Friday lecture | 8 January 2021

Simultaneous analysis of ABF-ADF STEM images: quantification and atom detection

by: Jarmo Fatermans, Emat

Practical

Abstract 
For high-resolution annular dark-field (ADF) scanning transmission electron microscopy (STEM) images, techniques for quantification of the material structure and automatic and objective detection of the atomic columns have been developed. The quantification and detection of light-element atomic columns from ADF STEM images, though, is challenging since light elements only scatter electrons weakly to high detector angles. Therefore, a simultaneous acquisition of both annular bright-field (ABF) and ADF STEM images is an interesting option to investigate atoms of a large range of atomic numbers since ABF STEM offers improved light-element visualization in the presence of heavy elements. In this talk, quantitative methods based on statistical parameter estimation theory and model-order selection are applied to simultaneously acquired ABF and ADF STEM images. For this, an extension of the commonly used parametric models in single-image STEM is proposed, hereby taking the effect of specimen tilt into account. Image formation in ABF STEM is indeed sensitive to tilt of the electron beam with respect to the crystal zone axis. Using simulations as well as experimental data, it is shown that the proposed methodology can be successfully used to investigate light elements in the presence of heavy elements by simultaneously quantifying ABF and ADF STEM images.

Friday lecture | 15 January 2021

Following structure evolution of SrFeOx using in situ 3D electron diffraction (3DED) experiments

by: Maria Batuk, Emat

Practical

Abstract 
Strontium iron oxide is a candidate for many different energy applications, including solid oxide fuel cells, chemical looping and thermochemical energy storage. Upon the redox reactions, SrFeOx cycles between two end forms: an oxygen deficient form SrFeO2.5 with a brownmillerite structure and an oxidized form SrFeO3-d with a perovskite structure. Two intermediate structures are reported from ex situ and in situ X-ray and neutron powder diffraction. However, in real applications, submicron sized crystals are used and X-ray and neutron diffraction techniques are not able to access structural information on an individual submicron crystal. In situ 3D electron diffraction (3DED) is the only way to obtain single crystal data on all structural changes occurring during the actual redox reactions. Due to the single-tilt design of the environmental holders combined with the complexity of these structures, in-zone electron diffraction and high resolution imaging on random crystallites are unrealistic, but 3DED does not require in zone orientation and could thus be successfully applied to gather structural data on the different phases.

Using the TEM Climate holder from DENSsolutions, we performed a series of experiments in different conditions to monitor the phase transition from brownmillerite SrFeO2.5 to perovskite SrFeO3-d and back, and observed several intermediate structures. We were able to derive the different structures occurring at the different oxidation steps.

Friday lecture | 22 January 2021

FIB-SEM on polyolefins: influence of different ion beam conditions

by: Maciej Paśniewski, ExxonMobil Chemical Europe Inc. / Emat

Practical

Abstract 
Polyolefins are used in a multitude of applications, from food packaging to light-weightingvehicles, improving daily life of billions of people. Microscopy plays a crucial role in a betterunderstanding of the structure-property relationship by providing insight into themicrostructure. Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM) is an importanttool for in-depth analysis of the microstructure at the nanoscale; it can also be used to preparecross-sections with nanometer precision for other characterization techniques, such asTransmission Electron Microscopy (TEM) or Atomic Force Microscopy (AFM).Interaction of matter with ions brings concerns regarding the nature of the newly createdsurfaces and how representative they are of the bulk phase, especially for such beam-sensitivematerials as polyolefins. This project aims to assess the nature and extent of the surface damage,the fundamental processes involved, and ways to minimize the damage.Initial analyses with AFM and Time of Flight-Secondary Ion Mass Spectrometer (ToF-SIMS)performed on FIB'd surfaces show significant alteration on the new surface.The studies performed so far include preparation of cross sections of polypropylene (PP) withfive different focused ion beams (Ga, Ar, N2, O2 and Xe) with varying energies. The preparedsurfaces were then studied by ToF-SIMS to analyse the molecular structure (atoms and moleculefragments). New experiments on PP, polyethylene (PE) and poly(methyl methacrylate) (PMMA)are underway.

Friday lecture | 29 January 2021

Cultural heritage materials: chemical imaging at multiple length scales

by: Koen Janssens, AXES Research group, NANOlab Centre of Excellence, University of Antwerp

Practical

Abstract
In the AXES Research group, new instruments and methods are developed and use for non-destructive investigation of works of art, notably oil paintings. These ‘chemical imaging’ tools allow to inspect and analyze painted works of art in ways not possible circa one decade ago. To develop them, in several stages, transfers of technology between the field of materials characterization, based on X-ray fluorescence and X-ray diffraction towards the art/museum world were realized. This also involved a change in imaging scale: from the nano/micrometre to the decimetre/metre scale, a notion that will be illustrated by means of Rembrandt’s 17th C. masterpiece: The Nightwatch.

The new tools produce hyperspectral data cubes containing information that proves very useful in various art-related activity fields such as (technical) art history and art conservation, giving rise to new insights or resolving age-old controversies on how artists worked. A few examples involving artworks from the 15-19th century, created by well-known artists such as Peter Paul Rubens (17th C.), Francisco de Goya (18th C.) and Jackson Pollock (20th C.) will be briefly discussed to illustrate this. Also new information on historic pigment-manufacturing technologies, such as those employed by the painters Jan Van Eyck (15th C.) and Johannes Vermeer (17th C.) can be extracted.

One of the phenomena catching our attention is the spontaneous degradation of pigmented materials. Artist’s paint in all historic periods is a strongly heterogeneous assembly of organic and inorganic components and their mutual interaction, especially when photo-activated by impinging light, can lead to undesired chemical transformations, including redox reactions. By combining macro-level imaging with micro- and nanolevel analysis and with theoretical calculations it possible to better understand these phenomena and pinpoint analogous systems in other fields. As specific example, the reactivity of CdS-based paints in artworks by 19-20th century painters James Ensor, Vincent Van Gogh and Edvard Munch will be discussed.

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Please also find these fresh news articles and videos on the exciting ongoing work of Prof. Janssens and the AXES group that you can peruse before his presentation:  

Friday lecture | 5 February 2021

Marry 2D crystallization and thin-film polymorphism: case study for PbPc

by: Yansong Hao, Emat

Practical

  • Location (online link)
  • Time: 11.30

Abstract
Polymorphism represents the ability of elements or compounds to crystallize into different forms. These crystal forms are named as polymorphs and they normally differ in atomic arrangements or molecular packing. The study of polymorphism becomes very popular as it is closely related to the physical properties of the materials, such as solubility, mechanical properties and electronic properties. Among all types of crystals, polymorphs of organic molecules are abundantly diverse. This is because, within crystals, organic molecules interact often through weak van der Waals interactions. Up to now, controlling crystallization of organic molecules into the desired polymorph or predicting possible polymorphs is still quite challenging. 

Practically, most of the crystallization happens on the substrate, which gives rise to the heterogeneous nucleation. Beyond well-known catalyzing effect, some studies have reported that molecular materials could crystallize into new polymorphic forms only in the vicinity of the substrate and these new forms are called substrate-induced polymorphs (SIPs). Typically, SIPs extend over several molecular layers above the substrate. This makes it different from another conceptually connected research line: self-assembly monolayers (SAMs) of organic molecules. the SAMs of organic molecules have been studied extensively in the last decades. However, the exact connection between the SAMs and SIPs for organic molecules is still unclear. 

The compound of interest for this study is lead phthalocyanine (PbPc). We investigated the SIP of PbPc on highly oriented pyrolytic graphite (HOPG) by combining experiments and multiscale computational-chemistry modelling. Firstly, self-assembled PbPc molecules at solution/HOPG interface were studied by both in-situ scanning tunneling microscope (STM) and atomistic modelling. Next, the SIP of PbPc up to several molecular layers was built on top of the assembly and the modelled SIP agreed well with experimental observations. It is revealed that the SAM acts as a bridging layer between the substrate and the thin-film crystal. Therefore, the crystal structure of SIPs is templated by the SAMs. 

Furthermore, a thin film of PbPc has been prepared by physical vapor deposition and characterized by transmission electron microscopy (TEM). Exit wave reconstruction shows nicely resolved atomic columns in the reconstructed phase, which proves the great capability of TEM in imaging soft organic crystals. Currently, thinner samples are being prepared and a better match between experiments and modelled SIPs is promising.