What is an Electron Microscope? - Definition, Types & Uses

Instructor: Devyn Feinman
Find out how we can observe the arrangement of atoms in a crystal, or the small features on the antenna of a wasp. In this lesson you will understand how electron microscopes work and their applications in science and technology.

Electron Microscope

You've probably used a microscope in school -- maybe to observe the wings of an insect or to get a closer look at a leaf. If so, then you know microscopes are used in the classroom to illuminate the surface of your subject of study. These microscopes use transparent glass lenses to magnify the image of whatever you are observing.

However, the size of the smallest features that we can distinguish under the microscope is on the order of the wavelength of the light used. Visible light, which is the one our eyes are sensitive to, ranges between 390 and 700 nanometers (one nanometer is one billionth of a meter). This means that we cannot observe things that are smaller than a few hundred nanometers using our eyes and visible light.

With the advancement of science and technology, we realized there is a whole world of things that we can observe and study at small scales. For example, the size of atoms and molecules is just fractions of a nanometer. An electron microscope allows us to see at these small scales.

Electron microscopes work by using an electron beam instead of visible light and an electron detector instead of our eyes. An electron beam allows us to see at very small scales because electrons can also behave as light. It has the properties of a wave with a wavelength that is much smaller than visible light (a few trillionths of a meter!). With this wavelength we can distinguish features down to a fraction of a nanometer. Let's explore the different types of electron microscopes, how they work and some of their applications.

Types of Electron Microscopes

Scanning Electron Microscope (SEM)

In a scanning electron microscope or SEM, a beam of electrons scans the surface of a sample (Figure 1). The electrons interact with the material in a way that triggers the emission of secondary electrons. These secondary electrons are captured by a detector, which forms an image of the surface of the sample. The direction of the emission of the secondary electrons depends on the orientation of the features of the surface. There, the image formed will reflect the characteristic feature of the region of the surface that was exposed to the electron beam.

Figure 1. A Scanning Electron Microscope focuses a beam of electrons on the surface of a sample. Secondary electrons are emitted from the sample surface. A detector is used to form an image from the secondary electrons.
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Transmission Electron Microscope (TEM)

In a transmission electron microscope or TEM, a beam of electrons hits a very thin sample (usually no more than 100 nm thick). The electrons are transmitted through the sample (Figure 2). After the sample, the electrons hit a fluorescence screen that forms an image with the electrons that were transmitted. You can better understand this process by imagining how a movie projector works. In a projector, you have a film that has the negative image that will be projected. The projector shines white light on the negative and the light transmitted forms the image contained in the negative.

Figure 2. Transmission Electron Microscope. A beam of electrons is focused on a sample. The electrons pass through the sample to form an image on a fluorescent screen.
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Scanning Transmission Electron Microscope (STEM)

A scanning transmission electron microscope or STEM combines the capabilities of both an SEM and a TEM. The electron beam is transmitted across the sample to create an image (TEM) while it also scans a small region on the sample (SEM). The ability to scan the electron beams allows the user to analyze the sample with various techniques such as Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive X-ray (EDX) Spectroscopy which are useful tools to understand the nature of the materials in the sample.

Uses of the Electron Microscope

With electron microscopes we can observe the small scale world that makes up most of the things around us. Before the development of the electron microscope we did not know how all these things looked (shape, size, etc.). We were relying on our imagination to picture these objects in our minds.

Figure 3. SEM Images. A) The surface of the antenna of a wasp. B) A snow flake. C) Wood. D) Blood cells (size of 1-6 micrometers).
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