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You are in: Cavendish Outreach » Physics At Work » 2006
Electron Microscopy Facility
xl30sem.phy.cam.ac.uk
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When trying to understand how materials work it is important to determine their
internal structure. While it is easy to see the structure of a sponge, it is far
more difficult to see microstructures of viruses or even atomic structures. Our eyes
are not good enough to see atoms, which are only 0.0000000002 metres (2x10-10m)
in size, so we need to use microscopes to help us. An optical microscope that
uses visible light only has a resolution of ~ 10-6 m. Why? Because light only has a
wavelength of roughly 500 nm (5x10-7m) and our eyes can only see two points that are
wider apart than the used wavelength. Atoms are much smaller than the wavelength of
light, so we need another wave with a much shorter wavelength. Therefore, we use
electrons that have a wavelength of 0.037x10-10 m at 100kV. In theory, by using
electrons, we should easily be able to resolve the distance between atoms. In
practice it is much harder, as the resolution of an electron microscope is affected
by a lot of things, such as magnetic lenses, or the source of electrons. A picture
of a high resolution electron microscope, which is able to see atoms, is shown in
Fig. 1.
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To understand how small atoms are, imagine the Earth is the size of a football.
Using an optical microscope Wembley Stadium would look like a blurred blob. If
we use a high resolution electron microscope we would be able to see individual
people and even resolve their fingers, as in the world 2.5 cm is equivalent to
atom spacing.
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Fig. 1 HB501 STEM (Scanning Transmission Electron Microscope)
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Not everybody looks for atoms. In fact, most researchers study
microstructures that are in the range of 0.00001 mm to 0.01 mm.
Electron microscopes can be used to study a large variety of different
materials, such as computer chips, polymers, metals, bones or ceramics.
Some of these materials are shown in Fig. 2-4.
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Fig. 2 TEM (Transmission electron microscope) image of a thin ceramic film
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Fig 3a SEM (Scanning Electron Microscope) picture of an onion skin
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Fig 3b SEM (Scanning Electron Microscope) micrograph of a bundle of Teflon
fibres
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There are three main types of electron microscopes: transmission electron
microscopes (TEMs) are used for looking at internal structures (Fig. 2),
scanning electron microscopes (SEMs) are used for looking at surface structures
(Fig. 3), and scanning transmission electron microscopes (STEMs) are used for
high resolution imaging and analysis (Fig. 4). We use all three types to help
us in our research at the Cavendish Laboratory.
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Fig. 4 High resolution image of an atomic structure of a Silicon/Nickel
disilicide (Super-STEM at Daresbury Laboratory)
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A diagram of a scanning election microscope (SEM) is shown in Fig. 5. The
electrons are produced by the gun and then focused by several lenses and
brought to focus at the sample. The electrons interact with the sample and
the resulting electrons are then collected by an electron detector and
displayed on a TV screen.
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Fig. 5 Diagram of a Scanning Electron Microscope
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