Electron microscopes

Last updated: June 24, 2007.

What's the smallest thing you've ever seen? Maybe a hair, a pinhead, or a spec of dust? If you swapped your eyes for a couple of the world's most powerful microscopes, you'd be able to see things 100 million times smaller: bacteria, viruses, molecules—even the atoms in crystals would be clearly visible to you!

Ordinary optical (light-based) microscopes, like the ones you find in a school lab, are nowhere near good enough to see things in such detail. It takes a much more powerful electron microscope—using beams of electrons instead of rays of light—to take us down to nano-dimensions. Let's take a closer look at electron microscopes and how they work.

Photo: This electron microscope at Argonne National Laboratory can produce images 1000 times sharper than any conventional optical (light-based) microscope. By courtesy of US Department of Energy.

Seeing with electrons

We can see objects in the world around us because light rays (either from the Sun or from another light source, like a desktop lamp) reflect off them and into our eyes. No-one really knows what light is like, but scientists have settled on the idea that it has a sort of split personality. They like to call this wave-particle duality, but the basic idea is much simpler than it sounds. Sometimes light behaves like a train of waves—much like waves travelling over the sea. Other times, it's more like a steady stream of particles—a bombardment of microscopic cannonballs, if you like. You can read these words on your computer screen because light particles are streaming out of the display into your eyes in a kind of mass, horizontal hailstorm! We call these individual particles of light photons: each one is a tiny packet of electromagnetic energy.

Seeing with photons is fine if you want to look at things that are bigger than photons. But if you want to see things that are any smaller, photons turn out to be pretty clumsy and useless. Just imagine if you were a master wood carver, renowned the world over for the finely carved furniture you made. To carve such fine details, you'd need small, sharp, precise tools smaller than the patterns you wanted to make. If all you had were a sledgehammer and a spade, carving intricate furniture would be impossible. The basic rule is that the tools you use have to be smaller than the things you're using them on.

Photo: Electrons are the particles that orbit the nucleus (center) of atoms.

And the same goes for science. If you want to see finely detailed things that are smaller than photons, you need to use particles that are smaller than photons to start with: in other words, you need to use electrons. As you probably know, electrons are the minute charged particles that occupy the outer regions of atoms. (They're also the particles that carry electricity found circuits.) In an electron microscope, a stream of electrons takes the place of a beam of light and allows us to see things smaller even than light itself.

How electron microscopes work

If you've ever used an ordinary microscope, you'll know the basic idea is simple. There's a light at the bottom that shines upward through a thin slice of the specimen. You look through an eyepiece and a powerful lens to see a considerably magnified image of the specimen—it's typically 10-200 times bigger in the kin microscope. So there are essentially four important parts to an ordinary microscope:

• The source of light.
• The specimen.
• The lenses that makes the specimen seem bigger.
• The magnified image of the specimen that you see.

In an electron microscope, these four things are slightly different.

• The light source is replaced by a beam of very fast moving electrons.
• The specimen usually has to be specially prepared and held inside a vacuum chamber from which the air has been pumped out (because electrons do not travel very far in air).
• The lenses are replaced by a series of coil-shaped electromagnets through which the electron beam travels. In an ordinary microscope, the glass lenses bend (or refract) the light beams passing through them to produce magnification. In an electron microscope, the coils bend the electron beams the same way.
• The image is formed as a photograph (called an electron micrograph) or as an image on a TV screen.

That's the basic, general idea of an electron microscope. But there are actually quite a few different types of electron microscopes and they all work in different ways. The three most familiar types are called transmission electron microscopes (TEMs), scanning electron microscopes (SEMs), and scanning tunneling microscopes (STMs). Note that that's spelled "tunnelling" if you're British.

Transmission electron microscopes (TEMs)

Photo: Studying a specimen with a transmission electron microscope. The electron gun is in the tall gray tube at the top. By courtesy of NASA Glenn Research Center.

A TEM has a lot in common with an ordinary optical microscope. You have to prepare a thin slice of the specimen quite carefully (it's a fairly laborious process) and sit it in a vacuum chamber in the middle of the machine. When you've done that, you fire an electron beam down through the specimen from a giant electron gun at the top. The gun uses electromagnetic coils and high voltages (typically from 50,000 to several million volts) to accelerate the electrons to very high speeds. Thanks to our old friend wave-particle duality, electrons (which we normally think of as particles) can behave like waves (just as waves of light can behave like particles). The faster they travel, the smaller the waves they form and the more detailed the images they show up. Having reached top speed, the electrons zoom through the specimen and out the other side, where more coils focus them to form an image on screen (for immediate viewing) or on a photographic plate (for making a permanent record of the image). TEMs are the most powerful electron microscopes: we can use them to see things just 1 nanometer in size, so they effectively magnify by a million times or more.

Scanning electron microscopes (SEMs)

Photo: A typical scanning electron microscope. The main microscope equipment is on the extreme left. You can see the image it produces on the two screens. By courtesy of NASA Langley Research Center.

Most of the funky electron microscope images you see in books—things like wasps holding microchips in their mouths—are not made by TEMs but by scanning electron microscopes (SEMs), which are designed to make images of the surfaces of tiny objects. Just as in a TEM, the top of a SEM is a powerful electron gun that shoots an electron beam down at the specimen. A series of electromagnetic coils pull the beam back and forth, scanning it slowly and systematically across the specimen's surface. Instead of travelling through the specimen, the electron beam effectively bounces straight off it. The electrons that are reflected off the specimen (known as secondary electrons) are directed at a screen, similar to a cathode-ray TV screen, where they create a TV-like picture. SEMs are generally about 10 times less powerful than TEMs (so we can use them to see things about 10 nanometers in size). On the plus side, they produce very sharp, 3D images (compared to the flat images produced by TEMs) and their specimens need less preparation.

Photo: Typical images produced by a SEM. Left: An artificially colored, scanning electron micrograph showing Salmonella typhimurium (red) invading cultured human cells. Right: A scanning electron micrograph of the bacteria Escherichia coli (E.coli). Photos by courtesy of Rocky Mountain Laboratories, US National Institute of Allergy and Infectious Diseases (NIAID), and US National Institute of Health.

Scanning tunneling microscopes (STMs)

Among the newest electron microscopes, STMs were invented only in 1981. Unlike TEMs, which produce images of the insides of materials, and SEMs, which show up 3D surfaces, STMs are designed to make detailed images of the atoms or molecules on the surface of something like a crystal. They work differently to TEMs and SEMs too: they have an extremely sharp metallic probe that scans back and forth across the surface of the specimen. As it does so, electrons try to wriggle out of the specimen and jump across the gap, into the probe, by an unusual phenomen called "tunneling". The closer the probe is to the surface, the easier it is for electrons to tunnel into it, the more electrons escape, and the greater the tunneling current. The microscope constantly moves the probe up or down by tiny amounts to keep the tunneling current constant. By recording how much the probe has to move, it effectively measures the peaks and troughs of the specimen's surface. A computer turns this information into a map of the specimen that shows up its detailed atomic structure.

Photo: An STM image of the atoms on the surface of a solar cell. By courtesy of US Department of Energy/National Renewable Energy Laboratory (NREL).

A brief history of electron microscopes

• 1924: French physicist Louis de Broglie (1892-1987) realizes that electron beams have a wavelike nature similar to light. Five years later, he wins the Nobel Prize in Physics for this work.
• 1931: German scientists Max Knoll (1897-1969) and his pupil Ernst Ruska (1906-1988) build the first experimental TEM in Berlin.
• 1933: Ernst Ruska builds the first electron microscope that is more powerful than an optical microscope.
• 1935: Max Knoll builds the first SEM.
• 1965: Cambridge Instrument Company produces the first commercial SEM in England.
• 1981: Gerd Binnig (1947-) and Heinrich Rohrer (1933-) of IBM's Zurich Research Laboratory invent the STM and produce detailed images of atoms on the surface of a crystal of gold.
• 1986: Binnig and Rohrer share the Nobel Prize in Physics with the original pioneer of electron microscopes, Ernst Ruska.

Further Reading

Favorite websites

• Bugscope: Electron microscopy for schools.
• Mic-UK: A website for microscope enthusiasts, including a great microscopy page called The Smallest Page on the Web.
• Nanooze!: A brilliant children's nanotechnology website.