Abstract:
The main experimental difficulty in extracting the crystal structure and chemical information from individual nanostructures and interfaces is often associated with a generally weak underlying signal. Spatially resolved Analytical Electron Microscopy (AEM), which typically combines Scanning Transmission Electron Microscopy (STEM) with different spectroscopy techniques, such as Energy Dispersive X-ray Spectroscopy (EDXS) and Electron Energy-Loss Spectroscopy (EELS) has been revolutionised the characterization and understanding of nanostructures and internal interfaces in materials by providing atomic-scale structural and chemical information. Nowadays, modern probe aberration-corrected STEMs with the electron probe size below 1 Å allow combined atomic resolution imaging and spectroscopy of nanostructures and associated structural defects. In this presentation, the above described analytical capabilities will be discussed through the perspective of high-resolution AEM studies performed on three different metallic systems. These are complex metallic nanostructures with hollow interiors, heavy rare earth (HRE)-rich Nd–Fe–B-based magnets and (Ca,Gd)Cu5 single crystal.
Complex metallic nanostructures with hollow interiors: by the combined information of STEM imaging and EELS we were able to measure the nitrogen gas pressure within individual nanospheres, which provide us with vital clues on how gas-filled hollow spheres could be fabricated in various complex metallic systems via a single-step procedure by the ablation of a metallic or alloy-based target into ambient nitrogen gas.
HRE-rich Nd–Fe–B-based magnets: Small amounts of the HREs (Dy, Tb) that partially replace the Nd in the grain-boundary diffusion process have a large, positive influence on the coercivity of the whole Nd–Fe–B-based magnet. Their reliable quantification is essential for optimising hard-magnet processing, but often compromised not only because of the limitations due to the analytical detection limits, but also because of the strong signal overlap that is found in EDXS and EELS. In this study, we report on the possible pitfalls of the related measurement techniques. The study yielded a step-by-step procedure for correctly assessing the true concentration of the HREs in a TEM by applying quantitative EELS analysis of grain boundaries and triple pockets.
(Ca,Gd)Cu5 single crystal: This model complex alloy was selected to demonstrate the strength of two complementary atomic resolution imaging modes; High-Angle Annular Dark-Field (HAADF) and Annular Bright-Field (ABF) STEM combined with STEM image simulations based on a given structural model and experimental microscope electron-optical parameters. When these techniques are used in a combined manner they can provide comprehensive information about the underlying crystal structure in terms of atoms positions and atom types at the atomic scale.
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