Measuring and Managing Electron Dose
Figure 3: User interface of the AXON Dose Image Analysis tool, shown here with a zeolite sample. The fast Fourier transform (FFT) of the image with a radial line scan across the FFT is shown in the upper right of the figure. These tools, combined with cumulative dose and dose rate information, allow investigators to quantify changes in the intensity of the Bragg spots (“spot fading”) as an indicator of beam damage to the sample.
Operation An example of beam interactions producing visible changes
in a material is easily observed in liquid cell STEM. In situ studies have shown that rastering the electron beam through aqueous solutions causes radiolytic decomposition of water into reducing and oxidizing species, and aqueous electrons can drive the nucleation and growth of metallic nanoparticles. Te onset of nucleation, the particle growth rate, and other char- acteristics of the process are known to be dose- and dose rate- dependent. With AXON Dose, the impact of the electron beam on nanoparticle growth can be quantified.
Figures 4 and 5 show STEM images of HAuCl3 solution
imaged in a Poseidon liquid cell using AXON Dose. An area of the liquid cell was imaged at high magnification until a reac- tion was observed, and then the magnification was gradually reduced in steps. Tis created an image with high dose areas in the center surrounded by lower dose areas in the periph- ery. AXON Dose heatmaps allow these areas to be visualized as heatmaps, and the precise dose history for each pixel on each image is known. It is clear from the images that a correla- tion exists between cumulative dose and particle size and that higher electron dose produces larger particles.
Figure 4: A STEM image with a “heatmap” overlay showing the cumulative dose across a sample area. The HAuCl3 size (top right), whereas the low dose area (bottom right) has finer particles.
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area exposed to high dose has a large particle
www.microscopy-today.com • 2022 July
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