field-emission microscope

field-emission microscope:

see microscopemicroscope,
optical instrument used to increase the apparent size of an object. Simple Microscopes

A magnifying glass, an ordinary double convex lens having a short focal length, is a simple microscope. The reading lens and hand lens are instruments of this type.
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Field-Emission Microscope

a lensless electron-optical device for producing a magnified image of the surface of a solid with a magnification of 10⁵–10⁶. The field-emission microscope was invented in 1936 by the German physicist E. Müller.

The main components of a field-emission microscope are a cathode in the form of a point with a tip radius of curvature of ~10^–7–10^–8 m, a spherical or conical glass envelope whose face is coated with a phosphor, and an anode in the form of either a conductive coating on the walls of the envelope or a wire ring around the cathode. The phosphor-coated face of the envelope constitutes a fluorescent screen.

When the point is heated, its tip acquires both a single-crystal structure and a rounded shape. The envelope is evacuated to a residual pressure of 10^–9–10^–11 mm Hg. When the anode is supplied with a positive voltage of several thousand volts relative to the needle-shaped cathode, which is in the center of the envelope, the electric field strength in the immediate vicinity of the cathode tip reaches 10⁷–10⁸ volts per cm. Such a high field strength provides for strong field emission from the cathode tip. The emitted electrons are accelerated radially with respect to the tip, bombard the screen, and cause the phosphor to fluoresce, thus producing a magnified image of the cathode surface on the screen. The image reflects the symmetry of the point’s crystal structure.

The magnification in a field-emission microscope is equal to the ratio R/βr, where R is the cathode-screen distance, r is the radius of curvature of the point, and β is a factor that characterizes the departure of the shape of the electric field’s equipotential surfaces from a spherical shape. The resolving power of a field-emission microscope is limited by the presence of tangential electron velocity components at the cathode tip and, to a lesser extent, by electron diffraction. The limit of resolution amounts to (2–3) × 10^–7cm.

Field-emission microscopes are used, for example, to study field emission from metals and semiconductors and to determine the work functions of the different faces of a single crystal. For such purposes as the observation of phase transitions or the study of the adsorption of atoms of various substances on the surface of a metal or a semiconductor, the use of field-emission microscopes is rather limited because field-ion microscopes offer much greater possibilities for these purposes.