Convergent beam electron diffraction data collection with Gatan counted cameras

Convergent beam electron diffraction (CBED) is a powerful technique producing a wealth of information about the sample in the electron microscope. Its usefulness is expanded even further when combined with scanning of the electron beam to collect 4D STEM data. Here we will describe the steps required to collect high-quality diffraction data using Gatan's K3® or Metro® cameras. 

Camera setup for CBED or 4D STEM data collection

  1. With the microscope in STEM mode, reduce the beam's intensity by changing the spot size to one of the highest numbers (smallest spot) available. 
  2. Do basic STEM alignments and place the beam in a thin region of the sample just outside the region you will capture from.
  3. Choose the desired condenser aperture to define the convergence angle and center it on the Ronchigram center. 
  4. Center the undeflected diffraction beam on the camera.
  5. Set the camera length such that the desired range of reciprocal space is visible on the camera. For Metro, this camera length may be quite short. 
  6.  Verify that the brightest diffraction disk has a low enough intensity to be quantified accurately.
    • Place an ROI within the brightest region of the brightest diffraction disk and check the first value shown in the dose rate monitor at the bottom of DigitalMicrograph® software. This should be below 40 e-/pix/s (K3) or 80 e-/pix/s (Metro).
    • If the intensity is much less than these limits, consider changing the spot size again to increase the intensity.
  7. Capture
    • For CBED, place the STEM probe on the desired sample location and capture a long exposure image with the camera. 10 – 100 s is recommended. 
    • For 4D STEM, use the SI Acquisition palette in DigitalMicrograph, select CBED, set up the desired spectrum image position, size, step size, and pixel dwell time, then click Capture. A short pixel dwell time (high framerate) with a small step size is often better than a long pixel time and large step, though this will produce more data. Binning diffraction images (Metro) or selecting sub-area modes (K3) on the camera during acquisition can reduce the data size and enable faster acquisition.