The rapidly changing face of electron microscopy

Quantifying elemental colocation in nanostructured materials using energy-dispersive X-ray spectroscopy

Multicomponent nanostructured materials are key amongst others for energy and catalysis applications. The nanoscale proximity of different metals critically determines the performance of these functional materials. However, it is difficult to study the spatial distribution of different elements at the nanoscale, especially achieving a statistically relevant assessment. Additionally, common support materials like metal oxides are sensitive to electron beam damage when using high resolution local techniques, such as transmission electron microscopy. We present a robust strategy to quantitatively assess elemental distributions in 3D nanostructured beam-sensitive samples. Key elements are resin embedding, and elemental co-localisation building on a combination of electron tomography and energy-dispersive X-ray spectroscopy. We showcase the methodology with ∼ 3 nm Pd-Ni nanoparticles supported on mesoporous silica. Epoxy resin-embedding ensured sufficient sample stability under the electron beam for tomography-based quantification of different mano- and mesoscale elemental distributions in these samples. Reliable co-location results were obtained and practical guidelines are provided for acquisition and post-processing, relevant for elemental overlap analysis in multi-metallic samples.

Insights into Calcium Phosphate Formation Induced by the Dissolution of 45S5 Bioactive Glass

Although models have been proposed to explain the mechanisms of bioglass (BG) dissolution and subsequent calcium phosphate (CaP) mineralization, open questions remain. The processes in which phase transition occurs in aqueous solutions and their dynamics remain underexplored partly because traditional instruments/techniques do not allow for direct observations at the adequate time and length scales at which such phase transformations occur. For instance, given the crucial role of the silica gel in CaP formation during BG dissolution, uncertainty exists about how such a silica gel forms on the BG surface. In the case of CaP formation driven by BG dissolution, questions can also be added, i.e., how CaP develops into an apatitic-like structure, how many transient phases there are, and, in general, phenomena occurring in the solid-liquid interface during BG dissolution. Several approaches were taken to study CaP mineralization driven by BG dissolution, mainly examining the solid-liquid interface and the BG after-reaction surface. This paper focuses on gaining insight into silica gel formation on the BG's surface during dissolution. Electron microscopy techniques were used, including scanning electron microscopy and focused ion beam cross sections. Other analysis techniques, such as time-of-flight secondary ion mass spectrometry, were utilized. Cross sections of reacted BG-blocks gave essential insights into the BG dissolution, particularly its strong dependency on experimental conditions, and tentative evidence has shown that soluble silica from BG dissolution may not reprecipitate/repolymerize on BG blocks' surface; thus, we wonder where it precipitates. Additionally, complementary analysis techniques determined that CaP, during BG dissolution, transitions from amorphous calcium phosphate to a calcium-deficient nanocrystalline apatitic structure with minimal contents of Si4+ and Na+ ions that may be molecularly part of CaP. The Hench model has been the core guide for BG dissolution and subsequent CaP formation for many years. However, this study shows tentative evidence that contributes to and somewhat differs from it.

Atomic Structure and 3D Shape of a Multibranched Plasmonic Nanostar from a Single Spatially Resolved Electron Diffraction Map

Despite the interest in improving the sensitivity of optical sensors using plasmonic nanoparticles (NPs) (rods, wires, and stars), the full structural characterization of complex shape nanostructures is challenging. Here, we derive from a single scanning transmission electron microscope diffraction map (4D-STEM) a detailed determination of both the 3D shape and atomic arrangement of an individual 6-branched AuAg nanostar (NS) with high-aspect-ratio legs. The NS core displays an icosahedral structure, and legs are decahedral rods attached along the 5-fold axes at the core apexes. The NS legs show an anomalous anisotropic spatial distribution (all close to a plane) due to an interplay between the icosahedral symmetry and the unzipping of the surfactant layer on the core. The results significantly improve our understanding of the star growth mechanism. This low dose diffraction mapping is promising for the atomic structure study of individual multidomain, multibranched, or multiphase NPs, even when constituted of beam-sensitive materials.

High precision orientation mapping from 4D-STEM precession electron diffraction data through quantitative analysis of diffracted intensities

The association of scanning transmission electron microscopy (STEM) and detection of a diffraction pattern at each probe position (so-called 4D-STEM) represents one of the most promising approaches to analyze structural properties of materials with nanometric resolution and low irradiation levels. This is widely used for texture analysis of materials using automated crystal orientation mapping (ACOM). Herein, we perform orientation mapping in InP nanowires exploiting precession electron diffraction (PED) patterns acquired by an axial CMOS camera. Crystal orientation is determined at each probe position by the quantitative analysis of diffracted intensities minimizing a residue comparing experiments and simulations in analogy to x-ray structural refinement. Our simulations are based on the two-beam dynamical diffraction approximation and yield a high angular precision (∼0.03°), much lower than the traditional ACOM based on pattern matching algorithms (∼1°). We anticipate that simultaneous exploration of both spot positions and high precision crystal misorientation will allow the exploration of the whole potentiality provided by PED-based 4D-STEM for the characterization of deformation fields in nanomaterials.

Unsupervised machine learning applied to scanning precession electron diffraction data

Scanning precession electron diffraction involves the acquisition of a two-dimensional precession electron diffraction pattern at every probe position in a two-dimensional scan. The data typically comprise many more diffraction patterns than the number of distinct microstructural volume elements (e.g. crystals) in the region sampled. A dimensionality reduction, ideally to one representative diffraction pattern per distinct element, may then be sought. Further, some diffraction patterns will contain contributions from multiple crystals sampled along the beam path, which may be unmixed by harnessing this oversampling. Here, we report on the application of unsupervised machine learning methods to achieve both dimensionality reduction and signal unmixing. Potential artefacts are discussed and precession electron diffraction is demonstrated to improve results by reducing the impact of bending and dynamical diffraction so that the data better approximate the case in which each crystal yields a given diffraction pattern.

Nanoanalytics: history, concepts, and specificities

This article deals with analytical chemistry devoted to nano-objects. A short review presents nano-objects, their singularity in relation to their dimensions, genesis, and possible transformations. The term nano-object is then explained. Nano-object characterization activities are considered and a definition of nanoanalytics is proposed. Parameters and properties for describing nano-objects on an individual scale and on the scale of a population are also presented. They enable the specificities of analytical activities to be highlighted in terms of multi-criteria description strategies and observation scale. Special attention is given to analytical methods, their dimensioning and validation.

Towards 3D crystal orientation reconstruction using automated crystal orientation mapping transmission electron microscopy (ACOM-TEM)

To relate the internal structure of a volume (crystallite and phase boundaries) to properties (electrical, magnetic, mechanical, thermal), a full 3D reconstruction in combination with in situ testing is desirable. In situ testing allows the crystallographic changes in a material to be followed by tracking and comparing the individual crystals and phases. Standard transmission electron microscopy (TEM) delivers a projection image through the 3D volume of an electron-transparent TEM sample lamella. Only with the help of a dedicated TEM tomography sample holder is an accurate 3D reconstruction of the TEM lamella currently possible. 2D crystal orientation mapping has become a standard method for crystal orientation and phase determination while 3D crystal orientation mapping have been reported only a few times. The combination of in situ testing with 3D crystal orientation mapping remains a challenge in terms of stability and accuracy. Here, we outline a method to 3D reconstruct the crystal orientation from a superimposed diffraction pattern of overlapping crystals without sample tilt. Avoiding the typically required tilt series for 3D reconstruction enables not only faster in situ tests but also opens the possibility for more stable and more accurate in situ mechanical testing. The approach laid out here should serve as an inspiration for further research and does not make a claim to be complete.

Reflections on the value of electron microscopy in the study of heterogeneous catalysts

Electron microscopy (EM) is arguably the single most powerful method of characterizing heterogeneous catalysts. Irrespective of whether they are bulk and multiphasic, or monophasic and monocrystalline, or nanocluster and even single-atom and on a support, their structures in atomic detail can be visualized in two or three dimensions, thanks to high-resolution instruments, with sub-Ångstrom spatial resolutions. Their topography, tomography, phase-purity, composition, as well as the bonding, and valence-states of their constituent atoms and ions and, in favourable circumstances, the short-range and long-range atomic order and dynamics of the catalytically active sites, can all be retrieved by the panoply of variants of modern EM. The latter embrace electron crystallography, rotation and precession electron diffraction, X-ray emission and high-resolution electron energy-loss spectra (EELS). Aberration-corrected (AC) transmission (TEM) and scanning transmission electron microscopy (STEM) have led to a revolution in structure determination. Environmental EM is already playing an increasing role in catalyst characterization, and new advances, involving special cells for the study of solid catalysts in contact with liquid reactants, have recently been deployed.