Techniques Development
Optimal STEM Convergence Angle Selection Using a Convolutional Neural Network and the Strehl Ratio by N Schnitzer, SH Sung, and R Hovden, Microsc Microanal | doi:10.1017/S1431927620001841 Selection of the correct convergence angle is essential for
achieving the highest resolution imaging in scanning trans- mission electron microscopy (STEM). Use of poor heuristics, such as Rayleigh’s quarter-phase rule, to assess probe quality and uncertainties in measurement of the aberration function result in incorrect selection of convergence angles and lower resolution. We show that the Strehl ratio provides an accurate and efficient way to calculate criteria for evaluating probe size. A convolutional neural network trained on the Strehl ratio is shown to outperform experienced microscopists at selecting a convergence angle from a simulated electron Ronchigram ( Figure). Generating tens of thousands of simulated examples, the network is trained to select convergence angles yielding probes on average 85% nearer to optimal size at millisecond speeds. Assessment on experimental Ronchigrams with inten- tionally introduced aberrations suggests that trends in the optimal convergence angle are well modeled, but high accu- racy requires extensive training datasets. Tis near immediate assessment of Ronchigrams using the Strehl ratio and machine learning, highlights a path toward rapid, automated alignment of aberration-corrected electron microscopes.
Convergence angles selected by the convolutional network and a microscopist for a simulated electron Ronchigram and the corresponding probe sizes.
Material Applications
Nanoscale Visualization of Phase Transition in Melting of Sn–Bi Particles by in situ Hard X-ray Ptychographic Coherent Diffraction Imaging by N Ishiguro, T Higashino, M Hirose, and Y Takahashi, Microsc Microanal | doi:10.1017/S1431927620024332 Te phase transition in the melting of Sn–Bi eutectic solder
alloy particles was observed by in situ hard X-ray ptychographic coherent diffraction imaging with a pin-point heating system. Ptychographic diffraction patterns of micrometer-sized Sn–Bi particles were collected at temperatures from room temperature to 540 K. Te projection images of each particle were recon- structed at a spatial resolution of 25 nm, showing differences in the phase shiſts due to two crystal phases in the Sn–Bi alloy system and the Sn/Bi oxides at the surface (Figure). By quanti- tatively evaluating the Bi content, it became clear that the non- uniformity of the composition exists in the particle when the particles are synthesized by the centrifugal atomization method. Tis knowledge is useful for optimizing the gas atomization process for Sn–Bi alloy particles. We believe that the present system provides a novel opportunity for studying and monitor- ing changes in the nanoscale structure, chemical composition, and morphology of bulk materials, such as catalysts, batteries, rubbers, and magnetic materials.
58 doi:10.1017/S1551929520001492
Phase images of Sn–Bi Particles by in situ hard X-ray ptychographic coherent diffraction imaging, showing remarkable phase shift due to two different crystal phases in the Sn–Bi alloy system and the Sn/Bi oxides at the surface.
www.microscopy-today.com • 2020 November
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