A Beginner’s Guide to Scanning Electron Microscopy (SEM) Products
Across different fields of study, electron microscopy (EM) has been widely used to create high-resolution images of incredibly small materials, organic or non-organic. This method relies on the use of electrons–light, negatively-charged subatomic particle–to emit radiation and illuminate the target, which is then captured and recorded.
One of the most popular types of EM is the Scanning Electron Microscope, which focuses an electron beam to scan a defined rectangular area of the specimen in a process called raster specimen. From observing biological specimens in detail to studying the behavior of different materials, SEM has proven invaluable in advancing our understanding of biology, medicine, materials science, engineering, and more. To start, here’s what you need to know about SEM products.
How Does an SEM Work?
Like all electron microscopy technologies, SEMs use a beam of electrons to “see” the target. However, this method scans the objects using a raster scanning pattern. Then, the electrons that make contact with the specimen loses energy as it passes through, resulting in any of the following forms in addition to heat and light:
- Secondary electrons. This results from electrons that are ejected from the specimen because of the SEM-fired electrons. Constituting one of the most common SEM imaging methods, detecting these low-energy secondary electrons is usually done just a few nanometers from the surface of the specimen, through a specialized device called the Everhart-Thornley Detector.
- Backscattered electrons (BSE). Contrary to the low-energy secondary electrons from the sample material, BSE refers to high-energy electrons coming from the electron beam. These are either reflected or scattered out the back. They are also used in SEM products that are used for materials with varying chemical compositions because of varying reactions to elements based on their atomic number, which in turn is based on the number of protons in the atom.
An average scanning electron microscope fires its electron beam through an electron gun that contains a tungsten filament. This beam is then focused using condensers, narrowing it to a spot that ranges between 0.4 to 5 nanometers. In context, human hair is generally about 100,000 nanometers wide.
Then, the condensed electron beam goes through a set of scanning coils or deflector plates (depending on the setup) that deflects the beam to let it scan in a rectangular raster scanning pattern. Most SEM products available today store their images digitally as opposed to capturing images on film. Each pixel on the resulting image corresponds to a specific beam position on the specimen. It’s easier to think of an SEM image as a distribution map of the signals gathered during the scanning process, concatenated together to form a visually intelligible image.
Comparing SEM and TEM
SEM is one of the most popular types of scanning electron microscopy, and the other is TEM or Transmission Electron Microscopy. Unlike the scanning variant, TEM creates an image by directing a broader electron beam to illuminate a specimen, which it then magnifies to generate an image. It fundamentally operates to how light microscopes work, but with electrons as the light source.
Although the two types of electron microscopes were developed shortly apart–the first TEM being created in 1931 and the SEM in 1937–they have different purposes and advantages. To start, SEM products generally cost less compared to their TEM counterparts. Transmission electron microscopes are larger and bulkier and require more time to generate an image.
In terms of applications, SEM is commonly used for imaging the surfaces of the sample, organic or non-organic, making it great for analyzing morphology or the form of its samples. On the other hand, TEM is used for imaging thin sections of the specimen to see what’s inside. It can be used to observe internal composition and morphology.
Their applications mean that for using transmission electron microscopes, the sample has to be cut thinner to allow the electrons to pass through and illuminate the sample. Aside from requiring a certain level of skill to prepare specimens for TEM processes, there are also specialized processes and equipment to help make sure that the sample is as thin as needed and that no debris is included in the sample. SEM, on the other hand, only requires coating the sample, which requires less time. Some samples can even be mounted directly on the stub and onto the microscope. Without the need to thoroughly prepare samples, SEM processes can work on large amounts of samples, unlike TEM.
For their imaging capacities, TEMs definitely have a lot stronger magnifying power compared to SEMs. However, the field of view available for transmission electron microscopy is significantly smaller compared to the scanning type, which also accounts for the time it takes to analyze a sample of the same size.
Conclusion
Electron microscopy has helped advance a number of different fields of study. Understanding how a scanning electron microscope functions and how it differs from a transmission electron microscope can help students better understand how it operates and probably inform future decisions for sourcing important laboratory equipment.