By Harvey Sells, updated Mar 24, 2022
Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) are powerful tools for visualizing structures at the nanometer scale. Although both rely on electron beams, they differ markedly in specimen preparation, imaging principles, and typical applications.
TEM excites a specimen with a focused electron beam that passes through the sample. Because electrons must traverse the specimen, TEM requires ultrathin sections (typically <100 nm thick) and often uses heavy‑metal staining to enhance contrast. This approach makes TEM ideal for visualizing the internal architecture of viruses, cells, and tissue sections at sub‑nanometer resolution.
SEM, in contrast, scans a focused electron beam over the surface of a specimen. To prevent charge build‑up and to detect backscattered or secondary electrons, samples are coated with a thin conductive layer—commonly gold‑palladium, carbon, or platinum. SEM excels at revealing surface topography and is routinely used to examine macromolecular aggregates, tissue surfaces, and engineered materials.
An electron gun generates a high‑energy beam that is first condensed by a condenser lens. The resulting narrow beam is directed through the specimen; electrons that are not absorbed form an image on a phosphor screen via an objective lens. Areas that appear darker correspond to thicker regions where fewer electrons pass through.
SEM also starts with an electron gun and a condenser lens, but the beam is subsequently focused into a fine spot by a second lens. Magnetic coils then raster the beam across the specimen, while a third lens steers the electrons to the area of interest. By adjusting dwell time and scan rate—typically 30 scans per second—SEM captures high‑resolution surface images.
