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Nanobeam diffraction

Nanobeam diffraction (NBED) is a 4DSTEM experiment that uses a small convergence angle so the Bragg disks in each diffraction pattern stay separated and can be indexed. This page covers the beam settings that differ from the 30 mrad STEM default. For detector setup, acquisition, and end of session, follow 4DSTEM.

Prerequisite: Complete 4DSTEM Part 1: Detector setup and Part 2: Beam setup first. Then return to Part 3: Acquisition after these steps.

Why change the convergence angle for 4DSTEM?

The convergence angle depends on the type of 4DSTEM experiment. For ptychography, 30 mrad (the same as standard STEM) works well because overlapping disks are part of the reconstruction. For nanobeam diffraction, where separated Bragg disks are needed to index reflections, a much smaller angle is used, typically 1 to 10 mrad depending on the material and the required disk separation.

Interactive demo: Explore how convergence angle affects the CBED pattern at bobleesj.github.io/electron-microscopy-website/cbed

Configure beam for nanobeam diffraction

The default STEM setup uses C2 = 70 and 30 mrad convergence angle. For nanobeam diffraction, reduce both to get separated Bragg disks. The table below shows typical values:

ParameterSTEM defaultNanobeam diffraction
C2 aperture7050
C3 aperture100030
Convergence angle30 mrad~10 mrad
Beam current~0.150 nA~0.032 nA
Camera length91 mm230 mm
Why change the C2 aperture?

The C2 aperture limits the angular range of electrons entering the probe-forming optics. A smaller aperture (50 vs 70) blocks more off-axis electrons, producing a more coherent beam with cleaner diffraction patterns at each probe position. The C2 aperture size and convergence angle are proportional (approximately 7:1 ratio, e.g. C2 = 70 gives ~10 mrad).

TODO: Confirm the C2 aperture to convergence angle ratio with staff

  1. In TEMUI, go to Tune tab, then Apertures. Change C2 from 70 to 50, and C3 from 1000 to 30.

    Aperture settings after adjustment

  2. In Beam Setting, click MF-Y Convergence Angle. Use the mulY knob to adjust the convergence angle to 10 mrad, then click MF-Y Convergence Angle again to deselect.

    Beam Settings panel with MF-Y Convergence Angle selected

  3. Adjust beam current: in TEMUI, go to Mono, click Focus, and use the intensity knob to set the current to ~0.032 nA.

  4. Set the camera length to 230 mm (or 285 mm depending on the required angular range for your material).