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Overview of Electron Microscopy
 

Why are electron microscopes important?

The electron microscope (EM) is an important research-grade instrument that is used to examine specimens in the sciences to enhance our understanding of the world around us at a micro- and nanostructural level. EMs are used in medicine, industry, and academic settings around the world.

 

Applications in the life sciences range from supporting diagnoses in human pathology to the study of autoimmune diseases such as AIDS and arthritis, and extend from medical to pharmaceutical, agricultural, dental, and related fields.

 

Applications in the physical sciences range from failure analysis in semiconductor devices at IBM and jet engines at Boeing, to materials development for the space shuttle by NASA and Stealth aircraft by Sandia, to building better cars and bridges at Ford and U.S. Steel.

How does an electron microscope work?

The EM uses electrons as a source for imaging (analogous to light used in a light microscope). As a direct result, the EM offers a resolution capability thousands of times better than a light microscope – with images that may be magnified over a million times. As a corollary, the EM is correspondingly larger (occupying a whole room), more expensive, and more difficult to use than a light microscope.

 

There are two basic types of EM – the transmission electron microscope (TEM) and the scanning electron microscope (SEM). A TEM images electrons that pass through a thinned sample, and therefore can examine internal structure. An SEM images electrons emitted from a bulk sample, and therefore can study surface structure. Both types of EM can be used to study not only the appearance of a specimen, but its chemical composition as well.

Electron microscopes at Trinity

Trinity has four electron microscopes! All are available for student use in various courses and in independent study.

 

There are two TEMs, one used primarily in the life sciences in the Life Sciences Center, and another used primarily in the physical sciences in the McCook Building. There are two SEMs, one in the Chemistry Department and one in the McCook Building. All are used by all the sciences.

 

Besides the two basic types of EM (TEM and SEM), Trinity has two other microscopes that are known as Scanning Probe Microscopes (SPM). One is an atomic force microscope (AFM), located  in Engineering. The other is a scanning tunneling microscope (STM), located in Physics. These instruments enable surface characterization of samples at the atomic level.

 

To delightfully complicate matters - and create a wonderful blend of capabilities - one of the TEMs also has a high-resolution SEM mode that images secondary and backscattered electrons, and a scanning-transmission mode (STEM). One of the SEMs and one of the TEMs are also equipped with accessories known as x-ray energy dispersive spectrometers (EDS) that enable chemical composition analysis. The potential magnification capability currently on campus ranges as high as 12,000,000X!

Support Instrumentation

It takes a lot of support instrumentation to prepare samples and record data in electron microscopy. Think of it - an EM cannot exist in isolation; it is not a useful tool if it is the only piece of equipment in the lab. If you imagine that the EM sits at the center of the spectrum, and then consider either end...

 

On one end of the spectrum is specimen preparation. Samples intended for the electron microscope must be specially processed to withstand the high-vacuum conditions required by the electron beam, as well as the thermal and electrical bombardment of the electrons themselves. This processing is elaborate (often taking a number of days to complete) and it relies upon precision support instrumentation such as ultramicrotomes, which slice samples only 70 nanometers in thickness, and ion mills, which accelerate heavy ions against a sample to carve out thin regions.

 

On the other end of the spectrum is data recording. Data may be recorded in the form of either an image or a spectrum. The images may be acquired either digitally or photographically. Once results are acquired and recorded, then they must be interpreted! But that is another subject altogether, and forms much of the basis for the courses featuring EM that are currently taught at Trinity.