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Pollen grains taken on an SEM show the characteristic depth of field of SEM micrographs.
Scanning electron microscopy (SEM) uses a finely focused beam of electrons in order to produce a high resolution image of a sample. SEM images have a three dimensional appearance, which is very useful when examining the surface structure of a sample.
The First Microscopes
The microscope has existed, in one form or another, for almost 1000 years. In the earliest days of the technology, light focused through lenses produced 6 to 10x magnifying power, an impressive feat in pre-Renaissance Europe. Currently, the finest optical microscopes which get their power from complex systems of mirrors and lenses, can reach between 500 and 1000x magnification.
Optical microscopes are limited in their power by the properties of light. To surpass such primitive limits, scientists in the 1930s began working with electron microscopes. M. Knoll and Manfred von Ardenne were two of the pioneers in this field.
The History of Scanning Electron Microscopy
Early models of Scanning electron microscopes (SEMs) were weaker than many popular models of optical microscopes. At the time, SEMs were only capable of a resolution of around 200 Angstroms (where 1nm is equal to 10 Angstrom units), compared to the sub-50 Angstrom resolution of the Transmission Electron Microscope (TEM). This technology has seen drastic improvement since those early days, and today there are over 50,000 SEMs populate labs and classrooms worldwide.
As the electron microscope developed, it attracted more and more researchers, including Charles Oatley of Cambridge University's Engineering Department. Mr. Oatley and his students put together their first SEM in 1948, and four years later, it was producing three-dimensional images.
Oatley's dream was to put a relatively cheap SEM on the market, as he saw the vast implications that greater magnifying power would have on the pursuit of disciplines ranging from engineering to particle research. During the rest of his career, Oatley sponsored efforts to build simple microscopes, paving the way for the SEMs mass appeal.
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Scanning electron microscope image, showing an example of green algae (Chlorophyta).
How does the SEM Differ from Optical Microscopy?
As the name suggests, SEMs use an electron beam instead of a beam of light, which is directed towards the specimen under examination. An electron gun, located at the top of the device, shoots out a beam of highly concentrated electrons. There are two main types of electron guns used by SEMs.
The first, Thermionic guns, heat a filament until electrons stream away. Field emission guns, the other popular choice, rip electrons away from their atoms by generating a strong electrical field.
The microscope is composed of a series of lenses within a vacuum chamber. These lenses direct the electrons towards the specimen in order to maximize efficiency. The more electrons that are used, the more powerful the magnification.
The SEM usually requires a vacuum chamber to function, as the electron beam must not be obstructed as it passes through the body of the microscope. Small particles could deflect the electrons onto the specimen itself, obscuring the results.
When a specimen is hit with a beam of the electrons known as the incident beam, it emits X-rays and three kinds of electrons: primary backscattered electrons, secondary electrons and Auger electrons. The SEM uses primary backscatter electrons and secondary electrons.
Using a Scanning Electron Microscope
Using a Scanning Electron Microscope - University of Leicester
An electron recorder picks up the rebounding electrons and records their imprint. This information is translated onto a screen which allows three-dimensional images to be represented clearly. One of the SEM's greatest advantages is its ability to reproduce textual information in a consistent and coherent manner.
Traditional SEMs were not able to produce color images, as the electrons merely revealed information about the sample's size and topography. However, recent advances have allowed researchers to measure energy signatures in the sample's reaction to the incident beam impact.
These energy signatures are then colour-coded, with each different element producing its own specific signature. By referring to those colors, scientists can identify, in extreme detail, the borders of each element in the sample.
Sample Preparation for SEM
The process will only work if the sample is properly prepared. Metals require no preparation, as they already conduct electricity and will respond favorably when bombarded with electrons. However, non-metals need to be prepped with a material known as a sputter coater.
Sputter coaters provide the specimen with a thin layer of conductive material, usually gold. The gold is acquired through the use of an electric field and an argon gas. The electric field dislodges an electron from the argon, resulting in positively charged ions.
These positive ions are then attracted to gold foil, which is negatively charged. As they settle onto the gold, the argon ions expel gold atoms, which fall onto the specimen, covering it with a thin conductive coating.
In addition, preparation traditionally includes the removal of all water. Water molecules will vaporize in a vacuum, creating obstacles for the electron beams and obscuring the clarity of the image.
However, a number of newer SEMs no longer require a full vacuum to operate. These devices produce images at weaker resolution, but they open up the possibility of examining a whole range of previously untouchable sample, crucial for a number of different industries and disciplines.
The Applications of Scanning Electron Microscopy
SEMs have opened doors in fields ranging from chemistry to engineering, allowing scientists working on a wide range of projects to access new, useful information about microscopic processes with macroscopic implications.
The SEM uses Energy-Dispersive X-Ray Spectroscopy (EDS) in the production of elemental maps, which accurately represent the distribution of elements within samples. The most typical use is elemental analysis, mineral orientation, morphology and contrasts.
While the SEM has been surpassed in power by newer microscopes, it remains one of the most useful on the market, as few others can operate with a similarly broad range of samples. Today, researchers all over the world rely on SEMs for accurate visual information.
Sources and Further Reading
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