arXiv subjects

S. Van Aert

Publications and source records attributed to S. Van Aert.

5 recordsLinked to original sources

arXiv2023

Deep convolutional neural networks to restore single-shot electron microscopy images

State-of-the-art electron microscopes such as scanning electron microscopes (SEM), scanning transmission electron microscopes (STEM) and transmission electron microscopes (TEM) have become increasingly sophisticated. However, the quality of experimental images is often hampered by stochastic and deterministic distortions arising from the instrument or its environment. These distortions can arise during any stage of the imaging process, including image acquisition, transmission, or visualization. In this paper, we will discuss the main sources of distortion in TEM and S(T)EM images, develop models to describe them and propose a method to correct these distortions using a convolutional neural network. We demonstrate the effectiveness of our approach on a variety of experimental images and show that it can significantly improve the signal-to-noise ratio resulting in an increase in the amount of quantitative structural information that can be extracted from the image. Overall, our findings provide a powerful framework for improving the quality of electron microscopy images and advancing the field of structural analysis and quantification in materials science and biology.

physics.comp-ph↗

arXiv2019

The atomic lensing model: new opportunities for atom-by-atom metrology of heterogeneous nanomaterials

The atomic lensing model has been proposed as a promising method facilitating atom-counting in heterogeneous nanocrystals [KHW van den Bos et. al, Phys. Rev. Lett. 116 (2016) 246101] 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 annular dark field regime for crystals having columns containing dozens of atoms. By using the principles of statistical detection theory, it will be shown that this model gives new opportunities for detecting compositional differences.

cond-mat.mtrl-sci↗

arXiv2019

The maximum a posteriori probability rule for atom column detection from HAADF STEM images

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 selection using a Bayesian framework and has been shown to be especially useful for the analysis of the structure of beam-sensitive nanomaterials. In order to avoid beam damage, images of such materials are usually acquired using a limited incoming electron dose resulting in a low contrast-to-noise ratio (CNR) which makes visual inspection unreliable. This creates a need for an objective and quantitative approach. The present paper describes the methodology of the MAP probability rule, gives its step-by-step derivation and discusses its algorithmic implementation for atom column detection. In addition, simulation results are presented showing that the performance of the MAP probability rule to detect the correct number of atomic columns from HAADF STEM images is superior to that of other model-order selection criteria, including the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC). Moreover, the MAP probability rule is used as a tool to evaluate the relation between STEM image quality measures and atom detectability resulting in the introduction of the so-called integrated CNR (ICNR) as a new image quality measure that better correlates with atom detectability than conventional measures such as signal-to-noise ratio (SNR) and CNR.

physics.data-an↗

arXiv2014

Atomic resolution mapping of phonon excitations in STEM-EELS experiments

Atomically resolved electron energy-loss spectroscopy experiments are commonplace in modern aberrationcorrected transmission electron microscopes. Energy resolution has also been increasing steadily with the continuous improvement of electron monochromators. Electronic excitations however are known to be delocalised due to the long range interaction of the charged accelerated electrons with the electrons in a sample. This has made several scientists question the value of combined high spatial and energy resolution for mapping interband transitions and possibly phonon excitation in crystals. In this paper we demonstrate experimentally that atomic resolution information is indeed available at very low energy losses around 100 meV expressed as a modulation of the broadening of the zero loss peak. Careful data analysis allows us to get a glimpse of what are likely phonon excitations with both an energy loss and gain part. These experiments confirm recent theoretical predictions on the strong localisation of phonon excitations as opposed to electronic excitations and show that a combination of atomic resolution and recent developments in increased energy resolution will offer great benefit for mapping phonon modes in real space.

cond-mat.mtrl-sci↗

arXiv2006

Study of Correlation Effects in the High Formal Oxidation State Compound Sr$_2$CoO$_4$

Two recent reports confirm that the newly synthesized Sr$_2$CoO$_4$ (formal oxidation state Co$^{4+}$) shows a high Curie temperature (~ 250 K), but they report different moments of 1.8 $μ_B$ and 1 $μ_B$ per Co. Using both commonly used functionals in the correlated band approach (LDA+U) as well as the local density approximation (LDA),the combined effects of correlation and hybridization with O 2p states are calculated and analyzed. Sr$_2$CoO$_2$ is already ferromagnetic within LDA (M=1.95 $μ_B$). Increasing U from zero, the two LDA+U schemes affect the moment oppositely out to a critical value $U_c$=2.5 eV, at which point they transform discontinuously from different states to the same large U state. Fixing U at $U_c$, fixed spin moment calculations show similar behavior out to a minimum at 1$μ_B$ (a half metallic state), beyond which the fully-localized-limit scheme jumps to a state with energy minimum very near 2$μ_B$ very close to the LDA moment). Although the energy minima occur very near integer values of the moment/Co (1$μ_B, 2$μ_B$), the strong 3d-2p mixing and resulting 3d orbital occupations seem to preclude any meaningful S=1/2 or S=1 assignment to the Co ion.

cond-mat.str-el↗