Field emission microscopes

The field emission microscope (FEM), invented in 1936 by Muller [59, 60], has provided major advances in the structural study of surfaces. The subject is highly developed and has been reviewed by several groups [2, 61, 62], and only a selective, introductory presentation is given here. Some aspects related to chemisorption are discussed in Chapter XVII. 

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

The field emission microscope investigation of surface reactions. Physic. Rev. [2] 92, 854 (1953). 

Investigation of the surface reaction of oxygen with carbon on tungsten with the field emission microscope. J. chem. Physics 21, 1177—1180 (1954). 

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

Field emission devices, 17 49-50 Field emission FPDs (FEDs), 22 259 Field emission microscope (FEM), 16 503 Field emission microscopy (FEM), 24 74 Field emission scanning electron microscope (FESEM), 16 492 Field emission scanning electron... 

Among the other information which it can provide, the field-emission microscope gives direct evidence of the mobility of adsorbed layers. These results have been reviewed elsewhere by Gomer 118) and will not be further discussed here. 

Experimentally, work-function measurements which rely on the cold emission of electrons are carried out in the field-emission microscope (F.E.M.) (21). The apparatus, as shown in Fig. 11, consists of a W tip P sharpened by electrolytic polishing so that the radius of curvature is 10 cm., and an anode in the form of a film of Aquadag. A variable potential of 3 to 15 kv. is applied to the anode, and the electrons, pulled out from the point, travel in approximately straight lines to the fluorescent screen. The linear magnification obtained is of the order of 10 to 10. The secondary electrons from the screen are collected by the anode, and the field-emission current is measured by a sensitive microammeter. 

The dipole moment of adsorbed atoms can be derived by measuring the change in work function as a result of the presence of the adatoms. In macroscopic measurements, the states of the deposited adatoms are unknown. They may combine into clusters of different sizes and some of them may be absorbed into lattice steps. One can also use the field emission microscope for this purpose. However, similar uncertainties exist. To achieve a well characterized surface and a known number of individual adsorbed atoms on a surface, a combined experiment with field ion microscope observations and field emission Fowler-Nordheim (F-N) plots has been most successful.198,200... 

The field emission microscope was invented by E. W. Muller in 1937 and developed by him in the years following (1). For various reasons the device failed to attract general attention until the end of World War II. Its potentialities are now being recognized the field and ion microscopes, in conjunction with recently developed ultra high vacuum techniques, are rapidly becoming important tools for the study of physical and chemical surface phenomena. 

The field emission microscope utilizes the phenomenon of cold emission as follows A wire etched to a very sharp point is surrounded by a spherical anode, usually in the form of a fluorescent screen. The system is evacuated to pressures of the order of 10 to 10 mm, of Hg and the wire is heat-polished electrically. This outgasses the metal and produces a smoothly... 

Fig. 2. Schematic drawing of one form of the field emission microscope. E, glass envelope S phosphorescent screen M, metal backing A, anode lead-in T, emitter tip C, tip support structure V, vacuum lead.

Fig. 3. Schematic diagram showing the optics of the field emission microscope. r, radius of curvature of the tip x, tip-to-screen distance. A region of linear dimension S will appear as D on the screen.

E. W. Muller recently developed an ingenious modification of the field emission microscope, based on the field ionization of hydrogen (1,12). The device consists of an ordinary field emission tube containing hydrogen at a pressure of about 10 mm. When the tip is made the anode, adsorbed hydrogen in the form of ions can be pulled off at fields of the order of 2 X 10 v./cm. A pattern, corresponding to the electron emission, but with greatly increased resolution ( 3A.) results. 

The field emission microscope offers a very clear-cut and basically simple method of determining the mobility of adsorbates quantitatively. If it were possible to evaporate the gas under study from a suitable source (e.g., a heatable CuO filament for oxygen) in such a way that only a portion of the tip became contaminated, one could determine how, and at what temperatures of the tip, migration occurred. If one attempted to evaporate from a gas emitter placed on one side of the tip while the microscope tube was at room temperature, gas rebounding from the walls would instantly contaminate the whole tip and the experiment would fail. 

Fio. 9. Sketch of field emission microscope assembly for mobility studies. D, inner Dewar <8, screen A, anode T, tip, TA, tip assembly M, platinum foil mortar, filled with copper wires, oxidized in ssiln electric heating of M produces a controllable flux of oxygen MA, gas emitter assembly V, vacuum lead, sealed off. Outer Dewar and electrical leads are not shown. 

VI. Other Adsorption Experiments with the Field Emission Microscope... 192... 

A still more complete insight into the nature of adsorbed species can be obtained from experiments (I) on thermionic emission, ( ) with the field emission microscope, and (S) with the ion gauge. From some thermionic experiments, particularly with cesium adsorbed on tungsten, it is learned that (a) Cs can be adsorbed as positive ions as well as adatoms (b) as the concentration of adsorbed cesium increases, the ratio of adions to adatoms decreases (c) the forces produced by adions are long-range forces which have appreciable effects over distances of 10 to 20 atom diameters (d) adatoms and adions can migrate over the surface at much lower temperatures than those at which they evaporate from the surface. 

From experiments with the field emission microscope it is learned that for a system like oxygen on tungsten (a) the crystallographic plane of the tungsten has a marked influence on the adsorption properties (6) the heat of adsorption increases with the number of W atoms a particular 0 atom can contact (c) the heat of adsorption for the first layer, in which 0 atoms make first valence bonds with W atoms, is about 4 ev., for the second layer, in which 0 atoms make second valence bonds with W atoms, only about 2 ev. (d) at a constant pressure the rate of adsorption is constant until the first layer is complete, and for the second layer it is slower by a factor of 100 or more (e) beyond the second layer oxygen is adsorbed as admoles of O2, O4, Os. 

Experiments with the field emission microscope show in a striking manner that the adsorption properties of a metal vary considerably from one crystallographic plane to another. The sticking probability, which determines the rate of adsorption, may be about 100 times smaller on one plane than on another as a result, at a fixed low pressure one plane may build up to only one layer while another builds up to two layers at a higher pressure, both planes may build up to two layers before a steady state is reached. If the surface is exposed to a high pressure until the whole surface is covered, and if the pressure is then reduced to a very low value, the rate of evaporation at a particular temperature from one plane may be hundreds of times greater than from a second plane. From such experimental facts one may safely conclude that the catalytic activity too will be found to vary for different crystallographic planes. 

The cases of adsorption discussed in this section and the rather detailed picture of the forces due to the interaction of the adsorbate species with the structural arrangement of the adsorbent atoms could hardly have been obtained by conventional catalytic experiments. Yet these same forces which are found in the simpler cases of adsorption must be present in the more complex cases of catalytic reactions. In arriving at this picture, we have found thermionic emission, the field emission microscope, and the modern ion gauge very useful tools. The next three sections will describe these tools in greater detail. 

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