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Electron microscopes

by Chris Woodford. Concluding updated: October 26, 2021.

Due westlid'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 globe's almost powerful microscopes, you'd exist able to see things 100 meg times smaller: bacteria, viruses, molecules—even the atoms in crystals would be clearly visible to y'all!

Ordinary optical microscopes (light-based microscopes), like the ones you find in a school lab, are nowhere near skilful enough to see things in such item. It takes a much more than powerful electron microscope—using beams of electrons instead of rays of lite—to take united states of america down to nano-dimensions. Let'south take a closer await at electron microscopes and how they work!

Photo: This Hitachi S-4700 field-emission, scanning electron microscope can magnify over half a million times and resolve features but 2 nanometers across! Past courtesy of NASA Glenn Enquiry Centre and Internet Annal.

Contents

1. Seeing with electrons
2. How electron microscopes work
3. Transmission electron microscopes (TEMs)
4. Scanning electron microscopes (SEMs)
5. Scanning tunneling microscopes (STMs)
6. Atomic force microscopes (AFMs)
7. Who invented electron microscopes?
8. Find out more

Seeing with electrons

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

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

Seeing with photons is fine if you desire to await at things that are much bigger than atoms. But if you want to meet things that are smaller, photons plough out to be pretty clumsy and useless. Just imagine if you lot were a master wood carver, renowned the world over for the finely carved furniture you made. To cleave such fine details, you lot'd demand small, precipitous, precise tools smaller than the patterns yous wanted to make. If all you had were a sledgehammer and a spade, carving intricate furniture would be incommunicable. The basic rule is that the tools you employ have to be smaller than the things y'all're using them on.

And the same goes for scientific discipline. The smallest thing you can run across with a microscope is determined (partly) by the low-cal that shines through it. An ordinary lite microscope uses photons of calorie-free, which are equivalent to waves with a wavelength of roughly 400–700 nanometers. That's fine for studying something similar a homo hair, which is about 100 times bigger (50,000–100,000 nanometers in diameter). Simply what almost a leaner that's 200 nanometers across or a protein just 10 nanometers long? If you desire to encounter finely detailed things that are "smaller than lite" (smaller than the wavelength of photons), you need to utilise particles that have an even shorter wavelength than photons: in other words, you need to utilise electrons. As you probably know, electrons are the minute charged particles that occupy the outer regions of atoms. (They're also the particles that acquit electricity around circuits.) In an electron microscope, a stream of electrons takes the identify of a beam of light. An electron has an equivalent wavelength of just over one nanometer, which allows us to run across things smaller even than light itself (smaller than the wavelength of light's photons).

How electron microscopes work

If you've ever used an ordinary microscope, you'll know the basic thought is simple. There'south a light at the bottom that shines upward through a thin piece of the specimen. You look through an eyepiece and a powerful lens to run across a considerably magnified image of the specimen (typically 10–200 times bigger). So there are essentially 4 important parts to an ordinary microscope:

1. The source of light.
2. The specimen.
3. The lenses that makes the specimen seem bigger.
4. The magnified image of the specimen that you see.

In an electron microscope, these iv things are slightly unlike.

1. The lite source is replaced past a beam of very fast moving electrons.
2. The specimen commonly has to be specially prepared and held within a vacuum chamber from which the air has been pumped out (because electrons do not travel very far in air).
3. The lenses are replaced past a series of coil-shaped electromagnets through which the electron axle travels. In an ordinary microscope, the glass lenses curve (or refract) the light beams passing through them to produce magnification. In an electron microscope, the coils bend the electron beams the same way.
4. The image is formed as a photo (called an electron micrograph) or as an image on a Idiot box screen.

That's the basic, general thought of an electron microscope. Simply at that place are really quite a few dissimilar types of electron microscopes and they all work in different means. The three most familiar types are chosen transmission electron microscopes (TEMs), scanning electron microscopes (SEMs), and scanning tunneling microscopes (STMs).

Photo: 1) Studying a specimen with a manual electron microscope. The electron gun is in the tall grey tube at the summit. By courtesy of NASA Glenn Inquiry Center. ii) A typical scanning electron microscope. The principal microscope equipment is on the farthermost left. Yous tin can see the prototype information technology produces on the two screens. Past courtesy of NASA Langley Research Eye.

Manual electron microscopes (TEMs)

A TEM has a lot in common with an ordinary optical microscope. You accept to prepare a thin slice of the specimen quite carefully (it's a adequately laborious process) and sit it in a vacuum sleeping room in the middle of the motorcar. When you've done that, you burn an electron axle downward through the specimen from a giant electron gun at the peak. The gun uses electromagnetic coils and loftier voltages (typically from 50,000 to several one thousand thousand volts) to advance the electrons to very high speeds. Thanks to our old friend wave-particle duality, electrons (which we normally think of every bit particles) can behave similar waves (just as waves of light can deport like particles). The faster they travel, the smaller the waves they course and the more than detailed the images they prove upwardly. Having reached superlative speed, the electrons zoom through the specimen and out the other side, where more than coils focus them to form an paradigm on screen (for immediate viewing) or on a photographic plate (for making a permanent record of the prototype). TEMs are the most powerful electron microscopes: we can use them to see things but 1 nanometer in size, then 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 calorie-free in an optical microscope.
3. An electromagnetic curlicue (the first lens) concentrates the electrons into a more powerful beam.
4. Another electromagnetic scroll (the second lens) focuses the beam onto a certain part of the specimen.
5. The specimen sits on a copper grid in the center of the main microscope tube. The beam passes through the specimen and "picks upward" an image of it.
6. The projector lens (the third lens) magnifies the image.
7. The prototype becomes visible when the electron axle hits a fluorescent screen at the base of the machine. This is analogous to the phosphor screen at the front end of an old-fashioned Television set .
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)

Nigh of the funky electron microscope images you see in books—things like wasps belongings microchips in their mouths—are not made past TEMs only past scanning electron microscopes (SEMs), which are designed to brand images of the surfaces of tiny objects. Simply equally in a TEM, the peak 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 beyond the specimen'south surface. Instead of traveling through the specimen, the electron beam finer bounces straight off information technology. The electrons that are reflected off the specimen (known every bit secondary electrons) are directed at a screen, similar to a cathode-ray TV screen, where they create a TV-like picture. SEMs are generally almost 10 times less powerful than TEMs (so we tin can use them to meet things most 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 past a SEM. one) An artificially colored, scanning electron micrograph showing Salmonella typhimurium (crimson) invading cultured human cells. ii) A scanning electron micrograph of the bacteria Escherichia coli (E.coli). Photos past courtesy of Rocky Mountain Laboratories, Usa National Found of Allergy and Infectious Diseases (NIAID), and U.s. National Establish 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 epitome of an object. That's completely unlike from a TEM, where the beam of electrons goes right through the specimen.

1. Electrons are fired into the machine.
2. The main role of the auto (where the object is scanned) is contained within a sealed vacuum chamber considering precise electron beams can't travel finer 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. Some other coil, lower downwards, steers the electron beam from side to side.
6. The beam systematically scans across the object existence 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 show.
9. A hugely magnified prototype of the object is displayed on a TV screen.

Scanning tunneling microscopes (STMs)

Photo: An STM paradigm of the atoms on the surface of a solar jail cell. By courtesy of Usa Section of Free energy/National Renewable Free energy Laboratory (NREL).

Among the newest electron microscopes, STMs were invented by Gerd Binnig and Heinrich Rohrer in 1981. Unlike TEMs, which produce images of the insides of materials, and SEMs, which testify 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 precipitous metallic probe that scans back and forth beyond the surface of the specimen. As it does so, electrons endeavor to wriggle out of the specimen and jump beyond the gap, into the probe, by an unusual quantum phenomenon called "tunneling." The closer the probe is to the surface, the easier information technology 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 motion, it finer measures the peaks and troughs of the specimen's surface. A computer turns this information into a map of the specimen that shows upward its detailed diminutive structure. I large drawback of ordinary electron microscopes is that they produce amazing particular using high-free energy beams of electrons, which tend to damage the objects they're imaging. STMs avoid this problem by using much lower energies.

How a scanning tunneling microscope (STM) works

A scanning tunnelling microscope makes images using electrons that "tunnel" betwixt the probe and the specimen. Hither's how it works:

1. The sample (blue) is sealed within a vacuum chamber.
2. The chamber is cooled
down to virtually absolute naught by a cryogenic source, such as a liquid helium refrigerator.
3. A pump creates a very high vacuum in the sleeping room.
4. The sample beingness scanned serves as one electrode.
5. The probe tip, an incredibly modest distance to a higher place, serves as the other electrode. The two electrodes can exist scanned past one another by a bulldoze that moves in three dimensions.
6. The tunneling current output from the probe is analyzed by a measuring device.
7. Results tin be displayed on a screen or plotter, showing upward (in this case) the blueprint of atoms on the surface of the sample.

Artwork (profoundly simplified!) based on an STM device outlined in US Patent iv,343,993: Scanning tunneling microscope by Gerd Binnig and Heinrich Rohrer, IBM Corporation, patented Baronial 10, 1982.

Atomic strength microscopes (AFMs)

If y'all call back STMs are astonishing, AFMs (atomic force microscopes), likewise invented past Gerd Binnig, are even ameliorate! I of the big drawbacks of STMs is that they rely on electric currents (flows of electrons) passing through materials, and then they can only make images of conductors. AFMs don't suffer from this trouble because, although they use still tuneling, they don't rely on a current flowing between the specimen and a probe, so we can use them to make diminutive-scale images of materials such as plastics, which don't comport electricity.

An AFM is a microscope with a little arm called a cantilever with a tip on the stop that scans beyond the surface of a specimen. As the tip sweeps across the surface, the force between the atoms from which it'due south made and the atoms on the surface constantly changes, causing the cantilever to curve past minute amounts. The corporeality past which the cantilever bends is detected by bouncing a laser beam off its surface. By measuring how far the light amplification by stimulated emission of radiation beam travels, we can measure out how much the cantilever bends and the forces acting on it from moment to moment, and that data can be used to effigy out and plot the contours of the surface. Other versions of AFMs (like the ane illustrated hither) make an paradigm by measuring a current that "tunnels" between the scanning tip and a tunneling probe mounted simply behind it. AFMs tin can make images of things at the atomic level and they can too be used to dispense individual atoms and molecules—i of the central ideas in nanotechnology.

How an atomic force microscope (AFM) works

An atomic force microscope is similar to a scanning tunnelling microscope, only makes images using a tiny cantilever probe that's wiggled about by the force between itself and the specimen. Hither's how one version works:

• The specimen to be scanned (ane) is mounted on a bulldoze machinery (two) that tin movement it in iii dimensions.
• To forbid unwanted vibrations, that mechanism is fixed to a safety cushion (three) mounted on a firm aluminum base (four), which is further cushioned by multiple layers of aluminum plates and rubber pads (not shown).
• To create an image, the specimen is slowly moved around the sharp, fixed imaging signal (5), which is mounted on a spring cantilever fabricated of thin gold foil (6), attached to a piezoelectric crystal (seven), and fixed to the same aluminum base of operations.
• At the other end of the apparatus, a tunneling probe (8) is moved very close (to inside about 0.3nm) of the leap cantilever past a 2d drive mechanism (9), isolated by another safety absorber (10).
• As the sample (one) moves around the imaging point (five), the current that tunnels betwixt the leap cantilever (6) and the tunneling tip (8) is constantly measured. These measurements are converted into data that tin be used to describe a detailed surface map of the specimen.

Artwork: How Gerd Binnig'due south original AFM worked—greatly simplified. Based on an original drawing from Gerd Binnig'southward United states Patent iv,724,318: Atomic force microscope and method for imaging surfaces with diminutive resolution.

Who invented electron microscopes?

Here's a cursory history of the central moments in electron microscopy—and so far!

• 1924: French physicist Louis de Broglie (1892–1987) realizes that electron beams take a wavelike nature similar to light. Five years later, he wins the Nobel Prize in Physics for this piece of work.
• 1931: German language 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 crude SEM.
• 1935: Working at the University of Toronto, James Hillier and Albert Prebus build on Ruska's work to produce the offset commercially successful TEM for RCA in North America.
• 1941: German electric engineers Manfred Von Ardenne and Bodo von Borries patent an "electron scanning microscope" (SEM).
• 1965: Cambridge Instrument Visitor produces the first commercial SEM in England.
• 1981: Gerd Binnig (1947–) and Heinrich Rohrer (1933–) of IBM'due south Zurich Research Laboratory invent the STM and produce detailed images of atoms on the surface of a crystal of gold.
• 1985: Binnig and his colleague Christoph Gerber produce the commencement atomic force microscope (AFM) by attaching a diamond to a piece of gold foil.
• 1986: Binnig and Rohrer share the Nobel Prize in Physics with the original pioneer of electron microscopes, Ernst Ruska.
• 1989: The first commercial AFM is produced past Sang-il Park (founder of Park Systems of Palo Alto, California).

Find out more

On this website

• Tools, instruments, and measurement
• Light
• Microscopes (optical)

Websites

For younger readers

• Bugscope: Electron microscopy for schools.

For older readers

• Mic-United kingdom: A website for microscope enthusiasts, including a great microscopy folio called The Smallest Page on the Spider web.
• Ernst Ruska: A memorial site dedicated to the life and piece of work of the electron microscope pioneer (in German language and English language).
• Life Through A Lens: The story of Ernst Ruska's efforts to develop the electron microscope.

Electron microscope photos

• FEI: Electron Microscope Image Gallery: FEI is a leading electron microscope manufacturer and its website includes a gallery of astonishing photos taken with its 'scopes!
• Flickr: Scanning electron microscopy: A Flickr grouping pool of several hundred SEM images. Some of these are copyright, others are published under various Creative Commons licences permitting you to reuse them under certain conditions.

Videos

• Diminutive forcefulness microscope (AFM) at piece of work!: A great little video showing the cantilever and tip of an atomic forcefulness microscope (AFM) in activity. Note the green ruler scale on the left, which shows you the scale at which we're working as we zoom in and out.

Books

Like shooting fish in a barrel reading

• Heaven and Globe: Unseen by the Naked Eye: Past David Malin, Katherine Roucoux. Phaidon Press, 2007. Lots of stunning photos of the very big and very minor in this excellent coffee table book.
• Absurd Stuff and How it Works: By Chris Woodford et al. Dorling Kindersley, 2005. One of my own books, this 1 explains all sorts of everyday objects with stunning photography (and quite a few electron micrographs).
• Cool Stuff ii.0 (The Gadget Book): By Chris Woodford and Jon Woodcock. Dorling Kindersley, 2007. A follow-upward to Absurd Stuff, with more stunning photos (and a few more electron micrographs).

More than detailed and technical

• Electron Microscopy: Methods and Protocols: By John Kuo. Humana Printing, 2007. Covers STM and TEM techniques, mainly for biological sciences.
• Applied Electron Microscopy: A Beginner'due south Illustrated Guide: By Elaine Hunter et al. Cambridge University Press, 1993. Essentially a lab manual for advanced students and researchers.
• Transmission Electron Microscopy: A Textbook for Materials Science: By David Williams and Barry Carter. Springer, 2009. Comprehensive coverage of TEM techniques.

History

• The Development of the Microscope by South. Bradbury. Elsevier, 2014. A reprint of a 1967 book. The early history is obviously nevertheless applicable, merely the final chapter about electron microscopes is at present a fiddling dated.

Articles

• A Faster Style to Rearrange Atoms Could Lead to Powerful Quantum Sensors by Mark Anderson. IEEE Spectrum, June 3, 2019. A closer look at a new kind fo quantum imaging.
• Canadian Pioneers in Science: James Hillier and Albert Prebus, Pioneers of Electron Microscopy past Tonny Huang. Cyclica, July 28, 2017. How two Canadian scientists developed the showtime commercially successful TEM in Due north America. [Archived via the Wayback Auto]
• Plasmonics Enable Optical Microscopes to Perform Like Electron Microscopes by Dexter Johnson. IEEE Spectrum, July 21, 2016. Is it possible to become beyond the limitations of optical microscopy without switching to an electron beam?
• Super-Resolution Microscopes Crack the Diffraction Limit by Douglas McCormick, IEEE Spectrum, May vi, 2013. How scanning lasers tin can push the limits of optical microscopes.
• Power Tool for Making Nanoscale Objects by Saswato Das. IEEE Spectrum, July one, 2007. Introducing a micro-manufacturing technique called transmission electron beam ablation lithography (TEBAL).

Patents

A patent search is a good way of finding deeper technical details and drawings. Here are a few cardinal patents for starters:

• United states Patent 2,234,281: Shielded electron microscope by Ernst Ruska, filed February iv, 1939. This is ane of Ruska'due south improved microscope designs from the late 1930s. Numerous earlier patents cover his various improvements to electron tubes and his systems for deflecting cathode rays and deflecting electron beams with electric and magnetic fields.
• US Patent 3,191,028: Electron scanning microscope by Manfred Von Ardenne and Bodo von Borries, patented May 13, 1941. I think this is the original SEM patent, edifice on earlier work by Ruska and Knoll.
• US Patent 3,191,028: Scanning electron microscope by Albert V. Crewe, Usa Atomic Energy Commission, patented June 22, 1965. A higher magnification and resolution SEM blueprint from the mid-1960s. This is a much more than detailed description than the Ardenne patent with some peachy technical drawings.
• U.s. Patent four,343,993: Scanning tunneling microscope by Gerd Binnig and Heinrich Rohrer, IBM Corporation, patented August 10, 1982. Binnig and Rohrer'due south original STM patent.
• US Patent iv,724,318: Atomic strength microscope and method for imaging surfaces with atomic resolution by Gerd Binnig, IBM Corporation, patented February nine, 1988. A dandy technical description of Binnig's groundbreaking (and Nobel-Prize-winning) AFM.

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