ELECTRON MICROSCOPES

 (Medical physics, Molecules and nanoparticles)

What's the smallest thing you've ever seen? Maybe a hair or a pinhead? 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!

Seeing with electrons

Photo: Inside an atom: electrons are the particles that occupy shells (or orbitals) around the nucleus (center).

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 must 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.

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. 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

1. A high-voltage electricity supply powers the cathode.
2. 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.
3. An electromagnetic coil (the first lens) concentrates the electrons into a more powerful beam.
4. Another electromagnetic coil (the second lens) focuses the beam onto a certain part of the specimen.
5. 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.
6. The projector lens (the third lens) magnifies the image.
7. 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.
8. 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.

Photos by courtesy of Rocky Mountain Laboratories, US National Institute of Allergy and Infectious Diseases (NIAID), and US National Institute of Health.

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.

1. Electrons are fired into the machine.
2. 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.
3. A positively charged electrode (anode) attracts the electrons and accelerates them into an energetic beam.
4. An electromagnetic coil brings the electron beam to a very precise focus, much like a lens.
5. Another coil, lower down, steers the electron beam from side to side.
6. The beam systematically scans across the object being viewed.
7. Electrons from the beam hit the surface of the object and bounce off it.
8. A detector registers these scattered electrons and turns them into a picture.
9. A hugely magnified image of the object is displayed on a TV screen.