Microscopy101
Recipes for Consistent Selected Area Electron Diffraction Results: Part 2: Recording the SAED Pattern
Scott D. Walck SURVICE Engineering Co., contractor to U.S. Army Combat Capabilities Development Command Army Research Laboratory, Aberdeen Proving Ground, Aberdeen, MD 21001
scott.d.walck2.ctr@mail.mil
Abstract: Electron diffraction is an essential tool for materials scientists to have in their characterization toolboxes. When using the transmission electron microscope (TEM) to perform diffraction experiments, setting up the microscope for both calibration standards and unknown materials in a consistent method will ensure that dependable results are obtained. Care must also be exercised to protect digital cameras from intense transmitted and diffracted beams to avoid damage. In Part 2 of this series a procedure is presented for the proper recording of SAED patterns.
Keywords: TEM, selected area electron diffraction, SAED, calibration
Introduction In Part 1 of this series of articles proper procedures for
reproducibly setting up a transmission electron microscope (TEM) for selected area electron diffraction (SAED) patterns were discussed. It is critical that data from unknown samples are acquired using the same conditions that were used to cali- brate the microscope with known samples. In this segment, methods for recording the SAED patterns with digital cameras that provide high-quality centered patterns that are well suited for diffraction soſtware, while protecting the camera from damaging exposures, are presented.
Recording the SAED Pattern Procedure Tis is important: only the lowest binning setting of the
camera, that is, binning of 1, should be used to record diffrac- tion patterns. Tis camera setting is the least sensitive to the high intensity of the transmitted beam and intense diffracted beams that oſten occur during collection of SAED patterns.
1) An initial decision of whether to use a beam block is required. Tis may depend on the type of pattern being displayed or the type of recording system that is available. a. No Beam Block: record the pattern using an appropriate exposure.
i. Regardless of the type of system available, if a pattern is from a single crystal where the transmitted beam is about the same intensity as the diffracted beam(s), as may occur on a low-index zone axis or a two-beam condition, then a beam block is not necessary (Fig- ure 1). Simply record the pattern with the appropriate exposure.
ii. A beam block might not be necessary if a camera with sufficient dynamic range that will capture the intense
54 doi:10.1017/S1551929520000887
transmitted beam and the less intense diffracted spots is available.
iii.A pattern without a beam block present is preferred. For most SAED patterns working without a beam block results in an overexposed transmitted beam region that will oversaturate the camera in the center of the pattern. Typically, if a beam block is not used, gamma processing of the digital image is required to show low- intensity diffraction information in the pattern.
b. Beam Block: record the pattern with “double exposure.” i. If using film or an imaging plate, a double exposure on a single sheet of film can be acquired by first collecting a long exposure with the beam block stopping the trans- mitted beam and then a second very short exposure with the beam block removed. Tis creates a diffraction pattern with a small spot at the center of the pattern.
ii. Use one of the following methods to produce a “double-exposure” pattern using a digital camera.
Recording a “Double-Exposure” SAED Pattern with a Digital Camera A double exposure with film is taken by opening the shut-
ter twice with the same piece of film in place. Of course there is no controlled way of taking a “double exposure” with a digital camera that produces a single image. It is possible to take a “double exposure” with a digital camera by removing the beam block shortly before the end of a long exposure. However, this is not a very consistent and reliable method for collecting the correct exposure for the transmitted beam. However, a long- exposure digital image acquired with a beam block can be added to a short-exposure digital image acquired without one. In Gatan’s Digital Micrograph (DM) and with other acquisi- tion programs care must be taken to ensure the image tags (that is, the metadata that carry the calibration parameters) are carried along to the new SAED image. Te following proce- dure describes how to safely take a long- and a short-exposure SAED pattern to create a single image that shows a bright dif- fraction pattern with a superimposed transmitted beam (000) that marks the center of the pattern. Method to acquire two SAED patterns:
1) Center the transmitted beam as described in Part 1 of this series. Te pattern center should be checked every time. Te pattern center will move when changing from imaging
www.microscopy-today.com • 2020 May
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