Nanoscale mapping of point defects with 4D-STEM

The following 4D-STEM data sets were collected on the ThemIS and TitanX scanning transmission electron microscopes located at the National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. 1• Authinfilm_reference_R1.dm4, calibration_reference_R1_AlStd.h5 4D-STEM measurements were conducted in situ on an FEI ThemIS image corrected microscope at 300kV during a series of heating and cooling experiments. Nano-diffraction data was collected using a Gatan K2-IS (2k 2k) detector at 400 frames per second. Each dataset contains a set of electron diffraction patterns taken at each scan position with a ~ 1 nm probe step size (1 milli-radian convergence angle). Approximately 80 80 scan positions were recorded from each region with a dwell time of 0.0025 seconds per frame. A custom 40µm patterned “bullseye” circular probe forming aperture was used to enhance the accuracy of 4D-STEM strain analysis by facilitating the identification of the center of diffraction discs. A convergence angle of 3.20 milli-radians, spot size of 8, and diffraction pixel size of 0.16 Å^-1 was used in micro-probe lens configuration. The data was machine and software binned to 512 512 pixels to increase the signal to noise ratio before computational analysis. Data processing were performed using strain mapping scripts provided in the open source py4DSTEM software package and by a series of custom-made Python scripts. 2• irrAl_sacn1.dm4, calibration_irrAl_scan1.h5 4D-STEM measurements were conducted on an FEI TitanX microscope equipped to do high-angle STEM tomography and operating at 300kV. Nano-diffraction data was collected using a Gatan Orius 830 (2k 2k) detector capable of collecting 30 frames per second. Each dataset contains a stack of convergent beam electron diffraction (CBED) patterns taken at each scan position with maximum resolution equivalent to 1.6 nm probe size. Approximately 50 50 frame scan regions were recorded with a dwell time of 0.01 seconds per frame. A custom 70µm patterned “bullseye” circular C2 aperture was used to greatly enhance the accuracy of 4D-STEM strain analysis by facilitating the identification of the center of diffraction discs. A convergence angle of 2.7 milli-radians, spot size 10, and camera length 195 mm was used in micro-probe lens configuration. With a measured screen current of 300 pA in this configuration, the total sum of electrons incident in a region of the sample, commonly known as the fluence (total dose), was determined at 67,100 electronsÅ^-2 per 4D-STEM scan. The 4D-STEM data was machine and software binned to 512 x 512 pixels to increase the signal to noise ratio before computational analysis. Data processing and image analysis were performed using strain mapping scripts provided in the open source py4DSTEM software package and by a series of custom-made Python and MATLAB scripts.