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Transmission Electron Microscopy: Theory & Applications

Instructor: Damien Howard

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

A transmission electron microscope is an instrument used to create high magnification images of the internal structure of a sample being studied. In this lesson you will learn how the microscope is able to do this, and discover several applications for which it is useful.

Microscopes

To the average person a microscope might seem like little more than a really powerful magnifying glass, but there's a lot more to them than that. Even the fact that there are multiple types of microscopes might come as a surprise, and the fact that some don't even use light to create a magnified image is certainly unexpected to most people.

We often think of microscopes as just powerful magnifying glasses, but they can be much more.
magnifying glass

Let's learn about one of those microscopes that doesn't use light to create magnified images; a transmission electron microscope.

Transmission Electron Microscope

A transmission electron microscope (TEM) is a special type of microscope that uses electrons for magnification. The magnification in a standard optical microscope is limited by the wavelength of visible light. Electrons have a much smaller wavelength, which allows electron microscopes to achieve extreme magnification. An average TEM can achieve a maximum magnification level of about 1,000,000x.

In order to understand how a TEM uses electrons to create magnified images, we need to understand how the instrument operates. A TEM can be broken down into three major sections.

Transmission Electron Microscope
TEM

1. The electron gun is the part of the microscope that fires electrons off toward the sample the user is magnifying. There are multiple types of electron guns, but the most common uses a heated tungsten filament to create the electrons. The electrons are then drawn toward an anode at one end of the electron gun. A hole in the anode allows electrons to pass through it and be fired toward the sample.

2. Once the electrons leave the electron gun they enter the condenser system, which consists of one or two lenses. These lenses are not the typical glass type you think about, but are instead electromagnetic lenses that consist of coils of wires with electricity running through them. They create a magnetic field that focuses and constricts the electrons into a thin beam.

3. The electron beam then enters the image producing system. At the beginning of this system is the sample being imaged. The electrons are transmitted through the sample and into another series of electromagnetic lenses that focus the electrons onto a fluorescent or phosphorescent screen to create the image.

TEM Image Creation

The way the image is created is similar to how a shadow is created with visible light. When the electron beam is transmitted through the sample, not all the electrons make it out. Some electrons are absorbed or deflected as they try to pass through the sample.

The areas where more electrons made it through create bright spots on the screen below, and the areas where fewer electrons came through create darker spots. This in turn creates a magnified, shadow-like, black and white image of the sample.

TEM image of a plant cell
plant cell TEM image

The image producing system of a TEM is what distinguishes it from its sister microscope, the scanning electron microscope (SEM). Unlike a TEM, the sample being viewed in a SEM is at the very bottom of the microscope.

SEM images are created by electrons that bounce off or are ejected from the sample. Because of this, the SEM gets surface images of the sample, whereas the TEM gets images of the internal composition of the sample.

The downside of this in a TEM is that the sample must be cut very thin for the electrons to pass through, making sample preparation much harder than that of a sample used in an SEM.

TEM Applications

The main application of a transmission electron microscope is to provide high magnification images of the internal structure of a sample. Being able to obtain an internal image of a sample opens new possibilities for what sort of information can be gathered from it.

A TEM operator can investigate the crystalline structure of an object, see the stress or internal fractures of a sample, or even view contamination within a sample through the use of diffraction patterns, to name just a few kinds of studies.

TEM image of nanotubes
nanotubes TEM image

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