Microscopy101
Recipes for Consistent Selected Area Electron Diffraction Results: Part 1: Microscope Setup
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 transmis- sion 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. A procedure is presented that will allow a microscopist to reproducibly configure the microscope and sample to acquire selected area electron diffraction patterns so that these issues are addressed.
Keywords: TEM, selected area electron diffraction, SAED, calibration
Introduction As the subject matter expert for electron microscopy at the
CCDC-ARL Materials and Manufacturing Science Division at Aberdeen Proving Ground, MD, I work with researchers to help them use the transmission electron microscope (TEM) proficiently in their work. When using the electron microscopes, individual experience levels can vary from limited or none to fully skilled. One duty that I have assumed is to ensure that the TEM has been carefully calibrated for all imaging and diffraction modes. Regard- less of their experience level, every user receives individual training on our instruments, tailored to their needs to make them success- ful. Tis training obviously includes the procedure for aligning the TEM and ensures that the investigators know the conditions used to calibrate the microscope. Tis ensures they can duplicate the procedures and rely on their data. Consistency is the key for that reliability. In this three-part series of articles, I will go through my procedures for collecting, recording, and analyzing selected area electron diffraction (SAED) images in the TEM to assure that the data are reliable and presentable. Te microscope used for data acquired in this article was a JEOL 2100F TEM/STEM equipped with a Gatan 832 Orius SC1000 camera and Gatan’s Digital Micro- graph (DM) [1]. Te procedures outlined here are somewhat biased toward that platform and may require some modifications if other equipment is used. An advantage of DM is that it has the capability to run scripts, and I describe some simple scripts and readily available scripts that make life a bit easier (pun intended) when processing and saving the SAED patterns. In Part 1 I will present my procedure for setting up the
sample and the microscope for acquiring SAED patterns from both calibration standard and unknown samples. In Part 2, I will present methods for acquiring SAED patterns. In the pro- cess, I will try to add a few hints and tricks, while emphasizing
40 doi:10.1017/S1551929520000255
techniques that will protect the digital camera. Additionally, I will discuss how to optimize the data, using simple DM scripts, for easy use with electron diffraction analysis soſtware. In Part 3, I will discuss some of the analysis techniques available with existing soſtware, what features the packages should have, and then finally discuss the reliability of SAED measurement results.
SAED Acquisition Introduction Every user of the TEM has been taught the comparison
between the TEM and a light microscope. When training a user I ask a simple question, “How do you focus a light microscope?” Te answer, of course, is that you change the distance between the objec- tive lens and the sample, usually by changing the height of the sam- ple stage. Ten I ask, “Can you vary the focus of the objective lens of the optical microscope?” and the answer is that you can’t for a given objective lens. Ten, “How do you fix the focal length of the TEM when you have a knob that can change the strength of the objec- tive lens?” To fix the focal distance of the TEM objective lens, you must set the strength of the lens coils to the same value every time. When that happens, the back focal plane of the lens is fixed in space relative to the lens, just as it is for the objective lens of a light micro- scope. On modern microscopes, that is done by pushing a button labeled something like “standard focus,” and the user can read the hexadecimal value on the computer display and see that it is always the same. On older microscopes, it was common practice to have a five- or six-digit voltmeter read the voltage on the objective lens, and the users would adjust the focus until it read a particular value. Te lens strength is usually set so that a sample positioned at the stage’s eucentric position is in focus, but be careful; that position can change slightly with different holders. Te sample is focused first by setting the strength of the objective lens and then raising or lower- ing the sample until it is in focus, completely analogous to an optical microscope. If this is the condition in which all of the magnifica- tion and diffraction modes are calibrated with a standard sample, users are assured that when they use these modes on an unknown sample, their images and diffraction patterns will also be calibrated.
SAED Acquisition Procedure
1) Follow the manufacturer’s or your lab manager’s proce- dures for aligning the microscope. Remember to do this at the standard focus condition. Te gun, condenser, tilt compensation, shiſt compensation, condenser astigmatism, objective astigmatism, and high voltage center (or current
www.microscopy-today.com • 2020 March
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