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Stream 1: EMAG - 2D Materials 10:00 - 12:00 Tuesday, 6Th July, 2021 Sessions EMAG Conference Session Session Organiser Andy Brown, Sarah Haigh

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Stream 1: EMAG - 2D Materials 10:00 - 12:00 Tuesday, 6Th July, 2021 Sessions EMAG Conference Session Session Organiser Andy Brown, Sarah Haigh Stream 1: EMAG - 2D Materials 10:00 - 12:00 Tuesday, 6th July, 2021 Sessions EMAG Conference Session Session Organiser Andy Brown, Sarah Haigh 10:00 - 10:30 349 Atomic Imaging in 2D Material Heterostructures : Twist, Defects and Particle Synthesis Prof Sarah Haigh1, Dr Nick Clark1, Astrid Weston1, Dr Daniel Kelly1, Dr Matthew Hamer1, Dr Yichao Zou1, Dr Vladimir Enaldiev1, Dr Alex Summerfield1, Dr Victor Zólyomi1, Prof Denis Gebauer2, Prof Vladimir Falko1, Dr Roman Gorbachev1 1National Graphene Institute, University of Manchester, Manchester, United Kingdom. 2Institute of Inorganic Chemistry, Leibniz Universität Hannover, Hannover, Germany Abstract Text This talk aims to demonstrate how atomic resolution scanning transmission electron microscope (STEM) imaging is being used in Manchester to support and enable the development of 2D materials and their heterostructures. The possibility to create new ‘designer’ materials by stacking together atomically thin layers extracted from layered materials with different properties has opened up a huge range of opportunities, from new optoelectronic phenomena [1], modifying and enhancing electron interactions in moiré superlattices [2], to creating a totally new concept of designer nanochannels for molecular or ionic transport [3]. The impressive progress being achieved in the field crucially depends on knowledge of the atomic structure of these heterostructures [4], which in many cases can only be analysed by transmission electron microscopy (TEM) techniques. In this talk I will try to illustrate this with some of our recent work. I will demonstrate imaging of the unusual lattice reconstruction that occurs in twisted transition metal dicholcogenide bilayers [5]. We reveal that this behaviour is more complex than is seen for twisted heterostructures of graphene and/or hexagonal boron nitride. Complementary scanning probe microscopy (SPM) measurements show that such reconstruction creates strong piezoelectric textures, opening a new avenue for engineering of 2D material properties. I will also illustrate a new TEM support grid where an MoS2 wetting layer is added to improve adhesion, enabling sample transfer and TEM visualisation for even the most challenging 2D heterostructures [6]. Finally I will show that 2D heterostructures can themselves be used to enable new possibilities for STEM imaging. We present a new design of graphene based mixing cell where a monolayer 2D material membrane is fractured by the electron beam enabling the earliest stages of mixing to be observed. We apply this novel platform for the direct visualisation of the entire reaction timeline for calcium carbonate synthesis, including nanoscale imaging of liquid-liquid phase separation, the formation of amorphous calcium carbonate, and particle crystallization. [7] Keywords 2D materials, CaCO3, scanning transmission electron microscopy, in situ, liquid cell, graphene, transition metal dichalcogenides 10:30 - 10:42 147 Monitoring dynamics of defects and single metal atoms in functionalized graphene by temperature programmed in situ transmission electron microscopy Dr Rosa Arrigo1, Dr Takeo Sasaki2, Dr Manfred Erwin Schuster3 1Salford University, Manchester, United Kingdom. 2JEOL, Welwyn Garden City, United Kingdom. 3Johnson Matthey, Sonning Common, United Kingdom Abstract Text The reactivity of carbon materials in catalysis and electrocatalysis is due to the edges site terminations and point defects (vacancies, non 6-membered rings) on the basal planes of the graphene layers. These sites are often terminated by heteroatoms such as O, N, B, P which impart specific acid/base properties. Furthermore, these heteroatoms can be used as anchoring sites for metal atoms to prepare C-metal hybrid systems. Single metal atom sites and point defects are key structural entities determining the performance of graphene- based catalysts. Not only do they co-participate in the catalytic turn-over, but the carbon defects can also be used to tailor the reactivity and selectivity of the immobilized metal species. In this contribution we are concerned with the case of Fe on N-functionalized few-layer graphene which is widely investigated as alternative to platinum group metals systems in electro-catalytic applications such as the oxygen reduction reaction, the electrochemical CO2 reduction and the electrochemical NH3 synthesis. We apply in-situ thermal programmed scanning transmission electron microscopy to monitor dynamics involving Fe single atoms and their stabilization on N functionalized few-layer graphene. This allows us to identify the nature of the defects initially present on N-functionalised graphene and their property to coordinate Fe atoms. Furthermore, we study how these structural defects change upon annealing in the presence and absence of metal atoms. We will show a high mobility of both defects and metal atoms diffusing rapidly from one defect point to the other. Furthermore, we will show that the presence of metal atoms induces a stabilization of the graphene defects. This study is of relevance for the application of graphene-like materials for low and medium temperature catalysis and electrocatalysis. Figure 1: temperature induced formation of vacancies on few layer N-doped graphene: 500°C (a) vs 700°C (b) Keywords in-situ TEM, single site catalysis, graphene 10:42 - 10:54 289 Matching algorithms for elemental quantification and few tilt tomography in 2D materials Dr. Christoph Hofer1, Dr. Viera Skákalová2, Jonas Haas3,4, Prof. Jannik Meyer3,4, Prof. Timothy Pennycook1 1University of Antwerp, Antwerp, Belgium. 2University of Vienna, Vienna, Austria. 3Eberhard Karl University of Tübingen, Tübingen, Germany. 4Natural and Medical Sciences Institute at the University of Tübingen, Reutlingen, Germany Abstract Text Introduction Identifying the position and chemical identity of each atom in a specimen is the ultimate goal of structural characterization. With the rise of aberration correctors in scanning transmission electron microscopy (STEM) the Z-contrast based annular dark-field (ADF) imaging technique even allows to distinguish light elements in single layer materials [1]. However, residual aberrations are difficult to manually detect and correct under the low doses needed for beam sensitive materials hampering the analysis of ADF images. Moreover, a conventional tomographic approach for three-dimensional imaging is difficult due to the requirement of a large number of projections [2]. I will introduce the approach of using matching algorithms to overcome these difficulties. To perform quantification of images that is robust to the presence of both residual aberrations and noise, algorithmic matching of simulated and experimental data can be used [3]. A simulation is performed from an initial guess. Successive simulations are then iteratively optimized to match the experimental data as well as possible by adjusting the aberrations and atomic positions used in the simulation. Once convergence is achieved, the atomic scattering factors are finally included in the optimization. Matching algorithms also enable tomography with a much reduced number or tilt angles. I will present results illustrating 3D structure determination of 2D materials such as graphene [4] directly from just two tilt angles. This is indeed sufficient as long as each atom can be identified individually in each projection and the connectivity matrix can be obtained showing which atom is which in the comparison of each view. Under these circumstances, a very similar optimization process is realized where the model is matched so that the simulated projections in each tilt angle are matched to the corresponding experimental data set. The reconstruction is demonstrated using the ADF signal, but is also expected to work with any other signal providing atomic-resolution. Materials & methods ADF STEM measurements were conducted using a Nion UltraSTEM100 operated at 60~kV and a JEOL JEM- ARM200F operated at 80 kV at a convergence angle of ~30 mrad, and tilts separated by ~20 deg. Ptychography data will be collected using a microsecond dwell time capable custom camera at EMAT. Results and Discussion Analysis reveals that our novel intensity method achieves more reliable results compared to other quantification methods in the presence of small non-round aberrations. The method even allows us to extract quantitative atomic intensities if the aberrations are strong. As a specific example, this approach allows to quantify different light-elements in graphene in the presence of a slight three-fold astigmatism which wouldn’t have been possible via other established approaches. The three-dimensional analysis reveals significant deformation of defect sites in 2D materials. Specifically for graphene grain boundaries, a correlation between the deformation amplitude and the misorientation angle is demonstrated. Figure 1 shows one example of a reconstructed grain boundary using two atomically resolved STEM images captured at tilt angles separated by 20 degrees. The analysis also enables one to study single- atomic out-of-plane dynamics, which we have demonstrated for impurities in graphene. We will discuss the extension of the few tilt tomography method to thicker 2D materials such as transmission metal dichalcogenides, and the addition of simultaneous ptychographic data to the ADF signal to facilitate the simultaneous location of both heavy and light atoms. The dose-efficiency of ptychography should also allow a lower dose to be used for such tomography. Alternatively, the
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