Tungsten work function

Measuring the electron emission intensity from a particular atom as a function of V provides the work function for that atom its change in the presence of an adsorbate can also be measured. For example, the work function for the (100) plane of tungsten decreases from 4.71 to 4.21 V on adsorption of nitrogen. For more details, see Refs. 66 and 67 and Chapter XVII. Information about the surface tensions of various crystal planes can also be obtained by observing the development of facets in field ion microscopy [68]. 

Cesium was first produced ia the metallic state by electrolysis of a molten mixture of cesium and barium cyanides (2). Subsequentiy the more common thermochemical—reduction techniques were developed (3,4). There were essentially no iadustrial uses for cesium until 1926, when it was used for a few years as a getter and as an effective agent ia reduciag the electron work function on coated tungsten filaments ia radio tubes. Development of photoelectric cells a few years later resulted ia a small but steady consumption of cesium and other appHcations for cesium ia photosensing elements followed. 

Vacuum Tubes. In the manufacture of vacuum tubes for use in polarized ion sources, vaporized cesium is used as a getter for residual gaseous impurities in the tube and as a coating to reduce the work function of the tungsten filaments or cathodes of the tube. The cesium vapor is generated by firing, at about 850°C within the sealed and evacuated tube, a cesium chromate pellet and zirconium (12) (see Vacuum technology). 

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes - physical adsorption and activated adsorption -and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten. 

The significance and impact of surface science were now becoming very apparent with studies of single crystals (Ehrlich and Gomer), field emission microscopy (Sachtler and Duell), calorimetric studies (Brennan and Wedler) and work function and photoemission studies (M.W.R.). Distinct adsorption states of nitrogen at tungsten surfaces (Ehrlich), the facile nature of surface reconstruction (Muller) and the defective nature of the chemisorbed oxygen overlayer at nickel surfaces (M.W.R.) were topics discussed. 

Tungsten has the necessary high melting temperature (3660 K) to be employed as a thermionic source, and lanthanum hexaboride (LaB6) is also employed because of its low work function. 

Specimens for field emission sources are of a very fine needle shape, usually in the form of tungsten wire with a tip radius of <0.1 pm (Figure 5.4). Application of a potential of lkV thus generates a field of 106V/m which lowers the work function barrier sufficiently for electrons to tunnel out of the tungsten. FEG electron microscopes usually employ a gun potential of 3-4 keV. 

Fig. 8.1. Field ionization of a hydrogen atom (H). (a) close to a tungsten surface (W), (b) isolated. Conditions and symbols electric field 2 V A Pw image potential of W distorted by the field, Ph potential of the hydrogen atom distorted by the field, X work function, p Fermi level. Broken lines represent potentials in absence of the electric field. Adapted from Ref. [4] by permission. Verlag der Zeitschrift ftir Naturforschung, 1955.

Thoriated tungsten, whose surface has been carburized at high temperatures, have lower work functions than pure tungsten and emit equivalent electron beam currents at lower temperatures thereby extending the life and stability when compared to regular tungsten sources. These sources require a stable, high vacuum (10 to 10 torr) and are more difficult to fabricate. 

Swanson, L. W., and Grouser, L. G. (1967). Total-energy distribution of field-emitted electrons and single-plane work functions for tungsten. Phys. Rev. 163, 622-641. 

Fig. 4. Potential energy versus distance from the surface. Data is appropriate for He and tungsten. E, is the ionization potential for helium and ( > is the work function of tungsten. E (e") is the kinetic energy of an emitted secondary electron. The symbol He + nej implies a system composed of an helium ion and n conduction electrons in tungsten. The lower potential curve results from an Auger neutralization process where both electrons were originally at the Fermi level. (The figure is similar to one published in Ref. )...

Fiq. 17. Change in work function with coverage for the adsorption of CO on tungsten. 

If the work functions of every portion of the crystal constituting the tip were identical, one would simply observe a uniformly bright circular patch on the fluorescent screen. Different crystal faces have slightly different work functions, however, so that one sees a pattern showing different intensities for different faces. In general, closely packed faces have higher work functions than loosely packed ones. In fee cubic crystals like Ni the (111), (100), and (110) faces are the most closely packed and appear darker than the rest of the pattern (Plate IA and B). In bee cubic metals like tungsten, the (110), (100), and (211) faces are most closely packed and appear darkest (Plate IC). 

Values of the work functions of different faces are best known for tungsten. A review of thermionic measurements and their theoretical interpretation is given by Herring and Nichols (7a). The reader interested in the theory of the phenomenon is also referred to Smoluchowski (7b). Work functions of two crystallographic directions in tungsten have recently been determined by field emission technique by Drechsler and Miiller (7c). 

Fig. 5a. Potential energy diagram for the Is electron of a hydrogen atom at a distance of 5.5A. from a tungsten surface in the absence of external fields. I = ionization potential x = work function g = depth of Fermi sea Ph = proton-electron potential Pw = image potential.

First let us consider how the presence of adions will affect the electron work function. To do this quantitatively let us consider a 100 and a 110 plane of tungsten. Figure 10 shows a top view and a section view of the location and sizes of the cesium ion and the tungsten atoms. The Cs+ is shown in the position in which it contacts the largest number of tungsten... 

Figures 4, 5, and 6 were drawn. The results were checked against the experimental data where they were available. To test whether the work functions are indeed independent of temperature would require that the experiments be repeated using single crystal tungsten ribbons which are so treated that the surface exposes only a single plane. We urge more scientists to undertake this problem. The rewards would seem to be attractive.

The effect of the second difference, mentioned above, is that nitrogen acts as a negative dipole and increases the electron work function also, as the nitrogen atom accepts or shares more electrons with the tungsten, its size increases but even so its effective diameter is only slightly larger than that of a tungsten atom. 

Many theories of adsorption, following Langmuir, have assumed that the rate of adsorption is proportional to (1 — 0), i.e., to the fraction of the surface which is bare or not yet covered. Langmuir first proved the (1 — 0) law by measuring experimentally how the thermionic work function

changed with time as thorium reached the surface of a tungsten filament at a constant rate (10). He then assumed that tp decreased linearly with 0 and thus deduced that dd/di was proportional to (1 — 0). But this assumption has been shown to be incorrect for such cases as Cs on W, Ba on W, SrO on W, and other systems. Hence it follows that the (1 — 0) law is not valid. The experiments described above for N2 on W not only show that dQ/dt is not proportional to (1 — 0), but they show by a direct experiment that dd/dt for a constant arrival rate is independent of 0 between 0 = 0 and 1.0. 

The constants in this equation are evaluated by using the value of 4.4 volts for the work function of the 111 plane for normal clean tungsten. We thus calculate values of

heat-treating temperature T. The results are shown by the 11 IP curve in Figure 22. This figure shows also the results for the other planes or regions. 

Fig. 22. The electron work functions for various crystallographic planes versus the temperature of heat treatment for oxygen on tungsten.

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