In a scanning electron microscope (SEM), a fine beam of electrons is scanned back and forth across the surface of a specimen. The electrons do not pass through it. Instead, electrons are knocked off or bounced back from the surface, and a detector collects them to build up an image point by point.
Preparing the specimen
The specimen is fixed and dried, then coated with a very thin layer of a heavy metal such as gold. The metal coating reflects electrons well. Because electrons do not need to pass through, the specimen does not need to be cut into thin sections, so whole organisms such as insects, pollen grains or bacteria can be viewed.Strengths
The SEM produces a three-dimensional image of surfaces with a large depth of field. This makes it ideal for studying external shape and surface features, for example the microvilli on intestinal cells, the stomata on a leaf or the surface of a red blood cell.Weaknesses
• Its resolution (about 3 to 10 nm) is lower than that of the TEM, although still much better than a light microscope.
• It shows only surfaces, not internal structure.
• As with all electron microscopes, specimens are dead, the image is black and white (colour is added afterwards) and preparation can cause artefacts.To choose between them: use a TEM to see inside a cell, and an SEM to see the 3D surface of a cell or organism.
Preparing the specimenThe specimen is fixed and dried, then coated with a very thin layer of a heavy metal such as gold. The metal coating reflects electrons well. Because electrons do not need to pass through, the specimen does not need to be cut into thin sections, so whole organisms such as insects, pollen grains or bacteria can be viewed.Strengths
The SEM produces a three-dimensional image of surfaces with a large depth of field. This makes it ideal for studying external shape and surface features, for example the microvilli on intestinal cells, the stomata on a leaf or the surface of a red blood cell.Weaknesses
• Its resolution (about 3 to 10 nm) is lower than that of the TEM, although still much better than a light microscope.
• It shows only surfaces, not internal structure.
• As with all electron microscopes, specimens are dead, the image is black and white (colour is added afterwards) and preparation can cause artefacts.To choose between them: use a TEM to see inside a cell, and an SEM to see the 3D surface of a cell or organism.