Why does a transmission electron microscope require a thin specimen?

A transmission electron microscope (TEM) requires a very thin specimen because the instrument functions by passing an electron beam directly through the sample to visualize its interior structure.12

For the electrons to successfully penetrate the specimen and form an image, the sample must be thin enough to be transparent to the beam. If a specimen is too thick, the electrons cannot pass through it effectively, which prevents the formation of a clear image. Furthermore, because the image in a TEM is formed by the scattering of electrons by atoms within the specimen, maintaining a thin sample helps minimize energy losses caused by collisions between electrons and atoms, which can otherwise lead to image-degrading chromatic aberration.32

Typically, these samples must be prepared as films only a few hundred nanometers thick, though the required thickness can vary depending on the density of the elements present in the material.3