Electron Microscopy
What are Electron Microscopes?
Electron Microscopes are scientific instruments that use a beam of highly energetic electrons to examine objects on a very fine scale. This examination can yield the following information: Topography The surface features of an object or "how it looks", its texture; direct relation between these features and materials properties (hardness, reflectivity...etc.) Morphology The shape and size of the particles making up the object; direct relation between these structures and materials properties (ductility, strength, reactivity...etc.) Composition The elements and compounds that the object is composed of and the relative amounts of them; direct relationship between composition and materials properties (melting point, reactivity, hardness...etc.) Crystallographic Information How the atoms are arranged in the object; direct relation between these arrangements and materials properties (conductivity, electrical properties, strength...etc.)Where did Electron Microscopes Come From?
Electron Microscopes were developed due to the limitations of Light Microscopes which are limited by the physics of light to 500x or 1000x magnification and a resolution of 0.2 micrometers. In the early 1930's this theoretical limit had been reached and there was a scientific desire to see the fine details of the interior structures of organic cells (nucleus, mitochondria...etc.). This required 10,000x plus magnification which was just not possible using Light Microscopes.The Transmission Electron Microscope (TEM) was the first type of Electron Microscope to be developed and is patterned exactly on the Light Transmission Microscope except that a focused beam of electrons is used instead of light to "see through" the specimen. It was developed by Max Knoll and Ernst Ruska in Germany in 1931.
The first Scanning Electron Microscope (SEM) debuted in 1942 with the first commercial instruments around 1965. Its late development was due to the electronics involved in "scanning" the beam of electrons across the sample. An excellent article was just published in Scanning detailing the history of SEMs and I would encourage those interested to read it.
How do Electron Microscopes Work?
Electron Microscopes(EMs) function exactly as their optical counterparts except that they use a focused beam of electrons instead of light to "image" the specimen and gain information as to its structure and composition.The basic steps involved in all EMs:
- A stream of electrons is formed (by the Electron Source) and accelerated toward the specimen using a positive electrical potential
- This stream is confined and focused using metal apertures and magnetic lenses into a thin, focused, monochromatic beam.
- This beam is focused onto the sample using a magnetic lens
- Interactions occur inside the irradiated sample, affecting the electron beam
The above steps are carried out in all EMs regardless of type. A more specific treatment of the workings of two different types of EMs are described in more detail:
Transmission Electron Microscope
Scanning Electron Microscope
Other related topics
Electron Source (GUN)
Specimen Interactions
Specimen Interaction Volume
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NOTIFICATIONS
Electron microscopes were developed in the 1930s to enable us to look more closely at objects than is possible with a light microscope. Scientists correctly predicted that a microscope that used electrons instead of visible light as the illumination source could view objects at far higher resolution than a light microscope. This is because the wavelength of visible light is what limits the resolution of light microscopes, and the wavelength of electrons is far smaller.
Over time, specialised electron microscopes have been developed that can provide information about different aspects of an object being investigated. This means that scientists can choose the microscope that is most likely to answer their questions about their sample.
Nature of science
The kind of data that scientists can collect is heavily dependent on the tools that are available to them. As microscopes have become more sophisticated, scientists have been able to view objects in greater and greater detail. In turn, they have been able to answer new kinds of questions about the objects they are studying.
What is electron microscopy?
Electron microscopes use a beam of electrons rather than visible light to illuminate the sample. They focus the electron beam using electromagnetic coils instead of glass lenses (as a light microscope does) because electrons can’t pass through glass.
Electron microscopes enable us to look in far more detail at objects than is possible with a light microscope. Some electron microscopes can detect objects that are approximately one-twentieth of a nanometre (10-9 m) in size – they can be used to visualise objects as small as viruses, molecules or even individual atoms.
Unlike light microscopes, electron microscopes can’t be used to look directly at living things because of the special preparation that samples must undergo before they are visualised. Instead, electron microscopes aim to provide a high-resolution ‘snapshot’ of a moment in time within a living tissue.
I think the electron microscope has contributed more to science than any other scientific instrument that’s ever been invented.
Allan Mitchell, Microscopy Otago
Specialised forms of electron microscopy
Several types of electron microscope have been developed to help investigate different aspects of a sample.
The transmission electron microscope (TEM) was the first electron microscope to be developed. It works by shooting a beam of electrons at a thin slice of a sample and detecting those electrons that make it through to the other side. The TEM lets us look in very high resolution at a thin section of a sample (and is therefore analogous to the compound light microscope). This makes it particularly good for learning about how components inside a cell, such as organelles, are structured.
Electron tomography is a form of TEM that lets us see a three-dimensional view of the cell or tissue being studied. Seeing structures in three dimensions can make it much easier to understand how they relate to each other. Electron tomography can also give two-dimensional images at higher resolution than conventional TEM.
The scanning electron microscope (SEM) lets us see the surface of three-dimensional objects in high resolution. It works by scanning the surface of an object with a focused beam of electrons and detecting electrons that are reflected from and knocked off the sample surface. At low magnifications, entire objects (such as insects) viewed on the SEM can be in focus at the same time. That’s why the SEM is so good at generating three-dimensional images of lice, flies, snowflakes and so on.
CryoSEM is a specialised form of SEM that’s good for looking at things that contain moisture (such as plants or food). In cryoSEM, samples are frozen in liquid nitrogen before being viewed. This avoids the need for the complex preparation steps that are done before conventional SEM (largely to remove water from the sample). Scientists often choose cryoSEM because it gives a more accurate image of what the sample looked like before it was prepared for microscopy.
Electron backscatter diffraction (EBSD) is used to look in detail at the structure of minerals (such as those in rocks). Rather than being microscopes in their own right, EBSD detectors are add-ons to SEMs. After the electron beam is fired at the rock, the EBSD detects electrons that have entered the rock and been scattered in all directions. The pattern of scattering can tell scientists a lot about the structure of the mineral and the orientation of crystals within it.
Activity ideas
In Using shadows to build 3D images, students model how scientists interpret microscope data by using shadows of an object from different angles to build up a 3D image.
In Which microscope is best?, students learn about various types of microscopes and discover which microscope is best for a specific sample type.
Useful link
Learn more about the 1986 Nobel Prize in Physics, half of which was awarded to Ernst Ruska for the design of the first electron microscope.
Plant & Food Research (PFR) have a collection of images taken using a scanning electron microscope (SEM). The images were captured within the Plant & Food microscopy laboratory and curated and hand-coloured by PFR photographer Wara Bullot.
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