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Correcting for probe wandering by precession path segmentation

Gregory Nordahl, Lewys Jones, Emil Frang Christiansen, Kasper Aas Hunnestad, Magnus Nord

TL;DR

Problem: Probe wandering during precession electron diffraction degrades spatial resolution in SPED-derived VBF images. Approach: segment the precession path into $n$ parts and reconstruct from each segment, then rigidly align and sum to form a high-resolution VBF image. Key findings: with $\phi=1.0^{\circ}$ ($\approx17.5\mathrm{mrad}$) and $n=8$, segmentation reduces blur and improves contrast; edge-slope rises from $k=0.15\pm0.01$ to $k=0.38\pm0.03$ (≈2.5×). Additional discussion notes that approximately $46$ segments would be required to approach a single-probe-per-segment limit for $\alpha=1.2\mathrm{mrad}$ and $\phi=17.5\mathrm{mrad}$, implying tradeoffs with intensity and frequency. Impact: the approach can ease alignment requirements and enable higher-throughput SPED imaging with modern direct-detector systems.

Abstract

Precession electron diffraction has in the past few decades become a powerful technique for structure solving, strain analysis, and orientation mapping, to name a few. One of the benefits of precessing the electron beam, is increased reciprocal space resolution, albeit at a loss of spatial resolution due to an effect referred to as 'probe wandering'. Here, a new methodology of precession path segmentation is presented to counteract this effect and increase the resolution in reconstructed virtual images from scanning precession electron diffraction data. By utilizing fast pixelated electron detector technology, multiple frames are recorded for each azimuthal rotation of the beam, allowing for the probe wandering to be corrected in post-acquisition processing. Not only is there an apparent increase in the resolution of the reconstructed images, but probe wandering due to instrument misalignment is reduced, potentially easing an already difficult alignment procedure.

Correcting for probe wandering by precession path segmentation

TL;DR

Problem: Probe wandering during precession electron diffraction degrades spatial resolution in SPED-derived VBF images. Approach: segment the precession path into parts and reconstruct from each segment, then rigidly align and sum to form a high-resolution VBF image. Key findings: with () and , segmentation reduces blur and improves contrast; edge-slope rises from to (≈2.5×). Additional discussion notes that approximately segments would be required to approach a single-probe-per-segment limit for and , implying tradeoffs with intensity and frequency. Impact: the approach can ease alignment requirements and enable higher-throughput SPED imaging with modern direct-detector systems.

Abstract

Precession electron diffraction has in the past few decades become a powerful technique for structure solving, strain analysis, and orientation mapping, to name a few. One of the benefits of precessing the electron beam, is increased reciprocal space resolution, albeit at a loss of spatial resolution due to an effect referred to as 'probe wandering'. Here, a new methodology of precession path segmentation is presented to counteract this effect and increase the resolution in reconstructed virtual images from scanning precession electron diffraction data. By utilizing fast pixelated electron detector technology, multiple frames are recorded for each azimuthal rotation of the beam, allowing for the probe wandering to be corrected in post-acquisition processing. Not only is there an apparent increase in the resolution of the reconstructed images, but probe wandering due to instrument misalignment is reduced, potentially easing an already difficult alignment procedure.
Paper Structure (4 sections, 3 figures)

This paper contains 4 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Experimental methods
  3. Results and discussion
  4. Conclusion

Figures (3)

  • Figure 1: a) Eight precession path segments visualized in a diffraction pattern with de-rocking switched off. b) and c) VBF reconstructions corresponding to two different segments of the precession path.
  • Figure 2: VBF images from a precession path segmented SPED scan where an intentional pivot point misalignment has been introduced, showing the sums a) before and b) after rigid correction. c) Line profiles from the same region in both scans are included.
  • Figure 3: VBF images from a precession path segmented SPED scan with a proper alignment. Sum of individual VBF a) before and b) after rigid correction. c) and d) Intensity profiles from two different regions have been extracted from the two VBF sums.