Scanning Electron Microscope

• Why was a SEM developed?

  To overcome the problem of a flat, 2-D image from a TEM.
• What are the limitations of a SEM?

  The whole system must be in a vacuum therefore living species cannot be observed, a complex staining process is required, the image is in black and white, and the image may contain artefacts.
• What happens in a SEM?

  A beam of electrons is directed onto the surface of the specimen from above and passes back and forth across a portion of the specimen in a regular pattern.
• What don't electrons do in a SEM but do in a TEM?

  Penetrate the specimen.
• What does the specimen do with the electrons in a SEM?

  Scatters them with a pattern that depends on the contours of the specimen surface.
• What kind of image can we produce from a SEM?

  A 3-D image.
• What is the resolving power of a SEM?

  20 nm.