Acceptance Angle Control
Figure 3 : (a) A schematic of the experimental parameters and electron trajectories necessary to implement different signal collection modes. The working distance (WD) is the distance between the specimen and the polepiece (not shown). Illustrations demonstrating the aperture system modularity: (b) stacking two masks to obtain an annular detector with different inner and outer radii, and (c) enabling several imaging modes with different apertures positioned over each of the four STEM detector diodes.
and a mask/aperture ( Figure 2a ) that can be adapted to most commercially available STEM detectors and (2) the cantilever- style holder shown in Figure 2b at an arbitrary orientation with an edge-clamped sample. Signal collection modes . Signal collection modes,
described in Table 1 and shown schematically in Figure 3a with the optic axis and a mask centered over a single diode, can be implemented by using masks in diff erent ways. For example, a variable-annulus aperture scheme can be implemented by stacking masks with diff erent apertures ( Figure 3b ). T e mask on the upper leſt can be used alone to exclude small-angle scattering from DF images, or it can be stacked with the mask on the upper right to admit electrons scattered through a specifi c angular range. Note that BF imaging can still be implemented with the lower detector plate by including a small aperture in the center of the masks.
T e four rectangular detector diodes provide much utility when combined with the integral xyz -positioning stage. For example, if the detector stage is used to align individual diodes with the optic axis, each of those diodes can be used for diff erent imaging modes when appropriate apertures are employed ( Figure 3c ), provided that off -axis detector elements can be disabled or masked. A signifi cant advantage to locating apertures over individual detector diodes is that very small acceptance angles can be selected. In this way, the aperture system enables signal collection modes beyond basic BF and DF imaging ( Table 1 ): BF imaging with apertures other than the existing 100 μ m through-hole, annular brightfi eld imaging [ 11 ], thin annular detection schemes [ 12 ], marginal brightfi eld imaging akin to that described by Cowley [ 13 ] in which a thin annular detector collects electrons scattered into acceptance angles straddling the beam convergence angle,
Figure 4 : (a) A plot showing acceptance half-angles that can be obtained using all four elements of the upper STEM detector plate, and (b) a plot showing how camera length (CL) can be used to adjust acceptance half-angles for two different annular apertures that enable thin annular detection modes. When a mask is centered over a single detector element, the box representing the detector should be extended to the optic axis.
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www.microscopy-today.com • 2017 March
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