Acceptance Angle Control
be accessed with only two apertures and ~20 mm of CL. Although the difference between inner and outer angles changes somewhat as the CL is changed, the angular selectivity that the aperture system provides is apparent, especially when consid- ering that the SEM sample stage can adjust the CL in very small increments.
Detector masks . Masks with apertures can be fabricated several ways. Perhaps the least expensive way is to poke or scratch an aperture in a piece of aluminum foil and then wrap the foil over the detector. Aligning a foil mask with the detector, however, can be challenging. A more strategic and robust approach is to design a layout of masks with apertures of incremen- tally varying geometry, and then have that layout photoetched or otherwise precision manufactured.
Results Multi-wall carbon nanotubes .
Figure 5 : Micrographs of MWCNTs in residual organic solvent imaged with different detectors. BF STEM images were recorded using a 20 μ m diameter aperture, and DF STEM images were taken with an annular aperture ( R Ai = 0.25 mm, R Ao = 0.5 mm) centered over a single diode. (a) BF image (CL = 3.75 mm, WD = 18.2 mm), (b) BF image (CL = 19.75 mm, WD = 1.9 mm), (c) SE image (WD = 1.9 mm), (d) ADF image (CL = 5.88 mm, WD = 14.2 mm), (e) ADF image (CL = 19.38 mm, WD = 0.8 mm), and (f) SE image (WD = 0.8 mm).
as well as annular darkfi eld (ADF) imaging at low, medium [ 14 ], and high angles [ 15 ]. Example setups . Selecting and implementing specific signal collection modes is straightforward with the aperture system. The basic procedure is to choose a mask with specific aperture dimensions and then use the SEM sample positioning stage to adjust the CL and admit electrons scattered into desired acceptance angle ranges. For example, Figure 4a shows how β o varies with aperture radius R o and H , the sample-to-mask distance. Note that CL = H + t + h , where t is the mask thickness, and h is the mask-to-detector distance. As the figure indicates, β o = 200 mrad can be obtained using all four diodes with different apertures (that is, with R o ≈1.6 mm and H = 8 mm, or with R o ≈2.8 mm and H =14 mm). Acceptance angles can also be shifted around the desired values by using the sample positioning stage to change the CL. Figure 4b shows how two different annular apertures can enable thin annular detector configurations to select electrons scattered through different angles. The black lines encompass acceptance angles accessible with a large aperture (inner radius R Ai = 3.5 mm, outer radius R Ao = 4 mm) centered over all four diodes; the blue lines encompass acceptance angles accessible with a smaller aperture ( R Ai = 0.25 mm, R Ao = 0.5 mm) located over a single diode. As the figure indicates, a large acceptance angle range can
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Figure 5 shows two examples of how the aperture system and the large CL can be used to reveal diff erent information. Both image sets in this fi gure show MWCNTs in residual n-methylpyrrolidone. Figures 5 a and 5 b show BF images recorded using
a mask with a 20 μ m diameter aperture, and Figure 5c shows a conventional SE image. Figures 5 d and 5 e show ADF images recorded using a mask with an annular aperture ( R Ai = 0.25 mm, R Ao = 0.5 mm) centered over one of the STEM detector diodes, and Figure 5f shows a conventional SE image recorded simultaneously with 5e. Individual nanotubes are discernable in 5a and 5b. In Figure 5b , however, bends and other deformations along many tubes can be observed, and inner and outer tube diameter measurements are feasible. Tubes are discernable in 5c, but diameter measurements are not feasible because of the residual solvent. In Figures 5 d and 5 f, many MWCNTs are visible, but they are difficult to differentiate from the residual solvent. Nanotubes in Figure 5e , however, can be differentiated from solvent because bright lines delineate the MWCNTs. Note that β i = 14 mrad is sufficiently small to capture Bragg-scattered electrons, and therefore it is feasible that the bright lines are partially due to diffraction. It is unclear why Figures 5 a and 5 d exhibit poor resolution compared to the other images, but the reduced resolution is consistently observed at shorter camera lengths with or without the aperture system in place. Au and TiO 2 particles on lacey carbon . BSE detectors are indispensable for numerous applications, but for suffi ciently thin samples where atomic number contrast is desired,
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