How Does An Electron Microscope Work?
Last updated on February 28, 2022 by
In a recent blog post, we discussed light vs. electron microscopes. Now, we’re digging a bit deeper to share more about what electron microscopes do, the various types available, and how they work.
What Does an Electron Microscope Do?
Optical microscopes or light microscopes aren’t good enough to see bacteria, viruses, or molecules. Electron microscopes are much more powerful and allow us to see these things in nano-dimensions.
Electron wavelengths can be up to 100,000 times shorter than the wavelength of visible light photons so electron microscopes have a much higher resolution and make it possible to see the structure of smaller objects.
Electron microscopes are most commonly used to research the ultrastructure of biological and inorganic specimens such as biopsy samples, crystals, metals, microorganisms, and cells.
They can also be used industrially to help with quality control and failure analysis. Today’s modern electron microscopes produce micrographs with specialized digital cameras and frame grabbers to capture the image of the specimen.
How Does an Electron Microscope Work?
Like an optical microscope, there are four important parts, but with some differences. Instead of a light source, an electron microscope features an electron source, a beam of electrons powered by a filament.
The specimen needs to be prepared and held inside a vacuum chamber where there is no air. Electrons are easily scattered by air particles that include gas molecules like oxygen and nitrogen, so without a vacuum the electron beam would be knocked off of it’s course. Biological samples can evaporate immediately in a vacuum without preparing the sample ahead of time, so it is important to take some preparative steps.
Instead of lenses, you have a series of electromagnets the electron beam travels through, referred to as electromagnetic lenses. With an ordinary microscope, the glass lenses either bend or refract the light beams passing through them to magnify the specimen. With an electron microscope, the coil-shaped electromagnets bend the beams the same way.
Instead of looking through an eyepiece to see the magnified sample, the microscope image is formed as a photograph, also known as an electron micrograph, or displayed on a computer screen.
Different types of electron microscopes are on the market and each of them works in a slightly different way. All produce high-resolution images, though some types are better suited for certain materials than others.
Types of Electron Microscopes
Transmission Electron Microscope (TEM)
A transmission electron microscope (TEM) is also known as the original form of the electron microscope. It uses a high-voltage electron beam to illuminate the specimen and produce a flat image.
The beam is made by an electron gun that is commonly filled with tungsten filament as the source. They’re commonly used in electron diffraction mode but often require incredibly thin sections about 100 nanometers thick for the electrons to pass through.
Creating specimens this thin is often extremely difficult and technically challenging. Some specimens may require dehydration or chemical fixation before cutting into thin slices is even possible. Some also may need to be stained for easier visibility.
Serial-Section Electron Microscope (ssEM)
An ssEM Is a subset of TEM. It creates images of many thin sections in sequence.
Scanning Electron Microscope (SEM)
A scanning electron microscope (SEM) is similar to a key copying machine. When you get a key copied, the machine traces over the original key to cut an exact replica into a blank. The copy isn’t produced all at once but instead traced out from one end to the other. The specimen under examination can be considered the original key.
The SEM uses an electron beam to trace the object and create an exact replica of the original on a monitor. Instead of just tracing out a flat outline of the key, the SEM provides a 3D image that’s complete with grooves and engraving.
As the beam traces over the object and interacts with the surfaces to dislodge secondary electrons from the surface of the specimen in patterns. A secondary detector attracts those electrons. The number of electrons that reach the electron detector influences the brightness level shown on the monitor.
Reflection Electron Microscope (REM)
With a reflection electron microscope (REM), an electron beam is on a surface, but instead of using the transmission or secondary electrons, the beam of elastically scattered electrons is detected.
This type of microscopy is used to observe processes happening on a sample’s surface. The elastically scattered electrons hit the sample at different glancing angles, which in turn generates an image.
Scanning Transmission Electron Microscope (STEM)
A scanning transmission electron microscope (STEM) creates images with a focused beam of electrons on an incredibly small area on the specimen, typically .05 to .2 nanometers. It is then scanned over the sample in a raster illumination system so that the sample is illuminated at each point with the beam parallel to an optical axis.
This type of electron microscopy is suitable for analytical techniques. The typical STEM microscope is a conventional TEM microscope that has additional scanning coils, detectors, and circuitry to allow it to switch between operating as a STEM or a standard TEM. However, it is also possible to find dedicated STEMs.
Scanning Tunnelling Microscopy (STM)
STM microscopes create detailed images of the molecules or atoms on the surface of the sample. They work differently than TEM and SEM.
STM microscopes use an extremely sharp metallic probe that scans back and forth across the surface of a specimen. Electrons try to move out of the specimen and jump across the gap into the probe. The closer the probe is to the surface, the easier it is for the electrons to move into it and as more electrons escape, the greater the tunneling current becomes.
The microscope constantly moves the probe up and down by minuscule amounts to keep the tunneling constant. By recording how much the probe needs to move it measures the peaks and valleys throughout the specimen’s surface.
A computer converts this information into a map of the specimen that reveals its detailed atomic structure. Ordinary electron microscopes use high-energy electron beams to produce their highly detailed images but the intensity tends to damage the object they are imaging. STM uses lower energies to reduce the likelihood of damage.
Atomic Force Microscope (AFM)
One of the disadvantages of STM’s is that they rely on the electrical currents passing through materials to create their images. If the object is not a conductor of electricity, the microscope cannot create an image.
Atomic force microscopy (AFM) microscopes don’t have this problem because even though they still use tunneling, they are not reliant on a current flowing between the probe and the specimen. Using this type of microscope makes it possible to create atomic-scale images of plastics and other materials that do not conduct electricity.
Cryo-Electron Microscopy (Cryo-EM)
Also known as cryo-EM, this involves using frozen samples with gentler electron beams to work with samples that aren’t compatible with high-vacuum conditions and intense electron beams.
Recently, a special method to freeze water-based TEM samples was developed so that the water creates a disordered glass instead of crystalline ice. Ice crystals diffract the electron beam and obscure information about the molecules.
Cryo-EM eliminates this issue and makes it possible to use transmission electron microscopy with samples incompatible with a vacuum environment.
How Much do Electron Microscopes Cost?
It’s impossible to provide an exact number as to how much electron microscopes cost. Price varies according to the type you purchase as well as what you intend to use it for and your final configuration.
Different detectors and resolutions are available that you may or may not need, dependent on your application. On the low end of the spectrum, a fairly basic tabletop SEM would cost around $50,000 to $70,000.
A conventional SEM with a tungsten source could easily cost anywhere from $80,000 to $120,000. Other types are even more expensive.
Leasing vs. Buying Electron Microscopes
For many labs, that kind of money for a single piece of equipment isn’t feasible. Leasing the equipment through Excedr gives you the electron microscopes you need for your lab, without a major investment upfront, and helps save even more because maintenance and repairs are included in the cost of the lease.
Plus, if your lab needs other types of microscopes, we also lease x-ray microscopes, infrared microscopes, confocal microscopes, fluorescence microscopes, Raman microscopes, and multi-photon microscopes.
Contact us today to learn more about how our equipment leasing program can help you.