Answer to Question #196895 in Quantum Mechanics for Mathushika Murugia

Question

With a neat diagram, explain the working of electron microscopes. Briefly explain the role of each experimental component. Compare the resolution of electron microscopes and optical microscopes. [Hint: Explanation of SEM & TEM is required]

Answer 1

The transmission electron microscope (TEM) is the first kind of electron microscope, and it uses a high-voltage electron beam to illuminate the specimen and produce an image. An electron gun produces the electron beam, which is usually equipped with a tungsten filament cathode as the electron source. The electron beam is accelerated concerning the cathode by an anode that normally operates at +100 keV (40–400 keV), focused by electrostatic and electromagnetic lenses, and transmitted through a partially translucent specimen electron and partially scatters them out of the beam. When it emerges from the specimen, the electron beam holds information about the composition of the specimen, which is magnified by the microscope's objective lens system. By projecting the magnified electron image onto a fluorescent viewing screen covered with a phosphor or scintillator material such as zinc sulfide, the spatial difference in this detail (the "image") can be seen. Alternatively, the image may be photographed by exposing a photographic film or plate to the electron beam. A high-resolution phosphor can be connected to the screen of a digital camera through a lens optical filter or a fiber optic light guide. The picture captured by the digital camera can be seen on a tablet or monitor.

The resolution of TEMs is mainly restricted by spherical aberration. However, a recent generation of hardware correctors will eliminate spherical aberration to reduce resolution in high-resolution transmission electron microscopy (HRTEM) to less than 0.5 angstroms (50 picometres), allowing magnifications greater than 50 million times. (The highest optical microscope resolution is around 0.2 microns = 200 nm.)