field-emission microscope

field-emission microscope, type of electron microscope in which a wire with a sharpened tip is mounted in a cathode-ray tube. Electrons are drawn from the tip by a high electrical field and travel toward the screen on which the image is formed. Only strong metals, such as tungsten, platinum, and molybdenum, can be examined in this way, because the high field at the tip exerts a large mechanical stress. The magnification is proportional to the ratio of the radius of curvature of the screen on which the image forms to the radius of the metal tip; it may reach up to 1,000,000×.

A development of the field-emission microscope is the field-ion microscope, in which the tip is surrounded by a low pressure of helium gas. The gas is ionized at the atom planes on the tip and produces an image that can have a magnification of up to 10,000,000×. The field-ion microscope has been applied mainly to the study of metals and semiconductors, but a few biological images have been obtained. A further development of the field-ion microscope is the atom probe. In this instrument, individual atoms are removed from the tip by pulsing the electric field. The atoms pass through a time-of-flight spectrometer, which measures their energy and charge-to-mass ratio. In this way the chemical nature of each atom in the field-ion image may be determined.