Field ion microscope
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Field ion microscopy (FIM) is an analytical technique used in materials science. The field ion microscope is a type of microscope that can be used to image the arrangement of atoms at the surface of a sharp metal tip. It was the first technique by which individual atoms could be spatially resolved. The technique was pioneered by Erwin Müller. Images of atomic structures of tungsten were first published in 1951 in the journal Zeitschrift für Physik.
In FIM, a sharp metal tip is produced and placed in a vacuum chamber, which is backfilled with an imaging gas such as helium or neon. The tip is cooled to cryogenic temperatures (<50 K). A positive voltage of 5000 to 10 000 volts is applied to the tip. Gas atoms adsorbed on the tip are ionized by the strong electric field in the vicinity of the tip (thus, "field ionization"), becoming positively charged and being repelled from the tip. The curvature of the surface near the tip causes a natural magnification – ions are repelled in a direction roughly perpendicular to the surface (a "point projection" effect). A detector is placed so as to collect these repelled ions; the image formed from all the collected ions can be of sufficient resolution to image individual atoms on the tip surface.
Note: There is a common misunderstanding about the resolution of a microscope, that is, that the best possible spatial resolution of a microscope is on the order of the wavelength of the particles that are used for imaging. Although this is true for many types of microscopes, it is not true for the FIM or other microscopes in which magnification is due to point projection.
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