Electron Microscopy

Observing structures smaller than 1 ¡m is not possible with light because diffraction effects limit the resolution of optical spectroscopy. So, if information at considerable higher resolution is desired, electromagnetic radiation of shorter wavelengths should be used. Electron beams offer this possibility. Their development over the past 50 years has resulted in electron microscopes that routinely achieve magnification on the order of one million times and can reveal details with a resolution of up to about 0.1 nm.

When an electron beam of energy between 100 and 400 keV hits a sample, many measurable signals are generated. TEM uses the transmitted electrons to form the image while SEM uses the secondary electrons emitted from the sample. Depending on the sample thickness, a fraction of the electrons passes through it without suffering significant energy loss. Since the attenuation of the electrons depends on the density and thickness of the sample the transmitted electrons form a two-dimensional (2D) projection of the sample. This is the basis for TEM imaging. Electrons can also be diffracted by particles if these are favorably oriented toward the electron beam; the crystallographic information that can be obtained from these diffracted electrons is the basis for electron diffraction. Finally the electrons in the primary beam can collide with atoms in the sample and be scattered back; backscattering is more effective when the mass of the atom increases. If a region of the sample contains heavier atoms (such metal particles) that the surroundings, it can be distinguished due to higher yield of backscattered electrons. If the electron beam is rastered over the surface and the yield of secondary or backscattered electrons is plotted as function of the position of the primary electron beam it is possible to get three-dimensional images of the samples analyzed. This method is the basis for SEM.

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