A technique that uses field emission of electrons or positive ions from a needle-shaped emitter to produce a magnified image of the emitter surface on a fluorescent screen.
In the field electron microscope, the image reveals the variation in work function of the emitter surface. Due to the large lateral velocity of the emitted electrons, which arises from a diffraction effect of the de Broglie wave and the large kinetic energy of electrons inside the metal, a resolution of only about 2.5 nanometers can be achieved. The field electron microscope has been used to study adsorption and desorption of gases and vapor-deposited materials, surface migration of adsorption layers and absorbed atoms on single crystal faces, and surface reactions in catalysis. Medium-sized individual molecules such as phythalocyanin have been made visible also. See also De Broglie wavelength; Electron microscope; Field emission; Work function (electronics).
In the field ion microscope, the emitter is kept at a high positive potential while the microscope chamber is filled with helium, neon, or argon at a pressure of 10−5 to 10−4 torr (10−3 to 10−2 pascal) [see illustration]. Under a field of several tens of volts per nanometer, an image gas molecule can be ionized above a protruding surface atom when an atomic electron tunnels into the metal. The ion is then accelerated to the screen by the applied field. Every second about 1000 ions are formed above the same surface atom; thus the atom is continuously imaged. When the tip is cooled down to the temperature of liquid hydrogen or nitrogen, the thermal energy of the image gas molecules is greatly reduced. In combination with their very short de Broglie wavelength, a resolution of about 0.25 nm can be achieved. This is sufficient to resolve the atomic structure of most surfaces. In addition, surface atoms can be evaporated by the applied field if it is gradually increased. Therefore the atomic structure of lattice defects inside the bulk can be revealed also. By using a channel plate for image intensification and image gases of high or low ionization energy, a wide range of metals from beryllium to uranium, semiconductors, and some compounds such as high-temperature oxide superconductors can be imaged. See also Channel electron multiplier; Tunneling in solids.
Field ion microscope.
The field ion microscope has been used as a research tool for studying lattice defects such as vacancies, interstitials, dislocations, grain boundaries, and radiation damages in metals and alloys. It has also been used to observe directly the behavior of single adsorbed atoms on metal surfaces. See also Crystal defects.
The atom-probe field ion microscope combines the field ion microscope with a single ion detection sensitivity mass spectrometer, usually a time-of-flight spectrometer. The tip is mounted on a gimbal system, and a channel plate screen assembly with a small probe hole is used. Behind the probe hole is a flight tube 3–24 ft (1–8 m) long. The probe hole usually covers several atomic image diameters. The chemical identity of atoms chosen by the investigator from the field ion image can be analyzed one by one by adjusting the gimbal until the image of the atoms falls into the probe hole. Nanosecond high-voltage pulses or subnanosecond laser pulses are then applied to field-evaporate surface atoms one by one. Because of the ion optics, only those atoms that have their image covered by the probe hole can go through the probe hole and be detected. From the flight times of these ions, their mass-to-charge ratios are calculated and chemical species identified.
The atom-probe field ion microscope represents the ultimate in chemical analysis, that is, an atom selected by an investigator can be analyzed. As field evaporation starts from the edges of a surface layer, composition of a solid surface can also be analyzed atomic layer by atomic layer. The spatial resolution in atom-probe chemical analysis is approximately 5 to 10 angstroms in the lateral direction and much better than 1 angstrom in the vertical direction.
