Introduction to the Electron Microscope

Electron Microscope Magnification

The advantages of using an electron microscope over an optical microscope are much higher magnification and resolving power. The disadvantages include the cost and size of the equipment, the requirement for special training to prepare samples for microscopy and to use the microscope, and the need to view the samples in a vacuum (although some hydrated samples may be used).

The easiest way to understand how an electron microscope works is to compare it to an ordinary light microscope. In an optical microscope, you look through an eyepiece and lens to see a magnified image of a specimen. The optical microscope setup consists of a specimen, lenses, a light source, and an image that you can see.

In an electron microscope, a beam of electrons takes the place of the beam of light. The specimen needs to be specially prepared so the electrons can interact with it. The air inside the specimen chamber is pumped out to form a vacuum because electrons don't travel far in a gas. Instead of lenses, electromagnetic coils focus the electron beam. The electromagnets bend the electron beam in much the same way lenses bend light. The image is produced by electrons, so it is viewed either by taking a photograph (an electron micrograph) or by viewing the specimen through a monitor.

There are three main types of electron microscopy, which differ according to how the image is formed, how the sample is prepared, and the resolution of the image. These are transmission electron microscopy (TEM), scanning electron microscopy (SEM), and scanning tunneling microscopy (STM).

Transmission Electron Microscope (TEM)

The first electron microscopes to be invented were transmission electron microscopes. In TEM, a high voltage electron beam is partially transmitted through a very thin specimen to form an image on a photographic plate, sensor, or fluorescent screen. The image that is formed is two-dimensional and black and white, sort of like an x-ray. The advantage of the technique is that it is capable of very high magnification and resolution (about an order of magnitude better than SEM). The key disadvantage is that it works best with very thin samples.

Scanning Electron Microscope (SEM)

In scanning electron microscopy, the beam of electrons is scanned across the surface of a sample in a raster pattern. The image is formed by secondary electrons emitted from the surface when they are excited by the electron beam. The detector maps the electron signals, forming an image that shows the depth of field in addition to the surface structure. While the resolution is lower than that of TEM, SEM offers two big advantages. First, it forms a three-dimensional image of a specimen. Second, it can be used on thicker specimens, since only the surface is scanned.

In both TEM and SEM, it's important to realize the image isn't necessarily an accurate representation of the sample. The specimen may experience changes due to its preparation for the microscope, from exposure to vacuum, or from exposure to the electron beam.

Scanning Tunneling Microscope (STM)

A scanning tunneling microscope (STM) images surfaces at the atomic level. It is the only type of electron microscopy that can image individual atoms. Its resolution is about 0.1 nanometers, with a depth of about 0.01 nanometers. STM can be used not only in a vacuum, but also in the air, water, and other gases and liquids. It can be used over a wide temperature range, from near absolute zero to over 1000 degrees C.

STM is based on quantum tunneling. An electrical conducting tip is brought near the surface of the sample. When a voltage difference is applied, electrons can tunnel between the tip and the specimen. The change in the current of the tip is measured as it is scanned across the sample to form an image. Unlike other types of electron microscopy, the instrument is affordable and easily made. However, STM requires extremely clean samples and it can be tricky getting it to work.

Development of the scanning tunneling microscope earned Gerd Binnig and Heinrich Rohrer the 1986 Nobel Prize in Physics.