Electron Microscope

The electron microscope uses a beam of electrons, which are 2000 times lighter than protons. The electrons are accelerated by a voltage of 50,000 to 200,000 V and focused onto a desired object. The wavelength of the electrons results from the de Broglie equation and is as small as 5 pm (pm = 10-12 m = one billionth of millimeter for 50 kV acceleration). By technical reasons the achieved resolution is a factor of 1000 worse than the de Broglie wavelength, but nevertheless a factor of 1000 better than that of a light microscope (see discussion below).

Construction
The electrons are emitted from the heated cathode and accelerated by the "Wehnelt cylinders". The magnetic coils act like the glass lenses of the light microscope and serve to focus the electron beam on its way through the microscope. The picture is produced by scattered electrons and is observed optically on a screen or with a camera (e.g. digital).

There are two ways to use an electron microscope: For the Transmission Electron Microscopy (TEM), the studied objects must not be too thick to allow the electrons to pass through. Due to their charge, the electrons are decelerated rapidly within the target. In the other choice, the Reflection Electron Microscope (REM)  can examine the surface of thick samples. However, these samples must have a conductive surface. In addition, a high vacuum inside this microscope is required; therefore no living objects can be examined.

 

The limits of resolution The distance of the two points, which can be perceived separately, is called the resolution of the instrument. The light microscopes achieve a resolution comparable to the used wavelengths (~0.5 µm). For electron microscopes the stronger distortions at the edges of the magnetic lenses (aberration), introduces errors in the picture. This results in a limitation of the resolution far beyond the dimensions of the de Broglie wavelength of the electrons and is in the order of 5 nm. For magnetic lenses of an electron microscope, the effect of the aberration is fundamentally stronger than for optical lenses of a light microscope (see figure).

 

Aberration:
Due to deviations at the edge of the lens, the picture becomes blurred, limiting the resolution, which could be possible for the given wavelength.

The resolution of an electron microscope becomes better with the increasing acceleration voltage of the electron beam, because of the decreasing de Broglie wavelength of the electron. This fact can be explained only by the wave nature of the electrons.

Fullerens