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Acceptance Angle Control


Depending on the acceptance angles and the sample, STEM image contrast interpretation may or may not be straightforward because a sufficiently large agglom- erate of TiO 2 particles can elicit the same mass-thickness contrast as a single Au particle. This effect can be observed in the HAADF (Z-contrast) image of Figure 6c . Within the red dashed circle several regions of strong contrast are visible (both Au and TiO 2 ). The contrast is not particularly amenable to direct visual interpretation. However, Figure 6d shows that by moving the sample closer to the STEM detector (that is, reducing the CL) the acceptance angle increases, and the contrast between the Au particles and the TiO 2


particles increases.


Figure 6 : Au and TiO 2 particles on lacey carbon imaged with different detectors: (a) SE image (WD = 8.1 mm), (b) BSE image (WD = 8.1 mm), (c) HAADF STEM image ( R Ai = 3 mm, CL = 11.4 mm, WD = 8.1 mm), and (d) HAADF STEM image ( R Ai = 3 mm, CL = 7.4 mm, WD = 12.1 mm). STEM detector gain settings were unchanged for Figures 6 c and 6 d.


transmitted electron imaging may be worth considering because of the higher probability of forward-scattering than back-scattering. For example, Figure 6 shows images of Au and TiO 2 particles on a lacey carbon substrate recorded using the SE detector (6a), the BSE detector (6b), and the STEM detector in HAADF mode (6c and 6d). The STEM images were collected using the same gain settings at CL ≈11.4 mm ( Figure 6c ) and CL ≈7.4 mm ( Figure 6d ) with an aperture similar to the one shown on the top left of Figure 4a . The STEM and BSE images both show Z-contrast information complementary to the SE image in that isolated Au particles can be discerned from isolated TiO 2 particles. Although the BSE image shows the Au particles as bright spots, the STEM images simultaneously show the Au particles (four of which are circled in yellow) and the TiO 2 particles, which are generally less bright.


The persistent bright spots can be assigned to the Au particles. This assignment is supported by the BSE image, which shows bright spots in


the same locations as those in Figure 4d .


Figure 7 : SWCNT bundles with metal catalyst particles and amorphous carbon imaged with (a) the SE detector, (b) in HAADF STEM mode (aperture R Ai ≈0.5 mm and R Ao ≈1.25 mm), and (c) in marginal BF STEM mode (aperture R Ai ≈60 μ m and R Ao ≈0.22 mm). Note that β i is slightly less than the beam convergence semi-angle, α , and therefore the image is designated marginal BF.


16


Catalyst particles in bundled carbon nanotubes . Combining small apertures and long CLs to mix BF and DF signals can elicit useful contrast. For example, Figure 7 shows different ways to discriminate metal catalyst particles from carbon in a highly bundled SWCNT sample. Although some catalyst is visible in the SE image ( Figure 7a ), the HAADF STEM image ( Figure 7b ) directly reveals the catalyst particles as the bright spots. The amorphous carbon and SWCNT bundles, however, are generally not visible. In the marginal BF image ( Figure 7c ), residual catalyst and amorphous carbon, SWCNT bundles, and the carbon substrate can all be observed simultaneously, and the image features are generally sharper than those in Figures 7 a and 7 b. The catalyst particles appear dark in the marginal BF image because the STEM detector aperture only admits electrons scattered into acceptance angles between ~4 and 15 mrad. As observed previously [ 10 ], regions of the sample with greater mass-thickness can appear darker than regions with lesser mass-thickness. In this instance, the metallic catalyst particles scatter a significant fraction of electrons through angles larger than 15 mrad. Therefore, the signal due to the catalyst will be weak, and the particles will appear dark compared to the rest of the image. Scattering angles associated with the carbon are generally more shallow, and therefore a large fraction of the signal is collected resulting


www.microscopy-today.com • 2017 March


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