Scanning electron microscope
A scanning electron microscope (SEM) examines a specimen by moving a focused electron beam across it and detecting signals produced by the interaction. These signals can reveal surface shape, differences in composition and, with suitable equipment, crystallographic information. The image is assembled from measurements associated with successive beam positions.[1]
Signals and analysis
Secondary electrons are particularly useful for showing surface topography. Backscattered electrons can provide contrast between materials of different composition. Characteristic X-rays produced by the beam can be analysed to identify elements, while electron backscatter diffraction provides information about crystal orientation. The available measurements depend on the detectors fitted to the instrument.[1]
Specimens
Conventional SEM requires samples that can tolerate a vacuum. Electrically insulating specimens often receive a thin conductive coating; some low-vacuum or environmental instruments permit examination of materials unsuitable for conventional conditions. These variations make the preparation requirements dependent on both the specimen and the microscope.[1]
Biological specimens may be fixed and dried before coating. Drying can alter their shape, so preparation methods seek to limit such artefacts. The resulting micrographs often have a strong three-dimensional appearance, but a surface image is distinct from a complete reconstruction of the object's interior.[2]
In transmission electron microscopy, electrons pass through a very thin section to show internal features. SEM can instead examine the surfaces of larger objects, including pollen grains, as well as small structures. The two approaches therefore answer different questions about a specimen rather than forming a simple hierarchy of better and worse images.[2]
References
- ↑ 1.0 1.1 1.2 Susan Swapp. Scanning Electron Microscopy (SEM). University of Wyoming / Science Education Resource Center.
- ↑ 2.0 2.1 OpenStax. Instruments of Microscopy. Microbiology.
Attribution
Adapted with reference to English Wikipedia, revision 1378299674 (contributors and history), under CC BY-SA 4.0. The prose has been rewritten and checked against the cited sources; changes have been made.