Question

How does an electron microscope work? Please explain in detail.

Answer 1

When electrons interact with a specimen, information can be collected about the interaction events for further analysis. The figure below (Fig. 2) describes these events and how they are used in two of the most common types of microscope: transmission (TEM) and scanning (SEM) electron microscope.

To acquire images in the scanning EM, the specimen is coated with a metal, which will lead to the emission of secondary electrons. A 3-nm focal beam of electrons is then used to scan the surface of the sample. The image collected by the sensor will eventually look three-dimensional (it will not contain any internal information), as secondary electrons are emitted off the specimen and recorded during scanning time, providing details about the surface structure of the specimen. In transmission EM, by contrast, an image is acquired as a projection of the entire sample (including any internal information); however, due to electron absorption, typically only thin specimen sections are used to produce a two-dimensional image on the viewing screen (phosphorescent screen, layer of photographic film, or sensor such as a CCD camera). Moreover, the brightness of any specific area seen in the image is proportional to the quantity of electrons that penetrate through the specimen.