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 microscopes (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 traveling 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 smallerthan 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.
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 around 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.
Photo: Inside an atom: electrons are the particles that occupy shells (or orbitals) around the nucleus (center).
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 (typically 10–200 times bigger). 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 electromagnetsthrough 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.
Photo: Left: 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. Right 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.
Transmission electron microscopes (TEMs)
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.
How a transmission electron microscope (TEM) works
A transmission electron microscope fires a beam of electrons through a specimen to produce a magnified image of an object.
- A high-voltage electricity supply powers the cathode.
- The cathode is a heated filament, a bit like the electron gun in an old-fashioned cathode-ray tube (CRT) TV. It generates a beam of electrons that works in an analogous way to the beam of light in an optical microscope.
- An electromagnetic coil (the first lens) concentrates the electrons into a more powerful beam.
- Another electromagnetic coil (the second lens) focuses the beam onto a certain part of the specimen.
- The specimen sits on a copper grid in the middle of the main microscope tube. The beam passes through the specimen and “picks up” an image of it.
- The projector lens (the third lens) magnifies the image.
- The image becomes visible when the electron beam hits a fluorescent screen at the base of the machine. This is analogous to the phosphor screen at the front of an old-fashioned TV .
- The image can be viewed directly (through a viewing portal), through binoculars at the side, or on a TV monitor attached to an image intensifier (which makes weak images easier to see).
Scanning electron microscopes (SEMs)
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 traveling 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.
How a scanning electron microscope (SEM) works
A scanning electron microscope scans a beam of electrons over a specimen to produce a magnified image of an object. That’s completely different from a TEM, where the beam of electrons goes right through the specimen.
- Electrons are fired into the machine.
- The main part of the machine (where the object is scanned) is contained within a sealed vacuum chamber because precise electron beams can’t travel effectively through air.
- A positively charged electrode (anode) attracts the electrons and accelerates them into an energetic beam.
- An electromagnetic coil brings the electron beam to a very precise focus, much like a lens.
- Another coil, lower down, steers the electron beam from side to side.
- The beam systematically scans across the object being viewed.
- Electrons from the beam hit the surface of the object and bounce off it.
- A detector registers these scattered electrons and turns them into a picture.
- A hugely magnified image of the object is displayed on a TV screen.