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Microscopy 101


Figure 1: Si [011] SAED single crystal pattern showing that the beam block is not required when the intensity of the spots is uniformly distributed.


to diffraction mode, if it was previously centered, or if the camera length is changed.


2) With the screen lowered, insert the beam block and align it with the transmitted beam. Try to make the intensity that surrounds the beam block as uniform as possible. (Note: If the beam block driſts when inserted, have it tightened by a service engineer so that it doesn’t driſt. It should take some effort to move it, otherwise it will driſt because the vacuum is pulling it in. Exposure times of 60 s or longer are not uncommon for SAED patterns.)


3) Lift the screen and observe the illumination around the beam block. Set the exposure at about 0.1 second and carefully adjust the position of the beam block to obtain uniform illumination around it (Figure 2). Take care not to expose the transmitted beam while doing this. If you accidently move the beam block away from the transmitted beam, be prepared to quickly cover it again, lower the screen, or hit your beam blank button and then lower the screen. (It is important to avoid exposing the camera to a transmitted beam with an exposure that is too high!)


4) Increase the exposure time to obtain a better pattern with- out saturating the camera. Locate the brightest spot on the image as shown with the yellow circle in Figure 2 and determine the maximum exposure that can be used in the Record image.


a. Calculate the exposure in the Record image by taking into account binning for the View and Record images and the maximum counts to be included in the image. For example, the pattern in Figure 2 acquired with a Gatan Orius camera using a 1.5 s exposure with a binning of 3 gave 8,500 counts for the brightest spot in the (111) ring, indicated by the circle. Tis camera saturates at 15,000 counts. Calculation of the exposure to use in order to give a maximum count of 14,000 in Record image is given by the equation below:


 


 56


14 000 8 500 15 32222


, .. . ×


, ss‘


 


×=


Figure 2: Enlarged area of the central portion of the SAED pattern from polycrystalline gold showing the uniform intensity around the beam block. The beam block allows a longer exposure to give better SAED patterns. The yellow circle indicates the brightest pixel in the (111) ring of the pattern.


b. Tis is important! Prior to proceeding, reduce the View exposure to a very short time, similar to the exposure that was used to center the pattern on the crossed lines. Do not leave it at the exposure that was used to deter- mine the Record exposure. In DM, aſter an image is recorded, the live View image comes back. Aſter record- ing the image, remove the beam block. It is important that the camera is not exposed to the transmitted beam for a long period of time.


5) Record the SAED pattern with the calculated exposure using Binning 1 (Figure 3A). a. In DM, while the pattern is being recorded with the long exposure, change the Record exposure to a very short one. Tis will depend on the number of crystallites in the selected area, the intensity of the transmitted beam due to the diffraction aperture selected, and the amount of open area exposed to the transmitted beam. A very small transmitted beam that is not saturated is optimal. A typical starting value is 0.005 s, which can be adjusted if necessary.


6) Remove the beam block and immediately press Record to take the short-exposure pattern without the beam block present (Figure 3B).


7) As soon as the short-exposure recording is finished, lower the screen or, if available, press Beam Blank on the micro- scope first, which is faster. Tis will minimize the time that the camera is exposed to the transmitted beam without the beam block inserted.


8) Save the long-exposure recording and leave both images open in DM. It is not necessary to save the short-exposure recording.


www.microscopy-today.com • 2020 May


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