Katherine E. MacArthur

Probe integrated scattering cross sections in the analysis of atomic resolution HAADF STEM images

The physical basis for using a probe-position integrated cross section (PICS) for a single column of atoms as an effective way to compare simulation and experiment in high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) is described, and the use of PICS in order to make quantitative use of image intensities is evaluated. It is based upon the calibration of the detector and the measurement of scattered intensities. Due to the predominantly incoherent nature of HAADF STEM, it is found to be robust to parameters that affect probe size and shape such as defocus and source coherence. The main imaging parameter dependencies are on detector angle and accelerating voltage, which are well known. The robustness to variation in other parameters allows for a quantitative comparison of experimental data and simulation without the need to fit parameters. By demonstrating the application of the PICS to the chemical identification of single atoms in a heterogeneous catalyst and in thin, layered-materials, we explore some of the experimental considerations when using this approach.

Rapid estimation of catalyst nanoparticle morphology and atomic-coordination by high-resolution Z-contrast electron microscopy.

Heterogeneous nanoparticle catalyst development relies on an understanding of their structure-property relationships, ideally at atomic resolution and in three-dimensions. Current transmission electron microscopy techniques such as discrete tomography can provide this but require multiple images of each nanoparticle and are incompatible with samples that change under electron irradiation or with surveying large numbers of particles to gain significant statistics. Here, we make use of recent advances in quantitative dark-field scanning transmission electron microscopy to count the number atoms in each atomic column of a single image from a platinum nanoparticle. These atom-counts, along with the prior knowledge of the face-centered cubic geometry, are used to create atomistic models. An energy minimization is then used to relax the nanoparticles 3D structure. This rapid approach enables high-throughput statistical studies or the analysis of dynamic processes such as facet-restructuring or particle damage.

Dose limited reliability of quantitative annular dark field scanning transmission electron microscopy for nano-particle atom-counting

Quantitative annular dark field scanning transmission electron microscopy (ADF STEM) has become a powerful technique to characterise nano-particles on an atomic scale. Because of their limited size and beam sensitivity, the atomic structure of such particles may become extremely challenging to determine. Therefore keeping the incoming electron dose to a minimum is important. However, this may reduce the reliability of quantitative ADF STEM which will here be demonstrated for nano-particle atom-counting. Based on experimental ADF STEM images of a real industrial catalyst, we discuss the limits for counting the number of atoms in a projected atomic column with single atom sensitivity. We diagnose these limits by combining a thorough statistical method and detailed image simulations.

Optimal ADF STEM imaging parameters for tilt-robust image quantification.

An approach towards experiment design and optimisation is proposed for achieving improved accuracy of ADF STEM quantification. In particular, improved robustness to small sample mis-tilts can be achieved by optimising detector collection and probe convergence angles. A decrease in cross section is seen for tilted samples due to the reduction in channelling, resulting in a quantification error, if this is not taken into account. At a smaller detector collection angle the increased contribution from elastic scattering, which initially increases with tilt, can be used to offset the decrease in the TDS signal.

Predicting the oxygen-binding properties of platinum nanoparticle ensembles by combining high-precision electron microscopy and density functional theory

Many studies of heterogeneous catalysis, both experimental and computational, make use of idealized structures such as extended surfaces or regular polyhedral nanoparticles. This simplification neglects the morphological diversity in real commercial oxygen reduction reaction (ORR) catalysts used in fuel-cell cathodes. Here we introduce an approach that combines 3D nanoparticle structures obtained from high-throughput high-precision electron microscopy with density functional theory. Discrepancies between experimental observations and cuboctahedral/truncated-octahedral particles are revealed and discussed using a range of widely used descriptors, such as electron-density, d-band centers, and generalized coordination numbers. We use this new approach to determine the optimum particle size for which both detrimental surface roughness and particle shape effects are minimized.

Electron Microscopy (Big and Small) Data Analysis With the Open Source Software Package HyperSpy

Francisco de la Peña, Tomas Ostasevicius, Vidar Tonaas Fauske, Pierre Burdet, Petras Jokubauskas, Magnus Nord, Mike Sarahan, Eric Prestat, Duncan N. Johnstone, Joshua Taillon, Jan Caron, Tom Furnival, Katherine E. MacArthur, Alberto Eljarrat, Stefano Mazzucco, Vadim Migunov, Thomas Aarholt, Michael Walls, Florian Winkler, Gaël Donval, Ben Martineau, Andreas Garmannslund, Luiz-Fernando Zagonel and Ilya Iyengar

Compositional quantification of PtCo acid-leached fuel cell catalysts using EDX partial cross sections

The new generation of analytical electron microscopes with aberration correction and new EDX detectors provide improved possibilities for microanalysis. Here, we apply a new approach to quantitative EDX based on using partial cross sections. Our quantification method is applied to alloy PtCo nanoparticles, which have been acid leached to provide Pt enrichment or rather Co depletion at the particle surface. Such surface Co depletion is absent at the vertices of the more facetted nanoparticles. The leaching process demonstrates very little change in Co composition when investigating the whole particle but produces a localised surface depletion, which can only be determined by the high-resolution EDX elemental maps.

From High-precision Imaging to High-performance Computing: Leveraging ADF-STEM Atom-counting and DFT for Catalyst Nano-metrology

Z-contrast imaging in the scanning transmission electron microscope (STEM) is a powerful tool to image precious metal heterogeneous catalysts at the atomic scale. When the annular dark-field (ADF) images from the STEM are quantified onto an absolute scale (Figure 1), it has been shown that it is possible to count the number of atoms in individual atomic columns of metallic nanoparticles and to estimate their three-dimensional structure [1]. In recent years further progress has been made in identifying the possible sources of error in the recording and analysis of quantitative annular dark-field (ADF STEM) images [2], in experiment-design, and in verifying the metrology by tomographic techniques. Of these developments, the move to fast multi-frame image-acquisition and -averaging has enabled the correction of experimental scanning-distortions, reductions in electron beam-damage of samples, and improvements in signal-noise ratio (SNR) [3]. Very recently, a new ADF image analysis best-practice, melding the benefits of both reference-simulation and unbiased statistical interpretation based analysis methods, has produced an atom counting method with even greater robustness [4,5]. Exploiting these recent technical developments, we obtain optimised raw data which is fed into highthroughput image processing tools revealing particle size, atom-counts etc. Unfortunately, our increased analysis throughput merely shifts the investigation bottleneck from data-processing to interpretation. To remedy this, we have developed a computationally-efficient genetic-algorithm based structure solving code (requiring a few tens of CPU hours per structure on a standard desktop PC) to retrieve likely lowenergy 3D particle structures which match the experimental observations.

Probing the Effects of Electron Channelling on EDX Quantification

A myriad of advances in microscope stability, aberration correction and improved detectors now make it routinely possible to achieve atomic resolution energy dispersive X-ray (EDX) mapping [1], showing atomic-scale qualitative detail as to where the elements reside within a sample. Whenever we analyse crystals down a low-order zone axis we are exploiting electron channelling to provide us with atomic resolution. The aligned columns act like miniature lenses, providing a focusing effect on the electron beam. This is beneficial because it causes our convergent beam to stay more focused onto one atomic column, providing increased signal from an aligned column of atoms than we would expect just by summing the intensity of each of the component atoms [2]. However, the focusing effect further means the contribution from atoms at different depths differs, a phenomenon often referred to as the ‘TopBottom Effect’, complicating the quantitative interpretation.

Opportunities in Angularly Resolved Dark-field STEM using Pixelated Detectors

Pixelated sensors in the detector plane of the scanning transmission electron microscope (STEM) offer many opportunities for extracting valuable information from the bright-field (BF) disk including synthesising annular bright-field images, differential phase-contrast images, and ptychographic phase reconstruction [1]. However, by reducing microscope camera-length, these pixelated detectors can also be used to record the scattered electron flux outside the BF disk. This scattering has previously been recorded using annular dark-field (ADF) detectors however there the operator must choose what annular range the DF signal is to be integrated over at the point of the imaging and this can limit operational flexibility. Some instruments offer two or more annular DF detectors to record more than one angular range but in this case the ratio of collection angles of these two detectors is fixed. Use of two detectors allows low-angle or medium-angle (LAADF and MAADF) images to be recorded which have been found to yield useful strain or structural information [2], [3].