Microscopy Conference 2023

Prof. Dr.

Session chairs

This session will cover recent advancements of instrumentation for imaging and analytical purposes of transmission electron microscopy (TEM and STEM). Part of this session will focus on time-resolved electron microscopy and its various aspects to study optical excitations and nonequilibrium phenomena in materials. Contributions are welcome to describe the state of the art of instrumentation for high resolution imaging as well as all variants of analytical techniques.

Prof.

Micron-sized magnetic multipole devices for charge particle optics

Abstract text (incl. figure legends and references) Introduction: Tunable electromagnets and corresponding devices, such as magnetic lenses or stigmators, are the backbone of numerous charged particle optical (CPO) instruments, such as electron microscopes. However, the built-in electromagnets are typically macroscopic conducting coils, which cannot generate swiftly changing magnetic fields due to their large resistivity and inductivity, require active cooling, and are structurally bulky. These restriction make them unsuitable for fast beam manipulation and miniaturized applications. To overcome these limitations, miniaturization of tuneable magnetic devices is required. Objectives: We seek on-chip microsized magnetic charged particle optics, which can be fabricated by microprocessing technologies, such as lithography. These devices should generate alternating magnetic fields of several 100 mT at frequencies up to GHz, supplying sufficiently large optical power for CPO applications. That particular includes fast spatio-temporal electron beam modulation such as electron beam deflection, focusing, and wave front shaping. Methods: Recently developed microcoil technology [1] allows overcoming the obstacles haunting miniaturization of magnetic CPO devices[PR3]. These microcoils (diameter: several 10 µms) are fabricated via lithography-patterned microelectronic structures, which assemble into 3D tubular architectures due to strain engineering within the multilayered polymer structure (Fig. 1a). The current-carrying copper layer of more than 100 nm thickness sustains more than 100 mA current in vacuum that generates a field of several mT depending on the number of windings. That field is further increases by introducing a soft-ferromagnetic wire (radius 5μm, saturation magnetization 0.2T, relative permeability 1000) into the zero-pitch microcoils (Fig. 1a). Results: Adapting the previously described microcoil technology, the whole platform was fabricated directly around an aperture plate in a multipolar arrangement (i.e., as monopole, dipole and quadrupole, Fig. 1b). The micron-sized devices were then mounted in dedicated high-frequency aperture holders, facilitating fast switching of the devices within a TEM. With that, alternating magnetic fields of about ±100 mT up to a hundred MHz, supplying sufficiently large optical power for numerous CPO applications is feasible (Fig. 2), including fast beam blanking and focusing for stroboscopic imaging. Notably, the quadrupole devices allow switchable line foci of a focal length of ca. 5 cm at 300 kV acceleration voltage, which is comparable to macroscopic bulk devices [2]. [1] D. Karnaushenko, T. Kang, V. K. Bandari, F. Zhu, and O. G. Schmidt. Adv. Mat. 32.15 (2020), 1902994. [2] R. Huber, F.L. Kern, D. Karnaushenko, E. Eisner, P. Lepucki, A. Thampi, A. Mirhajivarzaneh, C. Becker, T. Kang, S. Baunack, B. Büchner, D. Karnaushenko, O. Schmidt, A. Lubk, Nat. Comm. 13 (2022), 3220 [3] We acknowledge funding from the European Research Council (ERC) under the Horizon 2020 research and innovation program of the European Union (grant agreement no. 715620). Fig.1: Fabrication of microcoil charge particle optic devices: (a) strain-engineered multilayer architecture and rolling process. (b) Schematics of quadrupole devices and SEM image of central part of QP device. Fig.2: Performance characteristics of quadrupole device. Switchable line foci at 70 mA coil current and high-frequency deflection measurement demonstrating ca. 100 MHz cut-off frequency.

IM 1: Progress in instrumentation and ultrafast EM

IM 7: In situ/operando electron microscopy

Invited talk

Coupling electron microscopes with lasers: ultrafast charge dynamics and the laser-assisted nanoscale Kirkendall effect

Abstract text (incl. figure legends and references) While transmission electron microscopy (TEM) has evolved into a powerful tool for atomic-scale structure determination, temporal resolution in TEM has received far less attention, with detector and beam-current limits being on the order of microseconds. In this talk, I will describe how time resolution limits can be pushed by coupling ultrafast pulsed lasers to otherwise conventional TEMs. After briefly describing the experimental setup, I will present some of our recent work on transient lensing from photoemitted electron clouds on the picosecond time scale. I will also show how lasers coupled to TEMs can be used to induce nanoscale structural changes in nanomaterials on much slower time scales.

Prof. Dr. rer. nat.

ER-C 2.0: An update on plans for new electron microscopes at the Ernst Ruska-Centre in Forschungszentrum Jülich

Abstract text (incl. figure legends and references) The Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons (ER-C) in Forschungszentrum Jülich has operated a user facility in advanced transmission electron microscopy (TEM) jointly with RWTH Aachen University since 2004. Following a competitive selection process, the ER-C succeeded in being placed on the National Roadmap for Research Infrastructure of the German Federal Ministry of Education and Research in 2019 based on its &quot;ER-C 2.0&quot; application for a &quot;National research infrastructure for high-resolution electron microscopy&quot;. The ER-C 2.0 project is based on five next-generation electron microscopes, which are not currently available commercially and will be delivered in 2024 and 2025. The microscopes will be operated jointly by Forschungszentrum Jülich, RWTH Aachen University and Heinrich Heine University in Düsseldorf. Their development and operation will also be supported by associated partners in Humboldt University Berlin, the University of Göttingen, Maastricht University and the Max-Planck-Institut für Eisenforschung in Düsseldorf. The basic specifications of the five instruments, which are presently being designed by different column manufacturers in consultation with companies specializing in aberration corrector, detector and sample transfer technologies, as well as in ultra-high vacuum (UHV) equipment and correlative techniques, are as follows: TOMO: An aberration corrected UHV TEM combined in a single instrument with a co-located laser-equipped atom probe tomography capability and a cartridge-based liquid-helium-cooled specimen holder. OPERANDO: An aberration corrected fully bakeable UHV TEM equipped with in situ deposition of a wide range of materials using the technique of thermal laser epitaxy, with connectivity to a cryo UHV focused ion beam scanning electron microscope equipped with a secondary ion mass spectrometer and to a scanning probe microscope. FEMTO: Two time-resolved laser-driven aberration corrected field emission TEMs for ultrafast and dynamic studies of reversible and irreversible processes in materials over the widest range of temporal resolutions, featuring unique electron gun designs, magnetic-field-free imaging with atomic spatial resolution, an ultra-wide pole-piece gap and cryo workflows. SPECTRO: An ultra-high-energy-resolution aberration corrected scanning TEM equipped with light injection and collection, helium cooling of the specimen, magnetic-field-free imaging with atomic spatial resolution, gas injection and connectivity to a high pressure scanning electron microscope equipped with co-located cathodolouminescence and Raman spectroscopy. BIO: A chromatic and spherical aberration corrected next generation cryo TEM equipped with liquid helium cooling of the specimen and additional electron optical elements for novel phase plate designs. Access to the microscopes will be provided based on scientific proposals, which will be assessed by an external review panel. Successful applicants will be supervised by experienced scientists, who will offer hands-on support with experiments, analysis and data science. In this talk, I will present an update about the current status of the project.

Interference gating: a novel way for the dynamic investigation of switching semiconductor nanostructures by time-resolved electron holography

Abstract text (incl. figure legends and references) Off-axis electron holography deepens TEM&quot;s insight even further by providing access to amplitudes and phases of reconstructed electron waves. The latter contain spatially resolved information about projected electromagnetic potentials, making electron holography (EH) an excellent tool for the investigation of electric potential distributions in semiconductor nanodevices, especially when they are operated during investigations using special in situ specimen holders. Since existing methods for the realization of time-resolved measurements in a TEM by stroboscopic illumination have proven to be disadvantageous for EH [1,2], such investigations have so far been limited to static measurements. Recently, a simple, yet promising approach to realize time-resolved measurements of periodic processes with nanosecond time resolution in an electron holographic setup by means of interference gating (iGate) was presented [3]. As shown in fig. 1a, the basic idea of interference gating is a synchronized destruction of the interference pattern, realized by introducing random phase shifts φg(t) to the electron wave, for a defined period of time during an interferometric measurement such as electron holography (EH). The holographic reconstruction process acts as a temporal filter that only retains the information of the undisturbed interferogram outside this period (fig. 1b). By shifting the undisturbed time interval ? (gate) along an synchronized externally controlled periodic process (e.g. due to an applied voltage signal), the whole period can be sampled in a pump-probe (or lock-in) manner. In a first materials scientific application, interference gating was used to investigate dynamic electric potential distribution in an externally driven nanostructured general purpose silicon diode during switching in reverse-biased condition shown in fig. 2. An added value of this type of investigations is the simultaneous combination of high spatial and temporal resolutions. In the analysis of single frames of the investigated process (every 11.1 ns), different potential dynamics are revealed depending on the location. Thus, switching times in vacuum between free-standing electrodes (grey circles) correlate very well with the applied signal as measured in parallel by an oscilloscope (blue line), whereas switching delays (transients) appear as expected within the space charge region (red circles). Interference gating, thus, paves the way for the investigation of dynamic electrical effects in semiconductor nanostructures directly at the site of the event. [1] A. Feist et al., Ultramicroscopy, vol. 176, pp. 63–73, 2017. [2] F. Houdellier et al., Ultramicroscopy, vol. 186, pp. 128–138, 2018. [3] T. Wagner et al., Ultramicroscopy, vol. 206, p. 112 824, 2019. Figure 1: a) Dynamic phase shift φg(t) producing time-dependent electron hologram Ihol(r, t) (red and blue lines represent exemplary wave fronts for different times t). b) Dynamic phase sequence for generating time-resolved electron holograms with gating length ? and respective Fourier transformations of Ihol(r, t) (disturbed case: only centerband, undisturbed: also sidebands). Figure 2: Plot of the control signal Uin(t) (blue line) applied to the prepared diode measured by the oscilloscope and plots of the phase slopes d/dx φel(x, tgi) (vacuum: grey circles, depletion region of the diode: red circles) calculated for 30 gate positions tgi in two different areas with the standard deviation as error bars.

IM 6: Phase-related techniques & 4D STEM

Abstract talk

Evaluation a 48 pixel programmable phase plate for electron wavefront shaping

Abstract text (incl. figure legends and references) We report on advances in the development of a versatile programmable phase plate for use in electron microscopy. The current design provides up to 48 individually addressable electrostatic phase shifting elements that can be laid out in multiple design patterns depending on the target application. The phase plate state can be synchronously updated at up to 100 kHz rate providing an attractive route for high speed adaptive algorithms shaping the electron probe while optimizing a goal function. We discuss different phase plate implementations and their usefulness towards applications. We show the realization of dipolar, quadrupolar and vortex phase symmetries in sub nm electron probes when placing the phase plate in the condenser plane of a modern STEM microscope. We demonstrate adaptive algorithms maximizing for contrast in HAADF STEM images as well as discussing their prospects for focusing inside thick samples, increasing depth of field and use in ptychography. <table cellspacing="0" cellpadding="0" align="left"> <tr> <td width="45" height="0"></td> <td width="515"></td> <td width="8"></td> <td width="515"></td> </tr> <tr> <td height="1"></td> <td rowspan="2" align="left" valign="top"></td> </tr> <tr> <td height="473"></td> <td></td> <td rowspan="2" align="left" valign="top"></td> </tr> <tr> <td height="1"></td> </tr> </table> Figure 1. Example of two prototype phase plate realisations showing a 36 pixels cartesian design (left) and a polar segmented 48 pixel design. [1] J. Verbeeck et al. "Demonstration of a 2×2 programmable phase plate for electrons." Ultramicroscopy 190 (2018): 58-65. [2] Patents EP3474308, US11062872 B2. [3] The authors acknowledge the financial support of the Research Foundation Flanders (FWO, Belgium) project G042820N. The ADAPTEM project is funded by the European Research Council as an ERC POC project under grant nr: DLV-789598.

Hexapole correctors for STEM: CESCOR, DCOR, ASCOR and beyond ...

Abstract text (incl. figure legends and references) The success of hardware aberration correction in transmission electron microscopy started 25 years ago with the first resolution improvements achieved with a two-hexapole design in TEM [1] and a quadrupole-octupole design in STEM [2]. The two-hexapole design was used in STEM later as well [3]. A careful redesign lead to the advanced two-hexapole corrector (DCOR/ASCOR) with full fourth-order correction and effectively vanishing fifth-order aberrations [4]. This design is still state-of-the-art – independent from using additional A5 stigmators or not [5,6]. After these achievements it has been argued, that the usable aperture semi-angle for two-hexapole and even three-hexapole correctors is finally limited by the three-lobe aberration of sixth order D6 [7]. For an objective lens with a medium gap size around 5mm, e.g. for analytical applications, the π/4-phase shift limit due to this intrinsic aberration is just below 40mrad (Fig. 1). As investigated theoretically [4,5] and verified experimentally [6,8,9], these advanced correctors are sufficient for modern CFEG-equipped microscopes to touch the high-resolution limit set by the chromatic aberration. D6 hardly limits the obtainable resolution, if additional D6 phase shifts up to several π/4 are reasonably-well counterbalanced by lower-order three-fold aberrations [6,10]. However, if the intrinsic D6 can be eliminated a-priori in the optics, the counterbalancing techniques wouldn"t be necessary in most situations. This is especially valuable in case of monochromated illumination, which allows for even larger apertures, enabling not only better xy- but also improved z-resolution [11]. Recently, we found first evidence that a novel three-hexapole design can eliminate the three-lobe aberration of sixth order D6 together with higher-order spherical aberrations. Consequently, the theoretical π/4-limit due to intrinsic residual aberrations is shifted further out to 70mrad. Again, compensation techniques can extend the usable aperture even beyond this value. Fig. 2 shows very promising results of experimentally obtained ronchigrams with a radius of ≥80mrad recorded at 200kV. [12, 13] [1] M. Haider et al., Nature 392, 768 (1998). [2] P.E. Batson, N. Dellby, O.L. Krivanek, Nature 418, 617-620 (2002). [3] H. Sawada et al, Microscopy 54 (2), 119-121 (2005). [4] H. Müller et al., Microscopy and Microanalysis 12 (6), 442–445 (2006). [5] P. Hartel et. al., Ultramicroscopy 206, 112821 (2019). [6] R. Sagawa et al., Ultramicroscopy 233, 113440 (2022). [7] S. Morishita et al., Microscopy 67 (2018) 156-163. [8] M. Watanabe et al., Microsc. Microanal. 22 (2016) 310-311. [9] M. Bischoff et al., Microsc. Microanal. 24 (2018) 1134-1135. [10] S. Morishita et al., Phys.Rev.Let. 117 (2016) 153004. [11] R. Ishikawa et al., Ultramicroscopy 151 (2015) 122-129. [12] We acknowledge and highly appreciate the collaboration and support by JEOL company. [13] CEOS GmbH has received funding from the European Union"s Horizon 2020 research and innovation program under grant agreement No. 823717 – ESTEEM3. &lt;fig1&gt; Figure 1: Hexapole corrector generations. The typical size of the intrinsic aberrations and the resulting π/4-limits for 30..300kV are indicated, assuming a medium-size objective lens gap. &lt;fig2&gt; Figure 2: A comparison of experimental in-focus ronchigrams at 200kV: a) advanved hexapole corrector with D6 ≈ 2mm b) calibration with gold c) new corrector with D6 ≈ 0.

Energy-filtered ultrafast electron microscopy with chirped photoelectrons

Abstract text (incl. figure legends and references) Introduction Ultrafast electron microscopy (UEM) enables time-resolved imaging with a spatial resolution approaching (sub)nanometer precision. The temporal resolution has been governed by the duration of the imaging electron pulse at the specimen. The energy spread of the electron pulses develops a chirp, an energy‒time correlation, resulting in the temporal broadening of the pulses because the leading electrons with higher energies accelerate and those with lower energies are retarded during propagation. Objectives We demonstrate energy-filtered UEM (EFUEM), which exhibits a femtosecond resolution with a conventional energy filter for use in a TEM without pulse compression. The temporal resolution is controlled by selecting chirped photoelectrons of distinct kinetic energies using the post-specimen energy filter. We demonstrate ultrafast real-space imaging using EFUEM, observing the phase-transition dynamics of VO2 to study its heterogeneous nature with different strains exerted on individual nanoparticles (NPs) in an ensemble. Materials &amp; methods A proof-of-concept experiment was performed with a polycrystalline VO2 film, which exhibits insulator-to-metal transition (IMT) from a low-temperature semiconducting phase with a low-symmetry monoclinic structure, M1, to a high-temperature metallic phase with a high-symmetry rutile structure, R. Excitation using femtosecond optical pulses provides a route to the ultrafast transition from the M1 to the R phase. To demonstrate the temporal resolution approaching the time window of IMT, time-resolved micrographs were obtained by energy filtering the photoelectrons with various slit widths. By gating the detection range of electron energy, the frame time of images is controllable. Results The temporal resolution is determined by the slit width of the energy filter, reaching the resolution of ~700 fs, which is mainly limited to the duration of optical excitation pulses (550 fs). The concept enables resolution of ultrafast structural responses in matter beneath temporally broad photoelectron bunch without delicate instrumental modification or compromising the large number of electrons. In our approach, the temporal resolution of ultrafast imaging was additionally limited by the degree and dispersion of the chirps of probing electron pulses, the spectral jitter of the energy filter, and the duration of the optical excitation pulses to initiate structural change. If these factors are improved, the temporal resolution of ultrafast imaging may reach &lt;100 fs, which surpasses the timescale of serial atomic displacement. Using the energy-filtered photoelectrons, the ultrafast IMTs of VO2 NPs were visualized, revealing their unique behaviors associated with nanoscale strain. A NP under a negligible strain, such as the bulk single-crystalline VO2, transforms from the initial M1 phase directly to the R phase upon photoexcitation. When a NP in the M1 phase is partially strained, its metallic M2 phase emerges for the first few picoseconds, retaining the parent lattice structure, which then subsequently undergoes the structural transition to the R phase. Conclusion Our approach enables electron microscopy to access the timescale of elementary nuclear motion to visualize the onset of the structural dynamics of matter at the nanoscale

IM 1
Progress in instrumentation and ultrafast EM

Appointment

Session chairs

Description

Programme

IM 1Progress in instrumentation and ultrafast EM

Appointment

Session chairs

Description

Programme

IM 1
Progress in instrumentation and ultrafast EM