STEM
Overview
Scanning transmission electron microscopy (STEM) combines the scanning of Scanning Electron Microscopy with the transmitted electrons of Transmission Electron Microscopy to create images or analytical maps. This technique may be used in either type of microscope.
Theory
When a well aligned electron probe is focused on a sufficiently thin specimen, most electrons are transmitted through the specimen rather than scattering back through the surface. These electrons may be scattered according to the local elements and topography to generate diffracted beams, or they may pass through relatively unaffected as the primary beam. Collected signals are correlated to the location of the beam when the signal was generated, which allows for the creation of mapped datasets or images.
In the TEM
In bright field STEM, the transmitted beam of relatively unaffected electrons, also known as the primary beam, creates the image. This results in a bright background with dark features where particles, etc. are present.
In dark field STEM, the highly scattered electrons (diffracted beams) form the image. These electrons carry information about the elemental makeup of a specimen because heavier atoms scatter electrons at higher angles.
Analytical modes such as EDS and EELS provide specific information about the atomic makeup of specimens based on x-ray spectra or electron energy.
In the SEM
The primary beam is imaged with a detector beneath the specimen. Specimens must be mounted on a TEM grid with a specialized specimen holder. Particles appear dark where they scatter electrons. Layering may be visible in sufficiently thin specimens. This is comparable to bright field STEM in the TEM.