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Electron microscopes are instruments that use accelerated electrons under vacuum conditions in order to generate highly magnified images of specimens. We can distinguish roughly between two types of instrument:

Transmissions electron microscopes (TEM)

Electrons pass through a very thin sample and produce an image from the sample volume on a screen or camera beneath the sample.

Scanning electron microscopes (SEM)

Electrons scattered by or emitted from the surface of a sample are used for image generation.  This type of instrument provides information from the surface of a sample.

Modern electron microscopes are not only used for making pictures. Equipped e.g. with X-ray detectors or energy filters in TEM, they provide element-specific information from the sample and are used for chemical microanalysis. The detection limit for trace elements is < 10 atoms that can be localized at an area < 1 nm square.

TECNAI F20 S-Twin at USTEM, Vienna University of Technology. The resolution of 0.24 nm guarantees a broad range of imaging and analysis possibilities.

Transmission electron microscopy (TEM)

At the beginning of the 20th Century the physicist Ernst Mach could invalidate Boltzmann's idea that all matter is set up of individual atoms by asking "Have you ever seen one?".

Today, a view into a transmission electron microscope would suffice to give Mach a due answer. Modern TEMs, like the TECNAI F20 at USTEM, enable imaging, diffraction and chemical analysis with atomic resolution in a single instrument.

That's no big science, no mile-long accelerator, containments, just an air-conditioned room with heavy curtains, damped illumination and a whispering, user-friendly machine that awaits operator commands.

The first transmission electron microscope was set up in Berlin in 1931 by Ernst Ruska at Siemens. It could already show details half the size that was possible with the best optical microscopes at that time. Ruska was honored for his epoch-making work with the Nobel price in 1986.

A transmission electron microscope works like a slide projector consisting of a light bulb, slide and a screen.

Slide projector

Transmission electron microscope

Light bulb

A heated tungsten tip, or with modern instruments, a hot  LaB6 single crystal or a field emission tip, act as an electron source.  Like in a TV tube electrons are accelerated and reach up to 70% of the speed of light.

Slide

In a TEM the object is a thin sample with a thickness of less than 300 nm (several hundred layers of atoms).

Optical system with glass lenses
Instead of glass lenses, magnetic lenses are used to deflect the beam of electrons. Sometimes electrostatic lenses are also used.
Screen
A fluoroscopic screen, photographic film, CCD cameras or imaging plates replace the screen.

Why is such a high electron velocity required?

Besides the necessity of penetrating the sample, there is a second important reason:  the wave properties of the electrons.

The resolution of an optical microscope is limited by the wavelength of the light and is approximately 200 nm. In 1925 Louis de Broglie had the idea that electrons should also have similar properties like photons. The high velocity and the mass of the electrons lead, according to quantum theory, to very small wavelengths (about 10-12 m) and therefore make possible very high resolutions.

During the fifty years between the invention of the TEM and the Nobel price, the resolution of TEMs was improved by 1000 times. This seems to be small compared to the fast development that occurred in the field of integrated circuits. Nevertheless it means that modern instruments are capable of resolving details with a size of just tenths of a millionth of a millimeter. That is less than the distance between atoms in a crystal.

The next step seems already be given:  protons, electrons, quarks, strings, ....

A typical electron diffraction pattern from a polycrystalline material.

An advantage of modern TEMs is that just by pressing a button, the image of the sample is replaced by the corresponding diffraction pattern which can be obtained from a region of the sample as small as a few nanometers. Diffraction patterns allow, just like in X-ray diffraction, the determination of crystal structure and electron densities (Patterson function).  Besides that, crystal defects (dislocations, stacking faults) can also be investigated by electron diffraction.

The accompanying image shows two overlapping micrographs in order to demonstrate the resolution of modern transmission electron microscopes.

A GaAs/InGaAs interface is shown with atomic resolution (~ 4,600,000-times), imaged with an FEI TECNAI F20, and overlapping is a scanning electron microscope image of a mite.  Such a mite is barely visible with the eye.  If the mite would have been imaged with the same magnification like the atomic interface it would have a size of about 100 m.

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Scanning electron microscope (SEM)

A scanning electron microscope

Scanning electron microscopes function similar to TEMs.  A beam of highly accelerated electrons is focused by magnetic lenses onto a small region on the sample.

In contrast to TEM, secondary electrons (SE) or backscattered electrons (BSE) from the surface of the sample, caused by the electron beam as it is scanned across the sample, are used for generating images of the sample surface.

The electrons of the beam do not need to pass through the sample in order to produce an image, therefore larger and thicker samples can be investigated in SEMs.  Usually a time consuming sample preparation like in TEM is not required.

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