A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused electron beam. The electrons interact with the atoms in the sample, producing several signals that contain information about the surface topography and the composition of the sample. The electron beam is scanned in a raster scan pattern, and the beam position is combined with the intensity of the detected signal to produce an image. In the most common SEM mode, secondary electrons emitted by atoms excited by the electron beam are detected using an Everhart-Thornley detector. The number of secondary electrons that can be detected, and therefore the intensity of the signal, depends, among other things, on the topography of the sample. SEM can achieve a resolution better than 1 nanometer.
The samples are observed in high vacuum in conventional SEM, or in low vacuum or humid conditions in variable or environmental pressure SEM, and in a wide range of cryogenic or high temperatures with specialized instruments.
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