Acceptance Angle Control
Figure 8 : STEM images of 2D exfoliated zeolites with the optic axis positioned at the edge of a small ADF aperture ( R Ai = 0.25 mm, R Ao = 0.5 mm): (a) an image simultaneously showing BF and DF regions, and the transition region between the two, and (b) an image of the sample in the transition region. The red markers in the insets indicate the optic axis position on the aperture.
in brighter regions in the image. The image background exhibits a moderate level of contrast because electrons in the outer fringe of the incident illumination cone (that is, primary electrons with incident angles between 4.2 and 5.3 mrad) are able to pass through the aperture and be collected by the STEM detector. Despite the unconventional contrast, discerning the different phases in Figure 7c is still straightforward.
Exfoliated 2D zeolite sheets . Another way BF and DF signals can be mixed is by moving the STEM detector laterally with respect to the optic axis. For example, Figure 8 shows STEM images of ~3 nm thick exfoliated zeolite sheets [ 9 ] on an ultra-thin carbon/lacey carbon substrate. Here, the detector was moved so the optic axis intersected the edge of a small ADF aperture ( R Ai = 0.25 mm, R Ao = 0.5 mm) at the point indicated by the red ‘×’ in the insets. Figure 8a shows both BF and DF regions, as well as the transition between them. Figure 8b shows a higher magnification image of the transition region. The image is different from conven- tional STEM images in that it appears to show topographic information.
Discussion
One feature that should be considered when implementing the proposed experimental setup is that by using the SEM sample positioning stage to change the CL, the working distance (WD) must change to maintain focus at the sample. As the WD changes to maintain focus at the sample, the beam convergence angle must also change. For the SEM used here, the beam convergence half-angle can be reasonably estimated as α ≈2.53 D a /( WD + 9), where D a is the beam condenser aperture diameter, and WD is the working distance (both with mm units). T e 30 μ m condenser aperture used here, combined with the ~ 1–20 mm CL, enables ~2.6 < α < ~7.5 mrad. T is can be advantageous in a microscope that does not directly enable beam convergence angle control: more parallel illumination can be obtained by employing a long WD and/or a smaller beam condenser aperture, and more convergent illumination can be obtained by using a short WD and/or a larger beam condenser aperture. Knowledge of this feature is useful for controlling which scattering mechanisms contribute to image contrast. For example, small-angle coherent scattering (due
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to Bragg diff raction) can be collected in ADF imaging mode by using a long WD, a small beam condenser aperture, and a small STEM detector aperture. Higher-angle incoherent scattering (that is, scattering that contributes to Z-contrast) can be collected by employing a short WD, a larger beam condenser aperture, and a HAADF STEM detector aperture with a large inner radius. T e micros- copist should be acutely aware that these are very generic operating recommendations, and image contrast will be highly dependent
on the combination of sample, STEM detector, primary electron beam condenser apertures, WD, CL, and primary electron energy. Each of these parameters should be carefully considered when collecting and interpreting images. If the eff ects of changing beam convergence angle are not desired for a particular experiment, then the sample can be held stationary and the STEM detector xyz -positioning stage can be used to change the CL by ~10 mm.
Most commercially available STEM detectors have inherently large acceptance angle ranges, and therefore multiple contrast mechanisms will likely contribute to image contrast, particularly at small acceptance angles. For example, solid-state STEM detectors generally employ a BF detector with an acceptance angle significantly larger than the beam convergence angle. The STEM detector used here has an integrated 100 μ m aperture for BF imaging that can enable acceptance half-angles up to 50 mrad. This is significantly larger than the 7.5 mrad beam convergence angle available with the 30 μ m condenser aperture. In conventional TEM parlance, this means multi-beam imaging conditions likely contribute to image contrast. While combined contrast mechanisms can lead to useful images as demonstrated by Figure 7c , it can also complicate image interpretation. A significant advantage to the modular aperture approach described here is that very narrow acceptance angle ranges can be obtained over a wide range of scattering angles, and, therefore, electrons scattered by different mechanisms can be selected in controlled manner. In the long-term, this ability should improve the understanding of transmission imaging in an SEM, make image interpretation straightforward, and lead to rigorous quantitative analyses with reduced uncertainty. In the short term, it is unclear why Figures 5 a and 5 d exhibit poor resolution and weak contrast compared to the other images. These phenomena have been consis- tently observed with diverse samples at very short camera lengths with and without the aperture system in place. It is unlikely that the reduced imaging performance is due to the aperture system.
In closing, an operational guideline is recommended for new STEM-in-SEM users. Most modern SEMs enable large beam shift capabilities that compensate for moderately coarse sample stage positioning steps. Using the beam
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