Sandra Van Aert

University of Antwerp | UA · Department of Physics

Quantitative structure determination is needed in order to study and understand nanomaterials at the atomic scale. Materials characterisation resulting in precise structural information is a crucial point to understand the structure-property relation of materials. Counting the number of atoms and retrieving the 3D atomic structure of nanoparticles...

We report a study of scattering dynamics in crystals employing momentum-resolved scanning transmission electron microscopy under varying illumination conditions. As we perform successive changes of the probe focus, multiple real-space signals are obtained in dependence of the shape of the incident electron wave. With support from extensive simulati...

In oxide heterostructures, different materials are integrated into a single artificial crystal, resulting in a breaking of inversion symmetry across the heterointerfaces. A notable example is the interface between polar and nonpolar materials, where valence discontinuities lead to otherwise inaccessible charge and spin states. This approach paved t...

In oxide heterostructures, different materials are integrated into a single artificial crystal, resulting in a breaking of inversion-symmetry across the heterointerfaces. A notable example is the interface between polar and non-polar materials, where valence discontinuities lead to otherwise inaccessible charge and spin states. This approach paved...

In electron microscopy, the maximum a posteriori (MAP) probability rule has been introduced as a tool to determine the most probable atomic structure from high-resolution annular dark-field (ADF) scanning transmission electron microscopy (STEM) images exhibiting low contrast-to-noise ratio (CNR). Besides ADF imaging, STEM can also be applied in the...

In this work, the maximum a posteriori (MAP) probability rule, which combines statistical parameter estimation theory and model-order selection, is applied to simultaneously acquired annular bright-field (ABF) and annular dark-field (ADF) scanning transmission electron microscopy (STEM) images. For this, an extension of the commonly used parametric...

In this work, the maximum a posteriori (MAP) probability rule, which combines statistical parameter estimation theory and model-order selection, is applied to simultaneously acquired annular bright-field (ABF) and annular dark-field (ADF) scanning transmission electron microscopy (STEM) images. For this, an extension of the commonly used parametric...

Three-dimensional strontium ruthenate (SrRuO 3) is an itinerant ferromagnet that features Weyl points acting as sources of emergent magnetic fields, anomalous Hall conductivity, and unconventional spin dynamics. Integrating SrRuO 3 in oxide heterostructures is potentially a novel route to engineer emergent electrodynamics, but its electronic band t...

Single atom detection is of key importance to solving a wide range of scientific and technological problems. Nowadays, due to improvements in aberration correction technology, transmission electron microscopy (TEM) has become an excellent technique to visualise nanomaterials down to sub-angstrom resolution. However, for radiation-sensitive and ligh...

Recently, the maximum a posteriori (MAP) probability rule has been proposed as an objective and quantitative method to detect atom columns and even single atoms from high-resolution high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images [1,2]. The method is based on a combination of statistical parameter estim...

Recently, the maximum a posteriori (MAP) probability rule has been proposed as an objective and quantitative method to detect atom columns and even single atoms from high-resolution high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images [1,2]. The method is based on a combination of statistical parameter estim...

Single atom detection is of key importance to solving a wide range of scientific and technological problems. Nowadays, due to improvements in aberration correction technology, transmission electron microscopy has become an excellent technique to visualize nanomaterials down to sub-angstrom resolution. However, for radiation-sensitive and light-elem...

Recently, the maximum a posteriori (MAP) probability rule has been proposed as an objective and quantitative method to detect atom columns and even single atoms from high-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images. The method combines statistical parameter estimation and model-order sele...

The atomic lensing model has been proposed as a promising method facilitating atom-counting in heterogeneous nanocrystals [1]. Here, image simulations will validate the model, which describes dynamical diffraction as a superposition of individual atoms focussing the incident electrons. It will be demonstrated that the model is reliable in the annul...

Geometric phases in condensed matter play a central role in topological transport phenomena such as the quantum, spin and anomalous Hall effect (AHE). In contrast to the quantum Hall effect - which is characterized by a topological invariant and robust against perturbations - the AHE depends on the Berry curvature of occupied bands at the Fermi lev...

Single atom detection is of key importance to solve a wide range of scientific and technological problems. In principle, scanning transmission electron microscopy (STEM) is a powerful technique to image single atoms. However, to avoid beam damage, the incoming electron dose should be kept low enough resulting into images exhibiting a very low signa...

Single atom detection is of key importance to solve a wide range of scientific and technological problems. Scanning transmission electron microscopy (STEM) is a powerful technique to image single atoms. However, to overcome beam damage, the incoming electron dose should be kept low enough resulting into images exhibiting a very low signal-to-noise...

Single atom detection is of key importance to solving a wide range of scientific and technological problems. The strong interaction of electrons with matter makes transmission electron microscopy one of the most promising techniques. In particular, aberration correction using scanning transmission electron microscopy has made a significant step for...

In principle, scanning transmission electron microscopy (STEM) is a powerful technique to image single atoms. However, to avoid beam damage, the incoming electron dose should be kept low enough resulting into images exhibiting a low signal-to-noise ratio (SNR). In general, visual inspection of such images leads to biased structure information. To o...

In principle, scanning transmission electron microscopy (STEM) is a powerful technique to image single atoms. However, to avoid beam damage, the incoming electron dose should be kept low enough resulting into images exhibiting a low signal-to-noise ratio (SNR). In general, visual inspection of such images leads to biased structure information. To o...

In quantitative scanning transmission electron microscopy (STEM), scattering cross-sections have been shown to be very sensitive to the number of atoms in a column and its composition. They correspond to the integrated intensity over the atomic column and they outperform other measures. As compared to atomic column peak intensities, which saturate...

In this paper, both the frozen lattice (FL) and the absorptive potential (AP) approximation models are compared in terms of the integrated intensity and the precision with which atomic columns can be located from an image acquired using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). The comparison is made fo...

An efficient model-based estimation algorithm is introduced in order to quantify the atomic column positions and intensities from atomic resolution (scanning) transmission electron microscopy ((S)TEM) images. This algorithm uses the least squares estimator on image segments containing individual columns fully accounting for the overlap between neig...

Structure parameters from scanning transmission electron microscopy (STEM) images are commonly estimated by using a physics-based model describing the image data. Hereby, the number of atomic columns needs to be specified beforehand. This number can be determined by visual inspection of the image if the signal-to-noise ratio (SNR) is sufficiently h...

Structure parameters from scanning transmission electron microscopy (STEM) images are commonly determined by using a physics-based model describing the image data. More precisely, the atomic column positions and intensities are estimated by performing a model-fitting approach. Such an approach is generally referred to as statistical parameter estim...

Recent Advances of the Open Source MULTEM Program to Provide Accurate and Fast Electron Microscopy Simulations - Volume 23 Issue S1 - I.P. Lobato Hoyos, J. Verbeeck, S. Van Aert

In order to fully exploit structure-property relations of nanomaterials, three-dimensional (3D) characterization at the atomic scale is often required. In recent years, the resolution of electron tomography has reached the atomic scale. However, such tomography typically requires several projection images demanding substantial electron dose. A newl...

Nowadays, aberration corrected transmission electron microscopy (TEM) is a popular method to characterise nanomaterials at the atomic scale. Here, atomically resolved images of nanomaterials are acquired, where the contrast depends on the illumination, imaging and detector conditions of the microscope. Visualization of light elements is possible wh...

Heterostructures formed by La0.7Sr0.3MnO3/ZnO (LSMO/ZnO) interfaces exhibit extremely interesting electronic properties making them promising candidatea for novel oxide p-n junctions with multifunctional features. In this work, the structure of the interface is studied through a combined experimental/theoretical approach, . Hetero-structures were g...

We present a study of the relation between the surface chemistry and nanocrystal shape of PbSe nanocrystals with a variable Pb-to-Se stoichiometry and density of oleate ligands. The oleate ligand density and binding configuration is monitored by NMR and FTIR absorbance spectroscopy allowing us to quantify the number of surface-attached ligands per...

Colloidal CsPbX3 (X = Br, Cl, I) perovskite nanocrystals (NCs) have emerged as promising phosphors and solar cell materials due to their remarkable optoelectronic properties. These properties can be tailored not only by controlling the size and shape of the NCs, but also by post-synthetic composition tuning through topotactic anion exchange. In con...

Thickness-driven electronic phase transitions are broadly observed in different types of functional perovskite heterostructures. However, uncertainty remains whether these effects are solely due to spatial confinement, broken symmetry, or rather to a change of structure with varying film thickness. Here, this study presents direct evidence for the...

Nano-sized gold has become an important material in various fields of science and technology, where control over the size and crystallography is desired to tailor the functionality. Gold crystallizes in the face-centered cubic (fcc) phase, and its hexagonal closed packed (hcp) structure is a very unusual and rare phase. Stable Au hcp phase has been...

Thickness driven electronic phase transitions are broadly observed in different types of functional perovskite heterostructures. However, uncertainty remains whether these effects are solely due to spatial confinement, broken symmetry or rather to a change of structure with varying film thickness. Here, we present direct evidence for the relaxation...

A hybrid statistics-simulations based method for atom-counting from annular dark field scanning transmission electron microscopy (ADF STEM) images of monotype crystalline nanostructures is presented. Different atom-counting methods already exist for model-like systems. However, the increasing relevance of radiation damage in the study of nanostruct...

The present contribution gives a review of recent quantification work of atom displacements, atom site occupations and level of crystallinity in various systems and based on aberration corrected HR(S)TEM images. Depending on the case studied, picometer range precisions for individual distances can be obtained, boundary widths at the unit cell level...

Self-organized gold nanostructures on Ge(001) surfaces are currently of special interest due to their applications for mono-molecular electronic devices and the growth of Ge nanowires. The understanding of electrical as well as physical properties of the system is of great importance and is strongly linked to its atomic structure. Here, we report o...

Nanoclusters play key roles in a wide range of materials and devices because of their unique physical and chemical properties. These properties are determined by the specific three-dimensional (3D) morphology, structure and composition. It is well known that extremely small changes in their local structure may result in significant changes of their...

Atomic resolution electron tomography using HAADF STEM has become a key tool to get 3D atomic-scale structural information about the sample under study [1–3]. Different reconstruction algorithms exist including filtered back projection, simultaneous iterative reconstruction (SIRT), discrete tomography [4, 5] and total variation minimization [2]. Ho...

Aberration-corrected STEM has become a powerful technique for materials characterisation of complex nanostructures. Recent progress in the development of quantitative methods allows us to extract reliable structural and chemical information from experimental images in 2D as well as in 3D. In quantitative STEM, images are treated as datasets from wh...

Nowadays, high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) is one of the most popular materials' characterisation techniques because of its ability to provide direct structural images at the atomic resolution [1]. Recently, Rosenauer et al. proposed a new imaging technique called imaging STEM (ISTEM) combining...

The ISTEM (Imaging STEM) method [Phys. Rev Lett. 113, 096101(2014)] presented here constitutes a novel way for the realisation of TEM imaging with spatially incoherent illumination. It is well-known that such incoherent image formation allows for an increased resolution and higher robustness towards chromatic aberrations compared to coherent illumi...

The study of novel physical properties appearing when two materials are interfaced has become one of the major fields of research in solid state physics over the last decade. For example, in the strive for novel non-silicon based electronics, the discovery of the formation of a conductive layer right at the interface region between 2 insulators (fo...

In the past few years a lot of research has been done to improve the imaging power to detect light atoms like oxygen, lithium, and hydrogen, since they play a key-role in interesting industrial applications such as lithium-batteries or hydrogen-storage materials. Since material properties crucially depend on the exact atomic arrangement, an estimat...

The most commonly used algorithms to reconstruct HAADF STEM data such as Filtered Back Projection (FBP) and iterative reconstruction algorithms such as ART, SART and SIRT assume a linear image formation model. However, the linearity assumption is only a crude approximation of the non-linear behaviour of the real image formation model. Moreover, the...

Quantitative scanning transmission electron microscopy (STEM) using an annular dark field (ADF) detector has become a widely used technique for the characterization of materials at the atomic level. The quantification process involves the comparison of experimental data with image simulations, the use of statistical tools in a parameter estimation...

In recent years scanning transmission electron microscopy (STEM) has attracted great attention due to its high sensitivity with respect to atomic number and specimen thickness. The great advantage of this technique is to determine the number of atoms in single atomic columns in the specimen [1,2]. For instance, the reconstruction of the atomic stru...

The most practical method for TEM image simulations is the multislice method, which is known to be an accurate numerical procedure for solving the quantum mechanical electron-specimen interaction. Although most simulation codes treat the scattering process as purely elastic and coherent, inelastic scattering cannot be neglected and it has to be inc...

The three dimensional (3D) structural characterization of nanoparticles is crucial in materials science since many properties heavily depend on size, surface to volume ratio and morphology. In addition, the ability to investigate the crystal structure is just as essential because the presence of defects and surface relaxation will directly affect p...

The performance of catalyst nanoparticles is generally dependent on their size, shape, strain, composition and support. However, the relationship between these parameters and the catalyst performance is not well understood. In most instances, the catalyst design process involves several design and testing iterations until the desired performance is...

Counting the number of atoms in each atomic column from different viewing directions has proven to be a powerful technique to retrieve the 3D structure of homogeneous nanostructures [1]. In order to extend the atom counting technique to heterogeneous materials, this work presents a new atomic lensing model facilitating both atom counting and 3D com...

In this paper, we investigate how precise atoms of a small nanocluster can ultimately be located in three dimensions (3D) from a tilt series of images acquired using annular dark field (ADF) scanning transmission electron microscopy (STEM). Therefore, we derive an expression for the statistical precision with which the 3D atomic position coordinate...

Aberration correction in scanning transmission electron microscopy (STEM) has greatly improved the lateral and depth resolution. When using depth sectioning, a technique during which a series of images is recorded at different defocus values, single impurity atoms can be visualised in three dimensions. In this paper, we investigate new possibilitie...

In this work, a recently developed quantitative approach based on the principles of detection theory is used in order to determine the possibilities and limitations of High Resolution Scanning Transmission Electron Microscopy (HR STEM) and HR TEM for atom-counting. So far, HR STEM has been shown to be an appropriate imaging mode to count the number...

Recently, imaging scanning transmission electron microscopy (ISTEM) has been proposed as a promising new technique combining the advantages of conventional TEM (CTEM) and STEM (Rosenauer et al., 2014 [1]). The ability to visualize light and heavy elements together makes it a particularly interesting new, spatially incoherent imaging mode. Here, we...

Oriented attachment of PbSe nanocubes can result in the formation of two-dimensional (2D) superstructures with long-range nanoscale and atomic order. This questions the applicability of classic models in which the superlattice grows by first forming a nucleus, followed by sequential irreversible attachment of nanocrystals, as one misaligned attachm...

Direct observations of the ferroelectric domain boundaries in LiNbO3 are performed using high-resolution high-angle annular dark field scanning transmission electron microscopy imaging, revealing a very narrow width of the domain wall between the 180° domains. The domain walls demonstrate local side-way meandering, which results in inclinations eve...

A common way to measure unknown structure parameters from electron microscopy images is to perform a model fitting approach to determine the optimal parameters of a physics-based model describing the image data [1,2]. A commonly used model represents Gaussian shaped atomic columns superposed on a background. An important assumption which is often m...

A common way to measure unknown structure parameters from electron microscopy images is to use statistical parameter estimation theory. This requires the use of a physics-based parametric model describing the image data [1,2]. A commonly used model to describe annular dark-field scanning transmission electron microscopy (ADF STEM) images represents...

Over the last two decades, three-dimensional (3D) imaging by transmission electron microscopy or electron tomography has evolved into a powerful tool to investigate a variety of nanomaterials in different fields, such as life sciences, chemistry, solid-state physics, and materials science. Most of these results were obtained with nanometer-scale re...

In the present paper, the optimal detector design is investigated for both detecting and locating light atoms from high resolution scanning transmission electron microscopy (HR STEM) images. The principles of detection theory are used to quantify the probability of error for the detection of light atoms from HR STEM images. To determine the optimal...

div class="title">Quantification of ADF STEM Image Data for Nanoparticle Structure and Strain Measurements - Volume 22 Issue S3 - P D Nellist, L Jones, A Varambhia, A De Backer, S Van Aert, D Ozkaya

The development of new nanocrystals with outstanding physicochemical properties requires a full three-dimensional (3D) characterization at the atomic scale. For homogeneous nanocrystals, counting the number of atoms in each atomic column from high angle annular dark field scanning transmission electron microscopy images has been shown to be a succe...

A new version of the open source program MULTEM is presented here. It includes a graphical user interface, tapering truncation of the atomic potential, CPU multithreading functionality, single/double precision calculations, scanning transmission electron microscopy (STEM) simulations using experimental detector sensitivities, imaging STEM (ISTEM) s...

Ru catalysts are part of a set of late transition metal nanocatalysts that have garnered much interest for catalytic applications such as ammonia synthesis and fuel cell production. Their performance varies greatly depending on their morphology and size, these catalysts are widely studied using electron microscopy. Using recent developments in annu...

In epitaxial thin film systems, the crystal structure and its symmetry deviate from the bulk counterpart due to various mechanisms such as epitaxial strain and interfacial structural coupling, which is accompanyed by a change in their properties. In perovskite materials, the crystal symmetry can be described by rotations of sixfold coordinated tran...

Controlled in-plane rotation of the magnetic easy axis in manganite heterostructures by tailoring the interface oxygen network could allow the development of correlated oxide-based magnetic tunnelling junctions with non-collinear magnetization, with possible practical applications as miniaturized high-switching-speed magnetic random access memory (...

The increasing need for precise determination of the atomic arrangement of non-periodic structures in materials design and the control of nanostructures explains the growing interest in quantitative transmission electron microscopy. The aim is to extract precise and accurate numbers for unknown structure parameters including atomic positions, chemi...