Microscopy Conference 2023

Prof.

Session chairs

Prof. Dr. rer. nat. habil.

Nowadays, in situ/operando electron microscopy is a rapidly growing field due to the implementation of nanotechnology towards the fabrication of greatly improved, stable and more flexible sample holders. Furthermore, data collection, analysis and recording of dynamic information is becoming increasingly faster. Recent advances include, for example, the use of microelectromechanical systems (MEMS) based chips for in situ transmission electron microscopy. The capability to perform multiple measurements while simultaneously analysing corresponding structural, chemical, or even electronic structure changes in functional and nano- materials at different length scales are opening exciting new opportunities at the forefront of modern materials science research. This symposium intends to review progress on in situ/operando TEM and SEM experiments that apply heating, cooling, electrical biasing, and mechanical testing to induce and probe phase transformations.

Prof. Dr.

Unveiling the dynamic behavior of electrocatalysts through operando microscopy and spectroscopy

Abstract text (incl. figure legends and references) Climate change concerns have spurred a growing interest in developing environmentally friendly technologies for energy generation, including green H2 production from water splitting. Moreover, concerted attempts are also being made to re-utilize CO2 by electrocatalytically converting it into value-added chemicals and fuels, offering the possibility to store renewable energy into chemical bonds. Thus, efficient, selective and durable electrocatalysts that can operate under mild reaction conditions are urgently needed. However, the latter challenging goal can only be achieved when fundamental understanding of the electrocatalyst structure and surface composition under reaction conditions becomes available. It should also be kept in mind that even morphologically and chemically well-defined pre-catalysts are commonly susceptible to drastic modifications under operando conditions, especially when the reaction conditions themselves change dynamically. Here, a synergistic experimental approach taking advantage of a variety of cutting-edge microscopy (EC-AFM, EC-TEM, LEEM/XPEEM), spectroscopy (NAP-XPS, XAS, Raman Spect., GC/MS) and diffraction (XRD) will be employed to unveil the complexity of energy conversion electrocatalysts. In particular, I will provide new insights into the electrocatalytic reduction of CO2 and Nitrate as well as the oxygen evolution reaction using operando characterization methods while targeting model pre-catalyst systems ranging from single crystals to metal nanoparticles. Some of the aspects that will be discussed include: (i) the design of size- and shape-controlled catalytically active nanoparticle pre-catalysts (Cu, Cu2O cubes, CoOx NPs, Fe/NiO octahedra), (ii) the understanding of the active state formation and (iii) the correlation between the dynamically evolving structure and composition of these electrocatalysts under operando reaction conditions and their activity, selectivity and durability. Our results are expected to open up new routes for the reutilization of CO2 through its direct conversion into industrially valuable chemicals such as ethylene and ethanol, the electrochemical synthesis of ammonia and the generation of H2 through water splitting.

IM 7: In situ/operando electron microscopy

Invited talk

Dynamic observations of compositionally complex alloys by in situ TEM

Abstract text (incl. figure legends and references) Introduction The development of multicomponent high entropy alloys (HEAs) has opened the door to sculpt advanced microstructures in the near-infinite compositional space, resulting in materials with unprecedented mechanical and functional properties. Understanding the underlying phase transformation mechanisms during processing and application of these compositionally complex alloys is a key requirement to further advance alloy design strategies. In situtransmission electron microscopy (TEM) techniques nowadays make it possible to probe even compositionally complex alloys under varying temperatures or strains, while being able to dynamically observe microstructure evolution down to the atomic level. Objectives The present talk focuses on in situ probing strategies in the TEM to observe plastic deformation and temperature dependent phase transformations in HEAs. I will show a novel mechanism of transformation induced plasticity in an FeMnCoCr alloy revealed by in situ straining in the TEM. In a further example, in situ heating experiments in the TEM will be used to track temperature induced phase transformations and secondary phase formation in a carbon (C) doped FeMnCoCrNi alloy. Materials &amp; methods The HEAs are fabricated by vacuum induction melting and investment casting using at least 99.5 wt.% pure metals. Thermo-mechanical treatment is used to homogenize the alloys and adjust the desired starting microstructure. In situ tensile straining experiments are performed using a custom-built straining inset in a model 654 Gatan straining holder. Samples are heated in situ in the TEM using a DENSsolutions Lightning heating/biasing holder, while using both TEM and scanning TEM to observe microstructural changes in Titan Themis (Thermo Fisher Scientific) microscopes. Results The dual-phase Fe50Mn30Co10Cr10 (at.%) alloy is primarily deforming via displacive phase transformation from the face-centered cubic (FCC) to the hexagonal close packed (HCP) structure. Our in situ tensile straining reveals that a bi-directional transformation is active, where FCC transforms to HCP and back to FCC upon straining. This strain induced phase transformation leads to dynamic microstructural refinement, which equips the alloy with outstanding work-hardening capabilities. Such a nanostructure provides excellent room temperature properties, but rapidly coarsens at elevated temperatures leading to a loss in mechanical properties. Interstitial doping with C is a way to stabilize the microstructure and our in situ TEM observations reveal the underlying phase transformation mechanisms. At intermediate temperatures of ~300ºC we observe the formation of nano-twinned FCC originating from an initial HCP grain. Upon further heating to 700ºC, elongated nanoscale carbides are forming along the FCC twin boundaries, which can stabilize the nano-twinned regions up to 900ºC. Atomic scale observations at extreme temperatures at &gt;900ºC reveal the dissolution of the carbide phases and associated de-twinning, providing insights into microstructural evolution in the compositionally complex alloys. Conclusions In the present talk, we showed that in situ straining and heating in the TEM provides unprecedented insights into the phase transformation mechanisms in compositionally complex alloy systems. In situ probing is pivotal in observing the underlying transformation dynamics and is able to reveal previously overlooked phase transformation mechanisms.

STEM EBIC mapping of ferroelectric polarization in thin film hafnia

Abstract text (incl. figure legends and references) In its primary imaging mode, a scanning transmission electron microscope (STEM) can locate individual atoms. With various spectroscopic attachments, a STEM can even determine the chemical identity of those atoms. However, while it excels at determining a sample"s physical and chemical structure, traditional STEM struggles to map electronic structure. Inter-atom electric fields are small compared intra-atom electric fields, so visualizing the former generally requires advanced techniques such as holography or differential phase contrast imaging. These techniques are analysis intensive and can produce complicated images that are difficult to interpret. In electron beam induced current (EBIC) imaging, a transimpedance amplifier electrically connected to the sample collects the current induced by a scanning electron beam. Associating that current with the beam position produces an image. In samples with intrinsic electric fields, a raw EBIC image can sometimes be quickly and quantitatively interpreted to give the local electric field magnitude and direction. To illustrate STEM EBIC imaging"s capabilities for high-resolution, in situ electric field mapping, we fabricate electron-transparent TaN/HZO/TaN capacitors (30/20/30 nm) on 20-nm thick silicon nitride membranes. The tantalum nitride is deposited via DC sputtering and the zirconium-doped hafnia (Hf0.5Zr0.5O2, HZO) is deposited via plasma-enhanced atomic layer deposition [1]. Using traditional transport measurements, we determine the dielectric's polarization as a function of the applied electric field in situ over various ranges of the electric field. Interleaved with the P(E) measurements, we perform STEM EBIC imaging to map the remanent polarization as a function of position in the capacitor. Collecting the EBIC separately from the two electrodes of the capacitor allows us to separate the standard EBIC, which depends on the electric fields internal to the sample, from the EBIC caused by the ejection of secondary electrons from the sample [2]. The former appears in the difference EBIC images (Fig. 1), while the latter appears in the sum (not shown). Subtracting a polarization "up" differential EBIC image from a polarization "down" EBIC image beautifully reveals the spatial variation of the switching fraction (Fig. 2). The switching fraction further varies with the switching history. References 1.Fields, S. S. et al. Compositional and phase dependence of elastic modulus of crystalline and amorphous Hf1-xZrxO2 thin films. Appl. Phys. Lett. 118, 102901 (2021). 2.Hubbard, W. A., Mecklenburg, M., Chan, H. L. &amp; Regan, B. C. STEM Imaging with Beam-Induced Hole and Secondary Electron Currents. Phys. Rev. Applied 10, 044066 (2018). This work was supported by National Science Foundation (NSF) award DMR-2004897 and by NSF Science and Technology Center (STC) award DMR-1548924 (STROBE). Figure 1. (left to right) Bright field, annular dark field, differential EBIC with polarization "up", and differential EBIC with polarization "down" images. The field of view is 11 μm on a side. Figure 2. (left) The difference between the fourth and the third images from Fig. 1. Areas with a switchable remanent polarization appear black. (right) A higher magnification view (acquired separately, but analyzed in the same way) of the region indicated by the red box reveals ferroelectric domains. Here the field of view is 1.9 μm on a side.

Plenary

Growth mechanisms of nanomaterials studied by atomic-resolution liquid-phase scanning transmission electron microscopy

Abstract text (incl. figure legends and references) Understanding the chemical and the physical processes of nanomaterials (NMs) in liquid media is a crucial challenge and of fundamental interest to modern materials science, chemistry and biology.1 For example, observing the initial steps of nucleation and the growth of solids and following the exact pathway of the formation of bimetallic NMs are of highest importance for the development of novel materials. This requires techniques allowing the study of NMs in their liquid environments with the corresponding resolution necessary to observe processes at the atomic scale. The most attractive and unique capabilities of a scanning/transmission electron microscope (S/TEM) is its high spatial resolution, enabling the study of materials at the atomic scale. Nevertheless, a key limitation of this technique is the need of high vacuum conditions to analyse samples. In recent years, a new frontier in the field of TEM has emerged to observe specimens in liquid environments under high-vacuum conditions, by separating the liquid sample from the vacuum column using thin impermeable/electron transparent membranes.2 Recent advances in microfabrication technology have led to a number of commercially available liquid cell TEM holders, where the liquid cells are made of two micro-fabricated silicon nitride electron transparent thin films. This liquid cell consists of liquids normally exceeding 100 nm in thickness encapsulated by SiN windows that are several tens of nanometer in thickness as well, and thus are unsuitable for atomic scale observations. Graphene-based liquid cell electron microscopy (GLC) has demonstrated a better imaging resolution3 (Fig 1). In this study, we optimized graphene-based liquid cells to realize in real time atomic scale observations of nucleation and growth of Platinum (Pt) nanoparticles (NPs) and the formation of Pt-Pd core-shell NPs in liquid mode. We investigate the exact growth mechanisms of platinum NPs from single atoms to final crystals by in-situ liquid phase STEM at atomic-scale. After nucleation, we show that the nanocrystals grow via two main stages: atomic attachment in the first stage, followed by the second stage of growth, which is based on particle attachment by different atomic pathways (Fig 2). In addition, we investigated the exact atomic mechanisms underlying the growth of Pt-Pd core-shell NPs, where growth mechanisms of the Pt shell on Pd nanocubes are studied in aqueous solution at the atomic level. We found that Pd-Pt core-shell NPs are formed via two distinct mechanisms: (i) at low concentration of Pt atoms, an ultra-thin skin of only a few atomic layers is formed via atom-by-atom deposition and (ii) at higher concentration of Pt atoms, inhomogeneous islands and thick shells are formed via attachment of Pt clusters (Fig 3).3, 4 Figure 1: Encapsulated Pt precursor with GLC: (a) schematic illustration, (b) formation of Pt atoms in liquid. Figure 2: Nucleation and growth of nanoparticles: (a) ADF-STEM time lapse and (b) schematic illustration. (Reproduced from ref. 3) Figure 3: Pd NCs core and Pt shell STEM analysis. Pd nanocube (a) with thin layer of Pt shell (b) with Pt shell islands. (Reproduced from ref.4) REFERENCES 1Unocic, R. R. UMicrosc. Microanal 2015, 21, 1972-1973 2Alloyeau, D.; Dachraoui, W.; Javed, Y.; et al. Nano Letters 2015, 5, 2574-2581 3Dachraoui, W.; Henninen, T. R..; Keller D.; Erni, R. Scientific Reports 2021, 11, 23965 4Dachraoui, W.; Bodnarchuk, M. I.; Vogel, A.; Kovalenko, M. V.; Erni, R. Applied Physics Reviews 2021, 8, 041407

MS 1: Energy-related materials and catalysts

Abstract talk

Investigation of the underlying mechanism of the Ag-induced layer exchange by in situ studies in SEM and TEM

Abstract text (incl. figure legends and references) Thin film semiconductors are indispensable in modern technology and a key element for future applications. In particular, highly crystalline thin film semiconductors are of great interest due to their outstanding performance by reducing the detrimental influence of grain boundaries [1]. Preparing such thin film crystalline semiconductors on foreign (amorphous) substrates can even improve their performance and enable future applications like flexible devices. However, the fabrication on amorphous and/or heat-sensitive substrates are quiet challenging and expensive. A smart approach is the crystallization of the semiconductor directly on the needed substrate. Here, the metal-induced layer exchange (MILE) process offers a direct crystallization route by contacting the amorphous semiconductor with a metal layer, which decreases dramatically the necessary crystallization temperature [2]. Applications of the MILE process are widespread due to the great variety of semiconductor – metal systems. Although this process has been known for a long time, the mechanism behind it is still under debate [3]. In our work, we investigated the layer exchange (LE) process within the silver-silicon system (AgILE). Especially, the early stages before the initial LE starts are studied by in situ microscopy. The sample composition comprises a poly-crystalline Ag layer on the substrate (SiN), an oxide layer and an amorphous Si top layer. During heating, both layers change their position and the Si crystalizes (illustrated in Figure 1a). Pre-characterization and analysis of the process dynamics are performed by in situ light microscopy (see Figure 1b) and ex situ electron microscopy (see Figure 1d). In situ annealing experiments in SEM are performed by our custom-built chip-based heating stage (illustrated in Figure 1c) [4]. Especially, STEM High Angle Annular Dark-Field (HAADF) imaging facilitates the discrimination between Ag and Si due to the Z-dependency of the signal. To study the occurring reactions in more detail, additional heating experiments in the TEM are executed. Both heating experiments (700 °C) revealed similar reactions before the initial LE occurs. Figure 2a-d displays the observed reactions in the SEM. After a short initial annealing time, Ag push-ups appear followed by a second reaction, where a new Si-rich phase grows within the appeared Ag push-ups. Afterwards, the LE starts exactly at one previously occurred push-up and grows subsequently through the whole sample. The final state consist of pure Si-crystals (black), reacted area (dark gray) and some unreacted regions (bright). To gain insights into the inner sample structure, lift-out analysis and non-destructively electron tomography are performed. The results of the lift-out samples revealed a crystalline Si phase in the bottom layer, whereas the top layer consist of initial amorphous Si and pushed-up Ag grains (see Figure 2e). Figure 2g indicates the Ag is pushed up by the lateral growing c-Si. Interrupted in situ heating experiments in the TEM were performed to acquire a tilt series of the present sample state. The resulting 3D tomograms verify the distribution of Ag and Si in both layers (see Figure 2f). Accordingly, we propose a refined model of the AgILE process due to our detailed microscopic study and observed reactions [5]. [1] 10.1063/1.91832 [2] 10.1088/1361-6463/ab91ec [3] 10.1002/adem.200800340 [4] 10.1016/j.ultramic.2020.112956 [5] Financial support by DFG via GRK

IM 3: SEM and FIB developments

In-situ correlation of the anomalous Hall effect with the occurrence of topological and non-topological magnetic phases in Mn1.4PtSn

Abstract text (incl. figure legends and references) Introduction (Anti-)skyrmions are potential future nanoscale information carriers, since they can be electrically manipulated and detected. So far, Hall effect measurements on skyrmionic samples are conducted on samples with different thicknesses and confinements as compared to those used for magnetic imaging in a transmission electron microscope (TEM) [1,2], which has proven extremely valuable for unveiling the details of skyrmionic spin textures even in 3D [3]. Since the stability of (anti-)skyrmions depends sensitively on the sample geometry, a correlation of magneto-transport and TEM data are problematic, if not conducted on identical samples. Objectives We have therefore devised an in-situ measurement platform that bridges this gap and allows for the conduction magneto-transport measurements in-situ in a TEM. The current investigation aims at correlating the anomalous Hall effect in the Heusler compound Mn1.4PtSn with the magnetic field dependent occurrence of non-topological (NT) and topologically protected magnetic phases such as the helical phase, NT bubbles and anti-skyrmions. Materials &amp; methods In-situ Lorentz-TEM (L-TEM) investigations were conducted on a JEOL F-200 micro&shy;scope (200 kV, cold FEG) equipped with a Gatan OneView camera. A Protochips Fusion Select holder with gold-plated spring contacts connected to electrical feedthroughs is used to measure the anomalous Hall effect. A TEM lamella was cut from a Mn1.4PtSn single crystal [4] using a focused ion beam system (FIB), deposited on a measurement chip with a SixN window, and soldered to the pre-defined contact pads by Pt deposition in the FIB. In Lorentz mode, the objective lens of the microscope was used to apply a magnetic field perpendicular to the sample plane. Acquisition of L-TEM images and concurrent measurements of the Hall voltage as function of the magnetic field were conducted automatically utilizing Python scripting. Results The L-TEM-images in Fig. 1 reveal that upon increasing the magnetic field B (i) the density of helices (as manifested by a stripe contrast) is reduced and (ii) the magnetization process is governed by growth of (preferred) helical domains – very alike that of conventional ferromagnets. At fields above roughly B = 300 mT, where the Hall voltage exceeds the otherwise linear increase with B (cf. Fig. 2), we observe (i) a transition from a continuous to a discontinuous helical phase and (ii) finally the formation of NT bubbles and/or anti-skyrmions, which then slow down the approach towards saturation. The formation of anti-skyrmion lattices was, however, only observed after subsequent variations of the magnitude and direction of B. The subtle effect of the latter on the Hall effect of the sample is subject of ongoing research and will be discussed. Conclusion Our new setup allows us for the first time to follow in detail the field dependence of the Hall voltage while simultaneously monitoring the magnetic phases in Mn1.4PtSn, thereby providing valuable insights into the existence and nature of an intensely debated electrical signature of skyrmionics structures. A.T. and B.R. gratefully acknowledge financial support by the DFG within SPP 2137. M.W. thanks the Max Planck Society for funding through IMPRS-CPQM. [1] M. J. Stolt et al., Adv. Func. Mater. 2019 (2019) 1805418. [2] R. Schlitz et al., Nano Lett. 19 (2019) 2366. [3] D. Wolf et al., Nature Nanotechnology 17 (2021) 250. [4] P. Vir et al., Chem. Mater. 31 (2019) 5876.

MS 3: Low-dimensional and quantum materials

Enabling In situ TEM biasing of Two-terminal Oxide-based Nanodevices using MEMS-based chips and gas cells

Abstract text (incl. figure legends and references) Introduction: Materials characterization of two-terminal oxide-based electronic devices at the nano and sub-nanometer scale in operative conditions is undoubtedly reliant on existing in situ transmission electron microscopy (TEM) methods. However, electrical biasing experiments using MEMS-based platforms have been highly challenging up-to-date due to the complex Focused Ion Beam (FIB)-based methodologies involved, in which stray leakage current paths formed during TEM lamellae preparation, might lead to over-estimated electric responses and therefore, ambiguous in situ observations. Moreover, the electrical properties of oxide-based stack devices in actual conditions are strongly influenced by, for instance, gas flow and/or temperature [1]. Hence, a systematic in situ TEM investigation of leakage current in operando conditions is crucial to understanding device behavior at the micro and nanoscale. Objectives: To reproduce the electrical responses of micrometer-sized oxide-based stack devices in the corresponding TEM lamella devices using a short-circuit-free FIB-based preparation approach. To establish a structure (chemical)-property correlations in operando conditions (i.e., gas and/or temperature) with the use of in situ (S)TEM techniques. Materials and Methods: Micrometer-sized epitaxially grown SrTiO3 (STO)-based memristors and BaSrTiO3 (BST)-based high-performance capacitors have been grown by Pulsed Laser Deposition (PLD). TEM lamellae of both types of heterostructures were prepared using a Focused Ion Beam (FIB). The TEM lamellas were mounted on DENSsolutions MEMS-based chips for biasing, heating, and gas experiments. High-angle annular dark-field (HAADF)-STEM, bright field (BF)-STEM, and electron energy loss spectroscopy (EELS) data were obtained while operating TEM lamella devices under gas and biasing stimuli. Results: Our short-circuit-free FIB methodology yielded current measurements of TEM lamellae devices as low as 10-12 A, mimicking the expected micrometer-sized electrical response. Operative TEM lamellae-based memristor devices could be measured inside the microscope. Additionally, high-performance capacitors (varactors) with high leakage currents have been exposed to an O2-rich environment and heated by the effect of electron beam irradiation while simultaneously tracking the suppression of the device leakage current inside a TEM (Fig. 1). Conclusions: Our methodology enables current measurements e.g., STO-based memristors of at least six orders of magnitude in leakage current in comparison with conventional-FIB approaches and reported literature values [2], allowing for realistic and reproducible biasing operations of various oxide-based TEM lamella devices inside a TEM under heating or gas stimuli. References:[1]L. Zeinar et al., &quot;Matching conflicting oxidation conditions and strain accommodation in perovskite epitaxial thin-film ferroelectric varactors,&quot; Journal of Applied Physics, vol. 128, no. 21, p. 214104, Dec. 2020, doi: 10.1063/5.0021097. [2] D. Cooper and M. Bryan, &quot;Reproducible in-situ electrical biasing of resistive memory materials using piezo-controlled electrical contacts and chip-based systems.,&quot; Microsc Microanal, vol. 27, no. S1, pp. 164–166, Aug. 2021, doi: 10.1017/S1431927621001197. Fig. 1. Suppression of the leakage current of a TEM-lamella extracted from a BST-varactor device exposed to an O2 environment inside a TEM. A systematic electron beam irradiation (heating) study was performed.

IM 7
In situ/operando electron microscopy

Appointment

Session chairs

Description

Programme

IM 7In situ/operando electron microscopy

Appointment

Session chairs

Description

Programme

IM 7
In situ/operando electron microscopy