Field electron microscope

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View of the screen image with the field emission microscope (school experiment)

The field electron microscope (also: field emission microscope , rarely: tip microscope) is one of the high-resolution microscopes with which a spatial resolution in the atomic range is possible. In this way, atoms can be made visible in the samples to be examined .

This instrument was developed in 1936 by Erwin Wilhelm Müller .

construction

In principle, this instrument consists of a highly evacuated glass bulb that acts as an anode . Inside there is a fine tip (its diameter is smaller than the wavelength of visible light, it is a few nm ) - for example a tungsten needle - which represents the cathode . A potential difference of a few kilovolts between the cathode and anode creates an almost spherically symmetrical electric field . At field strengths of over 10 9  V / m, electrons are released from the cathode ( field emission , promoted by the tunnel effect ) and accelerated towards the anode. An image of the cathode tip, enlarged millions of times, then appears on a luminescent layer in the glass bulb.

functionality

Scheme of a field electron microscope

On the screen you can see a kind of image of the atomic structure of the cathode crystal, because a particularly large number of electrons escape at points on the needle tip where the work function is at a minimum. Due to the distance between the cathode and anode and the spherical symmetry , a pattern is obtained on the fluorescent screen that represents the enlarged tip. The magnification can be up to 1 in 500,000.

If you vaporize other substances - for example barium - on the tip, you can get a picture of individual such atoms on the tip and also make the heat movement of these atoms visible.

The introduction of helium gas causes an electron to be torn away from the helium in the strong field of the needle tip and the positive helium ion is accelerated towards the luminescent layer. Since the de Broglie wavelength of the helium ions is smaller than that of the electrons, the resolution is increased at low temperatures. Magnifications of up to 2,000,000 times can thus be achieved. The whole structure is reminiscent of a helium-ion microscope ; In this case, however, the cathode is not the enlarged object, but the generated helium ion beam hits the object to be examined.

Related high resolution microscope types

literature

  • Erwin Wilhelm Müller: Experiments on the theory of electron emission under the influence of high field strength , Phys. Line 37 838 (1936)
  • Richard Phillips Feynman, Robert B. Leighton, Matthew Sands: Feynman lectures on physics. 2. Electromagnetism and the structure of matter. 2001 Oldenbourg Wissenschaftsverlag ISBN 3-486-25589-4 (available on the web: Googlebooks )

Web links