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What is Scanning Electron Microscopy? - Theory & Applications

Instructor: Damien Howard

Damien has a master's degree in physics and has taught physics lab to college students.

In this lesson we'll explore how scanning electron microscopes works. You'll also learn why you might want to use a scanning electron microscope instead of a standard optical microscope and see some practical uses for scanning electron microscopes.

Optical Limits

When it comes to scientific instruments, one of the most iconic is the microscope. Microscopes let you see objects that are normally much too small for the human eye to see. Even though this concept is simple, it's extremely useful. Microscopes are used in many different scientific fields including biology, chemistry, geology, and physics, just to name a few.

Scientist Using a Microscope
scientist using a microscope

The type of microscope that most people are familiar with is the optical microscope. A microscope uses lenses to magnify the object placed on a tray below it. Optical microscopes are capable of magnifying objects up to 1000 times their normal size; however, sometimes you need an even greater magnification of an object. The maximum magnification of optical microscopes is limited by the wavelength of visible light. To get an image at a greater magnification you need something with a shorter wavelength than visible light, such as electrons. One of the types of microscopes that uses electrons for magnification is called the scanning electron microscope (SEM). In this lesson we're going to learn how these microscopes work and some applications for their use.

Scanning Electron Microscope
picture of an SEM

How Scanning Electron Microscopes Work

When an SEM fires electrons at the sample you want to magnify several different signals can be given off as the electrons strike the sample. Among the various signals given off, three of the most important are backscattered electrons, secondary electrons, and x-rays. The x-rays are used for elemental analysis of the sample, but in this lesson we'll focus on the two types of electrons given off as they are used for the actual magnification of the sample.

The way the electron shot at the sample collides with it is what determines whether a backscattered or secondary electron is emitted. Backscattered electrons occur when the collision is elastic. The backscattered electrons are actually the electrons that were originally shot at the sample bouncing back off of it. Conversely, secondary electrons occur when the collision is inelastic. Unlike backscattered electrons, secondary electrons originate from the sample itself. They are electrons that have been jarred loose from inside the sample.

Scanning Electron Microscope Diagram
SEM diagram

We use these two types of electrons to make an image of the sample by scanning a beam of the fired electrons across the whole sample, hence the 'scanning' in scanning electron microscope. As the electron beam is scanned across the sample, detectors inside the microscope pick up the signals given off by this interaction. The detectors then use these signals to create the magnified image of the sample.

Whether you want to use backscattered or secondary electrons to create your image depends on your goal. Secondary electrons produce the highest quality images with the greatest possible magnification in the SEM. Backscattered electrons produce a worse quality image but also give information of the sample's composition.

SEM Applications

As we stated in the beginning of this lesson, microscopes are used in many different scientific fields. Let's look at a few concrete examples of the applications of scanning electron microscopy.

Scanning electron microscopy is extremely useful when working with nanomaterials such as nanoparticles, nanowires, and nanotubes. These materials are far too small to get detailed images using an optical microscope. With an SEM you can see how well formed these nanomaterials are and measure their dimensions such as diameter and length.

Colorized Image of Carbon Nanotubes
colorized nanotubes picture

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