Recent Advances in Transmission Electron Microscopy for Materials Science at the EMAT Lab of the University of Antwerp

Abstract

The rapid progress in materials science that enables the design of materials down to the nanoscale also demands characterization techniques able to analyze the materials down to the same scale, such as transmission electron microscopy. As Belgium's foremost electron microscopy group, among the largest in the world, EMAT is continuously contributing to the development of TEM techniques, such as high-resolution imaging, diffraction, electron tomography, and spectroscopies, with an emphasis on quantification and reproducibility, as well as employing TEM methodology at the highest level to solve real-world materials science problems. The lab's recent contributions are presented here together with specific case studies in order to highlight the usefulness of TEM to the advancement of materials science.

Keywords: ACOM TEM; EDS; EELS; STEM; TEM; atom counting; compressed sensing; electron diffraction tomography; electron tomography; nanomechanical testing.

Figure 1

Complementary [001] HAADF-STEM (a) and ABF-STEM (b) images of fully-charged Na2-xIrO3 with an O1-type structure. Magnified fragment of the ABF-STEM image (c) with marked projections of the IrO6 octahedra (red dots: Ir columns; blue: Na; and yellow lines: O octahedral projections). Intensity profiles measured in the areas marked in blue and red highlighting short and long projected O−O distances (d). Adapted with permission from [33]. Copyright 2016 American Chemical Society.

Figure 2

(a) Screen shot of the StatSTEM software, showing the atom-counting results on an experimental image of a Au nanorod; (b) Counting the number of atoms from a HAADF-STEM image of a Ag-coated Au nanorod; and (c) 3D atomic scale characterization of a Au nanodumbbell before and after heating.

Figure 3

Experimental realization of compressed sensing for STEM imaging. (a) Conventional high resolution STEM image; (b) image over the same region of the same sample, where 20% of the pixels are acquired and 80% are blocked; and (c) the reconstructed image.

Figure 4

(a,c) Refined models for two HAADF-STEM projection images of a Ag atomic cluster embedded in an Al matrix; (b,d) Number of Ag atoms per projected atomic column for both images; (e) 3D representation of the reconstructed volume of the Ag nanoparticle, based on discrete tomography [59].

Figure 5

(a) Slices through the atomic resolution 3D reconstruction of a Au nanorod, revealing the atomic lattice and the surface facets present; (b) 3D visualization of a core—shell Au@Ag nanorod, where the Au core is rendered green, and the Ag atoms are visualized in orange [2,74].

Figure 6

(a,b) εxx and εzz strain field showing surface relaxation in both directions. The colour scale indicates the expansion of the lattice parameter with respect to a reference; (c,d) The surface relaxation was confirmed by measuring the lattice parameter on slices through the reconstructions at the positions indicated in (a,b) [75].

Figure 7

Octahedral tilt and orbital hybridization in a perovskite thin film. (a) At the interface between perovskites with different octahedral tilt systems, the oxygen octahedra have to rotate to maintain corner connectivity across the interface; (b) inverted-contrast ABF-STEM image showing the octahedral tilt in a LSMO on NGO film. In the inset, the image simulated from the obtained atomic structure (at the centre); (c) B-O-B angle between the oxygen atoms and the B cations as a function of the atomic plane; (d) inverted ABF-image of a similar film grown on top of a STO buffer layer, displaying no tilt; (e) the fine structure of the EELS oxygen peak measured in the buffered film. Each spectrum is measured from a region with a one unit cell thickness. The numbers in the inset indicate the distance of the chosen layer from the interface, expressed in unit cells. There is an obvious dependence of the fine structure from the distance; and (f) the fine structure of the EELS oxygen peak measured in the non-buffered film. There does not appear to be any dependence of the shape of the fine structure on the distance from the interface.

Figure 8

(a–c) 3D visualization of the EDS reconstructions of three different nanoparticles at different stages of a galvanic process reaction. Au and Ag are represented by red and green, respectively; (d) 3D visualization of the EDS reconstruction of a Fe-Co nanodumbbell, revealing the distribution of the elements in 3D. Co and Fe are represented by blue and green, respectively [82,83].

Figure 9

(a) Example of ACOM-TEM on a nanocrystalline Pd thin film. The colour code inverse pole figure is shown in the inset; (b,c) a virtual BF image and the corresponding local strain field map of a fatigued single-crystal Ni micropillar with dislocation walls seen as black curved lines running between opposite sides and are highlighted with white arrows.

Figure 10

(a) Schematic of the PicoIndenter holder; (b) TEM BF image of a PTP device; (c) SEM image of a tensile sample mounted on a PTP device; (d) TEM BF image of a FIB near-defect-free single crystal Ni sample containing only a few dislocations, and (e) engineering stress-strain curve of the single crystal Ni sample, the images in the insets are snapshots from the recorded deformation movie, corresponding to the points at the arrows’ origin. Note the strain bursts induced by the operation of the individual SAS (single arm source).

Figure 11

Improvement of band gap measurement. (a) Low loss spectrum acquired with the improved setup (blue) contrasted with one acquired in a conventional experiment (red). The distortion of the data caused by the parasitic losses is clearly visible; (b) ADF-STEM image of a multilayer sample composed of several wide-band gap semiconductors, and (c) recorded spectra clearly showing the onset of the inter-band transitions and, hence, the band gaps, for the different portions of the sample.