Abstract
High-throughput grain mapping with sub-nanometer spatial resolution is demonstrated using scanning nanobeam electron diffraction (also known as 4D scanning transmission electron microscopy, or 4D-STEM) combined with high-speed direct-electron detection. An electron probe size down to 0.5 nm in diameter is used and the sample investigated is a gold–palladium nanoparticle catalyst. Computational analysis of the 4D-STEM data sets is performed using a disk registration algorithm to identify the diffraction peaks followed by feature learning to map the individual grains. Two unsupervised feature learning techniques are compared: principal component analysis (PCA) and non-negative matrix factorization (NNMF). The characteristics of the PCA versus NNMF output are compared and the potential of the 4D-STEM approach for statistical analysis of grain orientations at high spatial resolution is discussed.
Highlights
The last decade has borne witness to a surge in scanning transmission electron microscopy (STEM) experiments implementing scanning nanobeam electron diffraction (NBED), fueled by the development of high-speed, high-efficiency, direct-electron detectors, as well as advanced computational methods
Elemental maps obtained by STEM-X-ray energy-dispersive spectroscopy (XEDS) show gold- as well as palladiumrich nanoparticles, but with no clear distinction between individual grains under the electron dose and dwell times used for these experiments
We have shown that 4D-STEM using a sub-nanometer probe with a beam overlap of 50% combined with fast direct-electron detection enables efficient grain mapping at high spatial resolution
Summary
The last decade has borne witness to a surge in scanning transmission electron microscopy (STEM) experiments implementing scanning nanobeam electron diffraction (NBED), fueled by the development of high-speed, high-efficiency, direct-electron detectors, as well as advanced computational methods. In scanning NBED, the electron probe is rastered over 2D spatial co-ordinates (x, y) and an NBED pattern with 2D reciprocal space co-ordinates (kx, ky) is acquired at every dwell point. Virtual imaging by 4D-STEM with atomic resolution has been demonstrated By analyzing the spacing between diffraction peaks, measurements of crystal lattice strain can be made (Béché et al, 2009), and by acquiring 4D-STEM data during in situ deformation experiments, strain evolution can be probed locally in time-discrete steps (Pekin et al, 2018). Converging the electron beam such that the Bragg diffraction disks significantly overlap enables electron ptychography experiments in which phase information is retrieved from the beam interference for resolution-enhanced imaging (Jiang et al, 2018). In the work presented here, 4D-STEM is implemented in combination with fast electron detection to explore new possibilities for high-throughput grain mapping at high spatial resolution
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