCNTs) was fabricated and studied with UEM (Figure 5).

The Si3N4 cantilever was initially amorphous and was crystallized in situ by heating with a train of femtosecond laser pulses. Heating and crystallization occurred only where the MWCNT mat was present on the Si3N4 substrate. This is because Si3N4 does not absorb at the optical frequencies used in these experiments (800 nm laser light), thus selective absorption and heating occurred only where adhering mats of MWCNTs were present. This was confirmed in the UEM images via repeated exposures of various specimen regions, with and without MWCNTs, to trains of 800 nm femtosecond pulses. The heating and crystallization were therefore attributed to absorption of laser light by the MWCNTs and subsequent thermal transfer to the Si3N4.

By quantifying lattice spacings in electron diffraction patterns, it was found that mats of MWCNTs exposed to femtosecond laser pulses could be heated to 1000 °C, which is the temperature required to induce the amorphous-to-crystalline phase transition of Si3N4. A crystallization front is shown in panel (C) of Figure 5 illustrating the dramatic differences in contrast between amorphous and crystalline regions.

Figure 3 Displacement (nm) of the Cu(TCNQ) cantilever as a function of time (ns). The displacement is relative to the crystal position at rest (i.e. before laser excitation). Images were acquired at 10 ns steps, and a relative crystal position at each step was determined and plotted.

Figure 4 Fourier transform of the time-dependent displacement shown in Figure 3. The two frequencies highlighted (2.5 and 15 MHz) match the fundamental and second-order oscillatory modes, respectively, (f1 and f2, illustrated in the inset) expected for the Cu(TCNQ) single crystal shown in Figure 2.

ultrafast electron microscopy

S7MicroscopyandAnalysis | November 2013 Nanotechnology Issue

As with most ordered materials studied with TEM, the crystalline Si3N4 displayed strong diffraction contrast. Because diffraction contrast patterns can change dramatically with small changes in crystal orientation or position, it was used as a sensitive indicator of photoinduced motion of the Si3N4 cantilever. As such, variations in the observed diffraction contrast patterns in the UEM images were used to quantify the time-dependent motion caused by the ultrafast photoactuation of the system (Figure 5B). Deflections of 0.3° occurring in 300 ps could be quantified, and motions of contrast patterns spatially propagating over a few nanometers could be observed in the UEM images. In this way, both the fundamental mechanisms associated with the photoactuation process, as well as time scales associated with the resulting motions, were directly observed and quantified.

Atomic-Scale Dynamics [5]The basis for photoinduced mechanical motion must reside in initial ultrafast excitation of bound (in the case of covalently bonded materials) and free (metallic bonding) electrons in the lattice. The well-known two-temperature model states that these electrons will be initially much hotter than the atoms in the lattice. On the picosecond time scale, these hot electrons will transfer kinetic energy to the nuclei in the lattice which will lead to an equilibration of the system at an elevated temperature. Eventually over a period of nanoseconds or more, the energy will leave the system as it equilibrates with its surroundings. Though this is the generally accepted picture of what is

Figure 5 (above) Picosecond photoactuation and diffraction contrast dynamics of crystalline Si3N4 with an adhering mat of MWCNTs. (A) UEM image of crystalline Si3N4 showing diffraction contrast. Overlying MWCNTs are discernible as a fine, irregular network in the upper-right quadrant of the image. Two areas of interest are highlighted (#1 and #2). (B) Change in contrast strength as a function of time within the highlighted areas of interest. The solid black line is included to guide the eye. (C) UEM image showing the crystallization front and the selected region from which diffraction (D) was obtained to confirm the crystallinity of the Si3N4 and was indexed to the alpha phase. The scale bar in (C) represents 500 nm. (A) and (C) are different areas.

Figure 6 Series of UEM diffraction patterns obtained from a mat of MWCNTs. The upper left panel shows a UEM diffraction pattern of MWCNTs with two of the Debye-Scherrer rings labeled with the planes from which the observed scattering occurs. The scale bar represents 10 nm-1. The following series of difference images are diffraction patterns obtained by subtracting a reference frame from all subsequent time-dependent frames. Increases in contrast strength denote Debye-Scherrer ring motion and, hence, lattice motion of the MWCNTs.

ultrafast electron microscopy

S8 Nanotechnology Issue November 2013 | MicroscopyandAnalysis

biography David J. Flannigan is the Ray D. and Mary T. Johnson/Mayon Plastics Assistant Professor of Chemical Engineering and Materials Science at the University of Minnesota. Prior to joining the faculty at Minnesota in the summer of 2012, he was a senior postdoctoral scholar at Caltech where he worked with Prof. Ahmed H. Zewail on the development and application of ultrafast electron microscopy. He received a BS in chemistry from the University of Minnesota and a PhD in chemistry under the guidance of Prof. Kenneth S. Suslick at the University of Illinois at Urbana-Champaign, where he studied the chemical and physical conditions generated during acoustic cavitation.

abstractThe development of ultrafast electron microscopy (UEM) and variant techniques now allows scientists to record dynamic processes with temporal resolutions down to the femtosecond (10-15 s) level. The combination of sub-angstrom spatial and femtosecond temporal resolutions opens to investigation myriad fundamental, atomic-scale processes in fields that range from materials science to biology. In this article, we first describe the operating concepts that enable UEM, and then offer some examples that illustrate its power to explore ultrafast dynamic processes at a range of spatial scales from micrometers to atoms.

Corresponding author details Dr David J. Flannigan, Department of Chemical Engineering and Materials Science, University of Minnesota421 Washington Ave. SE, Minneapolis, MN, 55455, USATel: +1 612-625-3867Email: flan0076@umn.edu

Microscopy and Analysis 27(7):S5-S8 (AM), November 2013

©2013 John Wiley & Sons, Ltd

occurring during optical excitation of materials, the time scales, magnitudes, and spatial behavior of such processes is highly material and system-dependent.

Ultrafast spectroscopy has established that, during coherent electronic excitation of carbon nanotubes (CNTs), charge-carriers are generated and can be long-lived. Further, it was postulated that generation of these long-lived charge-carriers should result in some structural response from the CNTs at the atomic-level. Several studies have shown that pristine CNT bundles, as well as CNT/polymer composites, display photoactuation properties. These experiments, however, were conducted on time scales that were far too slow to resolve any initial ultrafast structural response that may arise from modulation in bond energies. Further, the spatial resolution of the optical imaging methods employed could not be used to probe the structures at the atomic scale, from where ultrafast dynamics and all subsequent motion would originate.

MWCNTs having an average diameter of 10 nm were studied with UEM in parallel-beam diffraction mode with femtosecond temporal resolution. A mat of CNTs was excited with a single ultrafast laser pulse having a wavelength of 515 nm, and the resulting atomic-scale structural dynamics were probed with single-electron packets.

Figure 6 shows a series of UEM diffraction patterns (reference frame acquired before time zero and difference frames at positive times). As can be seen, the Debye-Scherrer rings, arising from scattering of specific lattice planes in the CNTs, contract inward toward the center of the image which corresponds to an expansion in real-space. A plot of the (002) ring, which corresponds to the intertubule spacing in a MWCNT, as a function of time shows a four picometer expansion occurring on the picosecond time scale (Figure 7). On this time scale, electron-phonon coupling has occurred and diffusive heating has begun. Thus, with UEM, the structural effects of mechanisms associated with heating can be directly studied, and contributions by both electrons and phonons may potentially be isolated and quantified.

Interestingly, while ultrafast increases in the intertubule spacings were observed, little or no detectable expansion within the tubes themselves was seen. Note that the intertubule spacing is maintained by relatively weak van der Walls forces, while strong carbon-carbon covalent bonds occur within the tubes. This observation can be explained by invoking a scrolled structure for the MWCNTs, in which the increased space between the walls is accommodated by circumferential slipping between the layers as the scroll unrolls. It is more challenging to explain if the MWCNTs are coaxial and capped,

which would require ultrafast increases in the covalently bound lattice spacing.

Conclusions4D ultrafast transmission electron microscopy is capable of both angstrom-level spatial resolution and femtosecond temporal resolution, making it ideally suited for investigations of ultrafast dynamic processes over a range of spatial scales from micrometers to atoms. Such processes are fundamental to our understanding of the macroscopic properties of materials and the behavior of complex systems.

References1. Lobastov, V. A., Srinivasan, R., Zewail, A. H. Four-dimensional ultrafast electron microscopy. Proc. Natl. Acad. Sci. USA 102:7069-7073, 2005.2. Park, H. S., Baskin, J. S., Kwon, O.-H., Zewail, A. H. Atomic-scale imaging in real and energy space developed in ultrafast electron microscopy. Nano Lett. 7:2545-2551, 2007.3. 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. 9:875-881, 2009.4. Flannigan, D. J., Zewail, A. H. Optomechanical and crystallization phenomena visualized with 4D electron microscopy: Interfacial carbon nanotubes on silicon nitride. Nano Lett. 10:1892-1899, 2010.5. Park, S. T., Flannigan, D. J., Zewail, A. H. 4D Electron microscopy visualization of anisotropic atomic motions in carbon nanotubes. J. Am. Chem. Soc. 134:9146-9149, 2012.

Further Reading:Zewail, A. H., Thomas, J. M. 4D Electron microscopy: Imaging in space and time. Imperial College Press: London, 2010.Zewail, A. H. Four-dimensional electron microscopy. Science 328:187–193, 2010..Flannigan, D. J., Zewail, A. H. 4D Electron microscopy: Principles and applications. Acc. Chem. Res. 45:1828-1839, 2012.

Figure 7 Real-space and real-time response of the (002) Debye-Scherrer ring from UEM diffraction patterns of MWCNTs. The red line is shown to guide the eye. The dynamics shown here represent an increase in the intertubule spacing of the MWCNTs. Note the observed expansion is four picometers.