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!
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.
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
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
- 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
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.
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.
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
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.
typhimurium (red) invading cultured human cells.
Right: A scanning electron micrograph of the bacteria Escherichia 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 phenomenon 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
Photo: An STM image of the atoms on the surface of
a solar cell.
the scale at which we're working as we zoom in and out.
- Heaven and Earth: Unseen by the Naked Eye:
By David Malin, Katherine Roucoux. Phaidon Press, 2007. Lots of stunning photos of the very big and very small in
this excellent coffee table book.
- Cool Stuff and How it Works:
By Chris Woodford et al. Dorling Kindersley, 2005. One of my own books, this one explains all sorts of everyday objects
with stunning photography (and quite a few electron micrographs).
- Cool Stuff 2.0 (The Gadget Book):
By Chris Woodford and Jon Woodcock. Dorling Kindersley, 2007. A follow-up to Cool Stuff, with more stunning photos (and a few more electron micrographs).
More detailed and technical