Field emission microscopy

The field electron microscope (also: field emission microscope, rare: About microscope tip ) belongs to the high-resolution microscopes with which a spatial resolution on the atomic scale is possible. Such 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 from a highly evacuated glass bulb, acts as anode. This 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 is the cathode. A potential difference of several kilovolts between the cathode and the anode produces an approximately spherically symmetrical electric field. At field strengths of 109 V / m electrons are released from the cathode ( field emission, favored by the tunnel effect ), and accelerated towards the anode. Then a million times magnified image of the cathode tip will appear on a luminescent layer in a glass flask.

Operation

On the screen can be seen, the emissivity of the needle tip, and thus a type of image of the atomic structure of the cathode - crystal. Because the electrons of the locations of the surface where the work function is at a minimum, can be solved out easily and are used for anode ( phosphor screen ) accelerated toward. Due to the distance between the cathode and anode as well as the spherical symmetry is obtained on the phosphor screen, a pattern which represents the enlarged tip. The magnification can be up to 500,000 -fold.

If other substances - for example, barium - by vapor deposition on the top, then you can get a picture of such isolated atoms on the top and make the thermal motion of these atoms visible.

The introduction of helium gas causes the strong field of the needle tip to the helium, an electron is torn and the positive helium ion is accelerated toward the luminescent layer. Since the de Broglie wavelength of the helium ions is smaller than that of the electrons at a low temperature, the resolving power is increased, it can magnifications up to 2,000,000 times are achieved. The whole structure then resembling a helium ion microscope, in which, however, the generated helium ion beam is incident on the object to be examined, not the cathode increases the imaged object.

Related high-resolution microscope types

• Transmission electron microscope
• Scanning electron microscope
• STM
• Atomic force microscope
• Helium Ion Microscope
• Focused Ion Beam Microscope
• Field ion microscope