Scanning electron microscopy (SEM) is a powerful imaging technique that produces a widely enlarged image by using electrons and X-rays instead of light.
The primary electrons come from the electron gun located at the top of the microscope. The secondary electrons originate from the sample as a result of the displacement of the primary electrons. The scattered back electrons consist of high energy electrons that originate from the primary electron beam that are reflected or backscattered from the sample.
SEM can be coupled with an X-ray spectrometer called dispersive energy X-ray spectroscopy (EDX). At rest, an atom within the sample contains electrons in the fundamental state. The primary electrons in the electron cannon excite an electron in an inner layer, which causes its expulsion as a secondary electron, resulting in the formation of an electron hole within the electronic structure of the atom. An electron from an outer layer of higher energy fills the lower energy hole, and the excess energy is released in the form of an X-ray photon. Each element of the periodic table emits X-rays at distinctive energies due to the elementary electronic structure different. Therefore, the X-ray emission data can be analyzed to characterize the elementary composition of the sample and also perform an area mapping.
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