Materials engineer Dr Ruth Knibbe uses a scanning electron microscope (SEM) to image her own samples and samples for other scientists. Unlike a light microscope, an SEM uses a focused beam of electrons to produce images from the top surface of a sample. “So you don’t get lots of information from the bulk of your sample, it’s mostly topography that you’re getting information from,” says Knibbe, from Victoria University of Wellington.
An SEM can also be used to extract analytical information. “So you can also find out what atoms…or what elements you have in your sample,” she says. For example, whether a sample contains iron or oxygen.
The electron beams in an SEM come down a column, and when it hits the sample will either release secondary electrons or back-scatter electrons. Secondary electrons provide information about the topography of a sample, and the back-scatter electrons provide information about the composition of the sample, otherwise known as atomic number contrast.
The electron beam in an SEM is very finely controlled, and starts in the top left hand corner and moves quickly across the sample in a rastering format. Electrons from that particular area are detected which creates a signal.
“That’s why we call it a scanning electron microscopy detector…because you’re actually scanning across that surface,” says Knibbe.
So unlike a light microscope where the entire sample is saturated immediately, an SEM scans across a sample and every location gives a slightly different signal.
Samples generally need to be conducting, so non-conductive samples such as geological or biological samples are coated with a metal or carbon. “If you have an insulator, it would just flare up and charge and you just see lots of bright white. Not so interesting,” says Knibbe.
Samples are loaded onto circular stubs which have a pin underneath which can be tightened into the microscope with an Allen key. “You don’t want your sample to be moving when it’s in the microscope,” says Knibbe. “If it’s moving in the microscope and you’re imaging it 100,000s of times, you’re not going to see very much at all.”
Different scientists use an SEM for different purposes. “[Biologists] would use this microscope because you can get that high detailed topography information,” says Knibbe. Another big benefit of a scanning electron microscope for all types of scientists, as compared with a light microscope, is the huge depth of field which gives SEM images their characteristic, almost 3D-like, quality.
You can listen to an extended version of using a scanning electron microscope here:
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