Scanning electron microscopy (SEM) is a powerful imaging technique which produces a largely magnified image by using electrons and X-rays instead of light.

 

Primary electrons come from the electron gun situated at the top of the microscope. Secondary electrons originate from the specimen as a result of displacement by the primary electrons. Backscattered electrons consist of high-energy electrons originating in the primary electron beam that are reflected or back-scattered out of the specimen.

 

SEM can be coupled with an X-ray spectrometer which is called energy-dispersive X-ray spectroscopy (EDX). At rest, an atom within the specimen contains ground state electrons. The primary electrons from the electron gun excites an electron in an inner shell, prompting its ejection as a secondary electron, resulting in the formation of an electron hole within the atom’s electronic structure. An electron from an outer higher energy shell then 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 distinct elemental electronic structure. Thus, X-ray emission data can be analysed to characterise the elemental composition of the specimen and also perform area mapping.