Microscopy Conference 2023

Session chairs

Prof. Dr.

The revival of polymers, organic molecular/carbon-rich materials and composites thereof is not only due to the development of high-efficiency (semi)conductors and their enhanced environmental stability. Basic reactants appear unlimited prospectively yielding sustainable products at globally needed amounts. Those materials already contribute to the global energy revolution and facilitate a vast variety of applications such as energy conversion/storage, (flexible) printed electronics, catalysis, rapid prototyping, lightweight construction as well as drug delivery. The serious issue of microplastics in the environment needs to be addressed, e.g., by means of biodegradable material design.
This session will cover novel micro- and nanocharacterization techniques (like 4D-STEM, UHR-EELS, cryo-EM, X-ray micro-spectroscopy, fluorescence OM) and modern sample preparation (like plasma-FIB cross-sectioning, (cryo-)ultramicrotomy, plunge freezing) to investigate individual materials (molecules, thin films, particles, fibers), composites and complex devices for understanding structure formation, function, and degradation. It includes modern in situ methodology stressing strategies to minimize electron-beam induced alterations.


Characterization of organic materials using four dimensional-scanning transmission electron microscopy (4D-STEM)

MS 5: Functional organic materials

IM 6: Phase-related techniques & 4D STEM

Abstract text (incl. figure legends and references) Organic materials remain challenging to characterize in electron microscopy due to their weak scattering from and high dose sensitivity to the electron beam. This talk will show how advances in detector technology and computational imaging have enabled four dimensional- scanning transmission electron microscopy (4D-STEM) characterization of functional organic thin films [1]. By carefully tuning the size of the converged electron probe, the distance between adjacent measurements, and the total exposure of the electron beam, we can maximize the amount of measurable information. This talk will show experimental examples characterizing the morphology of crystalline and semi-crystalline polymers and other organic thin films. It will also show several methods which can improve 4D-STEM characterization including cryogenic cooling, custom probe-forming apertures, 4D-scanning confocal electron diffraction (4D-SCED), and using machine learning methods to invert noisy and nonlinear diffraction measurements. The primary goal of this research is to measure structural and morphological material properties from weakly-scattering and beam sensitive organic molecules. These properties include the degree of crystallinity, crystalline phase and orientation, and degree of short and medium range order present. We aim to map these properties over the functional length scale of the materials in question, where the field of view may range from nanometers to hundreds of micrometers. Our experiments are performed using several FEI/ThemoFisher Titan and Themis platform STEM instruments. We typically use 300 kV accelerating voltage, and probe convergence semiangles from 0.1 to 4 mrads. We also make use of custom amplitude apertures in the probe-forming plane [2]. Our electron detectors range from scintillator-based cameras such as the Gatan Ultrascan, and direct electron detectors such as the Gatan K2 and K3. We also show results from the Berkeley Lab 4D Camera operating at 87 000 images/second [3]. The 4D-STEM experimental geometry and an example 4D-STEM sdataset is shown in Figure 1. We also demonstrate alternative experimental geometries such as 4D-SCED which can minimize the electron beam damage induced by adjacent probe positions measurements [4]. Thick samples may also produce significant amounts of multiple scattering of the electron beam. In order to perform accurate measurements of these complex nonlinear diffraction signals at low electron dose, we have implemented machine learning methods [5]. Finally, all of our methods and datasets are freely available in the py4DSTEM open source analysis toolkit [6]. Figure 1 - 4D-STEM experimental geometry showing diffraction patterns measured at various probe positions for a small organic molecule sample. In each diffraction pattern, the pi-pi stacking direction(s) can be identified from the primary Bragg peak positions. The reconstructed sample morphology is shown above as an orientation map. Adapted from [7]. [C1] C Ophus, Microscopy and Microanalysis 25, 563 (2019). [C2] S Zeltmann et al., Ultramicroscopy 209, 112890 (2020). [C3] P Pelz et al., IEEE Signal Processing Magazine 39. 25 (2021). [C4] M Wu et al., Nature Communications 13, 1 (2022). [C5] J Munshi et al., arXiv:2202.00204 (2022). [C6] B Savitzky et al., Microscopy and Microanalysis 27, 712 (2021). [C7] K Bustillo et al., Accounts of Chemical Research 54, 2543 (2021)

Invited talk

Microscopy escorting the process from polymer synthesis to membranes fabrication

Abstract text (incl. figure legends and references) Tuning the morphology of block copolymer structures is an essential task in the development of polymer membranes.1 This is even more important when thermoresponsive block such as the poly((dimethyl acrylamide)-co-(isopropyl acrylamide)) as a copolymer with polystyrene (P(DMA-co-NIPAM)-b-PS) is involved. Such membranes are formed via a non-equilibrium self-assembly process in solution. In order to observe all the steps of the process in detail, from the synthesis to the membrane fabrication, a bundle of microscopic methods are necessary to be employed. From the formation of micelles during emulsion polymerization, via the morphology in the polymer bulk to the membrane&quot;s pore structure, microscopy is required to complement and confirm integral analysis methods. Starting from the polymer fabrication,cryoTEM was employed to observe micelles in the emulsion reversible addition fragmentation chain transfer (RAFT) synthesis of the block copolymer, a precursor for structure formation during the solidification of the membrane.2 Following that, TEM and AFM shed light onto the P(DMA-co-NIPAM)-b-PS bulk morphology as a first indicator for the final membrane structure. SEM investigations enable us to judge the quality of the final membrane. The images confirm the dynamic light scattering results regarding size and thermoresponsiveness, while also identifying the fabrication parameters for the final membrane. The structure of these thermoresponsive membranes would not have been identified without electron microscopic and atomic force microscopic imaging throughout their development. 1Müller and Abetz. Chemical Reviews 121 (22), 2021, p. 14189. DOI: 10.1021/acs.chemrev.1c00029 2Nieswandt et al. Polymers 14 (1), 2022, p. 62. DOI: 10.3390/polym14010062

Abstract talk

Prof.

Enhancing the photocatalytic performance of carbon nitrides through controlled local structure modification

MS 1: Energy-related materials and catalysts

Abstract text (incl. figure legends and references) Carbon nitrides are the most studied sustainable metal-free photocatalysts: they can be synthesized by simple routes from abundant organic precursors, they are visible-light-active as well as thermally and chemically stable in photocatalytic reactions. However, most of them are poorly crystalline, which leads to a high recombination rate and difficulties in precise control and adjustment of their band structure. Polyheptizine imides (M-PHIs) salts are one of the most crystalline carbon nitrides. They have high charge-separation efficiency, enabling storage of photogenerated electrons and the realization of so-called dark photocatalysis. In this work we show how the controlled modification of the local structure, obtained by varying the sizes and charges of metals as well as introducing a rotation between 2D heptazine layers, influences the energy band structure and allows to enhance the photocatalytic activity of M-PHIs. To realize this goal, we combined X-ray powder diffraction (XRD), low-dose HRTEM imaging, energy filtered electron radial distribution function (EF-eRDF) analysis obtained from rotational electron diffraction data and VEELS data with photocatalytic performance on M-PHI (M = Na, K, Mg, H) during the hydrogen evolution reaction (HER). From low-dose HRTEM and XRD data and general assumptions about the structure from earlier works starting models for crystal structure refinement were proposed. PHI salts were found to crystalize in a trigonal unit cell; their idealized structure consists of heptazine units that are placed on top of each other in AAA stacking, forming continuous channels along the c direction. Based on XRD only we could not locate metal atoms uniquely in Mg-PHI and Na-PHI, thus we proposed 2 possible structural models. In order to find an unambiguous solution, we compared short-range order and the degree of crystallinity in different M-PHIs by performing an EF-eRDF analysis using rotational SAED data. The latter allowed to compensate for the preferential orientation of domains found in the M-PHI flakes. Fitting of EF-ePDF of Mg and Na salts showed that only one model with metal cations equally distributed between interlayer positions and bridging nitrogen atom positions in the channels is correct. The coherence lengths increase in the series H-, Na-, Mg-, K-PHI (reaching ~30 Å), indicatingan increase in crystallinity (Fig.1). The ePDF curves of Na- and Mg-PHI display broadening of the peaks beyond 8 Å pointing to a less pronounced fine structure compared to K-PHI, which is related with twists of layers found in these compounds.The rotation of layers by a random (usually only 2-8°) angle leads to the formation of "flower-like structures", while at specific rotation angles Moiré pattern lattices are formed. DFT calculations showed that both defects modify the band structure of these materials and should influence the HER. Indeed, in tests as photocatalysts for the HER, Na- and Mg-PHI showed the best performance with the latter reaching an unprecedented quantum efficiency of 7.14%. This was attributed to the higher charge of Mg2+ compared to all other cations (Na+, K+, H+), which leads to a more efficient polarization of the heptazine backbone and the promotion of the cleavage of the H-O bond during the water splitting as well as a considerably faster interlayer charge transfer in the presence of rotational defects. We gratefully acknowledge ﬁnancial support by the Max Planck Society.

Revealing interfacial widths and line-edge roughnesses in cylinder-forming block copolymers using low keV analytical transmission electron microscopy

MS 4: Functional thin films

Abstract text (incl. figure legends and references) Lithography based on the self-assembly (SA) of block copolymers (BCP) is a competitive alternative to extreme UV lithography applied in semiconductor industries, since it allows to create regular arrays of motives with sub-ten nanometre feature size economically and on large areas [1]. During BCP-SA two blocks of immiscible polymer species, A and B, which are covalently bond to each other in a molecular chain, undergo a microphase separation. Depending on the BCP chain length ratio, this can result in various motives, e.g. hexagonally arranged cylinders of block A in a matrix of block B. For further lithography processes one of the blocks is selectively removed. Since the features after processing are a direct replica of their lithographic mask, the abruptness of the microphase interfaces (interfacial width) in the phase separated BCP film [2] and the line edge roughness (LER) of the resulting mask are crucial but difficult to measure. Previous studies have employed analytical techniques that either only allow to analyse the surface of the BCP mask or average the properties over a large area or provide comparatively low spatial resolution. However, future progress in BCP lithography requires high-resolution information on the internal interfaces of microphases at or below the single-nanometre scale. Here, we investigate the quality of the BCP masks by employing advanced analytical (scanning) transmission electron microscopy ((S)TEM) at 60 kV in combination with XPS and PM-IRRAS to derive information about the size of the polymer domains, the interfacial width and LER, as well as the chemical changes occurring upon mask development using different polymer removal techniques. BCP lithography masks are made from cylinder forming BCP consisting of the two polymer species polystyrene (PS) and polymethyl methacrylate (PMMA). Membranes of as-phase-separated as well as developed BCP films are studied, the latter after either wet chemical UV/acetic acid or Ar/O2 plasma treatment. Films are fabricated on a sacrificial SiO2 substrate which for TEM preparation is removed by HF etching, followed by skimming of the BCP membrane with a Quantifoil TEM grid. Domain sizes, interfacial widths and LERs are calculated from high-resolution ADF STEM images using a self-written MATLAB application which is based on the analysis of angularly resolved radial intensity profiles of individual pores. This allows to monitor the LER of cylindrical mask openings at unprecedented, sub-nm resolution in different stages of the process chain and to detect the presence of pockets of the opposite polymer species protruding through the PS-PMMA interfaces. In addition, energy-filtered TEM element and thickness maps are acquired to analyse changes in chemistry and thickness of the BCP masks and it is shown that Ar/O2 plasma treatment, while leading to smoother interfaces, also results in oxidised polymer surfaces [3]. [1] K. Brassat, J.K.N. Lindner, Advanced Materials Interfaces 7.5 (2020): 1901565. [2] J. Bürger et al., Nanomaterials 10.1 (2020): 141. [3] J. Bürger et al., Advanced Materials Interfaces (2022), accepted.

Understanding novel carbon-based sensor architectures by advanced STEM-EELS and 4D-STEM

Abstract text (incl. figure legends and references) Nitrogen-doped carbon materials (NCM) have emerged as cost-effective, environmentally friendly, and readily available starting materials for gas sensing device fabrication. They allow for the systematic tuning of properties for superior selectivity and sensitivity to specific analytes (volatile organic compounds and specific gases) by carefully choosing the precursor materials. CO2 sensing is one of the major applications where such materials have been implemented. So far, miniaturized CO2 sensors are based on sophisticated nanomaterials like selected, doped carbon nanotubes [1]. In this study, a one-step laser patterning procedure is applied to an optimized ink coating to directly generate complete and highly porous sensor architectures (~50 micrometer overall thickness) even on flexible PET substrates. The versatile ink is based on abundant organic precursors like adenine (nitrogen source for sensing functionality) and glucose as a pore-forming agent. Thermal laser treatment of the precursor material in an oxygen-containing environment allows us to fabricate complete sensor heterostructures with defined nitrogen and oxygen functionalities. The attenuation of the laser irradiation is depth dependent and results in the formation of a defined graphitic surface layer and a nitrogen-rich lower region (sensing layer) separated by a transition region. To understand the structure formation by laser treatment as well as device functionality, a thorough scale-bridging investigation was conducted. Microtomic cross sectioning is employed to prepare high quality TEM cross-sections (0.25 - 0.5 t/λ) [2]. Elemental distribution maps from STEM-EELS data depicted a clear separation between the upper and lower layers. STEM-EELS chemical and bond analysis across the whole device allowed the clear differentiation between distinct amorphous and crystalline phases, and thus between different functional layers of the heterostructure. Therefore, principal component analyses were applied due to the complexity of the laser-generated material. The distribution of the crystalline graphitic phase, the degree of ordering graphitization and the alignment of the basal planes were investigated in detail by 4D-STEM (see Fig 1.)as those parameters impact on the electrical conductivity of the sensor. In addition, the data analysis method employed, sparse correlation matching, as implemented in py4DSTEM [3] facilitates the detailed analysis of the structural peculiarities of the amorphous phases. Keywords: laser-patterned CO2 sensor, graphitic, nitrogen-doped carbon Acknowledgements: We acknowledge funding support from the Fonds der Chemischen Industrie and the Max Planck Society, use of the DFG-funded Micro-and Nanoanalytics Facility (MNaF) at the University of Siegen (INST 221/131-1) and the National Center for Electron Microscopy (NCEM) in the Molecular Foundry (MF) at the Lawrence Berkeley National Laboratory (LBNL) supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 [1] Inagaki, Michio, et al. Carbon 2018, 132, 104-140. [2] Hepp, Marco, et al. npj Flexible Electronics 2022, 6.1, 1-9. [3] Ophus, Colin, et al. Microscopy and Microanalysis 2022, 28.2, 390-403.

Advanced characterization of NP@MOF core-shell complexes using real-time electron tomography

IM 1: Progress in instrumentation and ultrafast EM

Abstract text (incl. figure legends and references) Metal-organic Frameworks (MOFs) are a diverse class of materials consisting of metal clusters and organic linkers arranged into a porous crystalline lattice. Due to their porosity, MOFs possess some of the largest surface areas of any known material. This is exploited for gas adsorption, catalysis and drug delivery. For drug delivery, a drug is incorporated within the MOF and release is initiated by stimuli induced MOF decomposition. Recent efforts have focused on incorporating stimuli responsive nanoparticles (NP) into the MOF, to induce thermal decomposition as a delivery mechanism. The morphology of these NP@MOF complexes is critical to their overall efficacy for drug delivery. [1] Electron tomography (ET) is invaluable for characterizing their 3D morphology. However, ET requires long exposure times and high electron dosages which cause artefacts during imaging of NP@MOFs. Limiting beam exposure can be achieved by limiting the number of images acquired. Unfortunately, reducing the image number also induces undersampling artefacts. Hence, an optimum image number must be obtained. Optimization could be performed by comparing multiple 3D reconstructions acquired with variable image numbers, a time-consuming process. Furthermore, since each acquisition must be performed on a new sample due to beam exposure from the previous acquisition, reconstructions are not directly comparable and a microscopist must perform a qualitative judgement with uncertain reliability. [2] Herein a protocol was propsed to estimate the optimum number of images from a single acquisition by combining a non-termination acquisition protocol, golden ratio scanning (grs) with real-time reconstruction. The protocol was tested using simulations and an experimentally acquired Au/Pd NP. Finally, the value of this procedure was highlighted by application to NP@MOF samples of ZIF-8 and NU-1000 containing Au nanoparticles. [3] Fig. 1. In ET , images are collected (a) incrementally from the lowest angle to the highest angle. E.g. image 1 is collected at -70°, image 2 collected at -65° etc. Early termination results in a missing wedge. By collecting images using (b) grs, the ,issing wedge is filled mostly in the first images and subsequent images improve sampling resolution. Real time reconstruction is used to monitor reconstruction quality and terminate acquisition. For Au/Pd@ZIF-8 viewed along the xy, xz and yz planes, undersampled (10 images) and oversampled (27) reconstructions have artefacts (red) not apparent in the determined optimum (13 images). References [1] Wang, L., Zheng, M., and Xie, Z. (2018) Nanoscale metal–organic frameworks for drug delivery: a conventional platform with new promise. Journal of Materials Chemistry B, 6 (5), 707–717 [2] Vanrompay, H., Béché, A., Verbeeck, J., and Bals, S. (2019) Experimental Evaluation of Undersampling Schemes for Electron Tomography of Nanoparticles. Particle and Particle Systems Characterization, 36 (7), 1–8. [3] This project receives funding from the European Union"s Horizon 2020 research and innovation programme grant agreement No 860942 (HEATNMOF).

MS 5
Functional organic materials

Termin

Session chairs

Beschreibung

Programm

MS 5Functional organic materials

Termin

Session chairs

Beschreibung

Programm

MS 5
Functional organic materials