Tungsten field emitter

Swanson, L. W., and Grouser, L. C. (1966). Anomalous total energy distribution for a tungsten field emitter. Phys. Rev. Lett. 16, 389-392. 

Meyer (3) showed some time ago that the color of hot sulfur melts is caused mainly by the presence of S3 and S4. In this connection we should also mention recent sophisticated studies by Block and co-workers (10). Sulfur molecules S with 2-22 sulfur atoms have been desorbed from a condensed sulfur layer on a tungsten field emitter of a field ionization time-of-flight mass spectrometer. The condensed sulfur layer is in a highly mobile liquid-like steady state. The observation of these large sulfur molecules is important to the current models of liquid sulfur. 

Menzel and Corner 147) observed desorption of gases from a tungsten field-emitter tip under slow-electron bombardment. They note that the process is in general inefficient, with a cross section about 10 of that for ionization or excitation in the gas phase. This desorption under bombardment with slow electrons was used by Zingerman and Ishchuk 148) as an analytical technique in a study of adsorption of oxygen on tungsten. Desorption by electron bombardment from copper and stainless steel was reported by Auslender and Minchenkov 148a). 

In general, the trend of electron-emission properties of the tungsten field-emitter surface as nitrogen is chemisorbed is consistent lyith the hypothesis that chemisorption is least favored by atomically smooth... 

Figure 3. Transmission electron microscopy of enzyme-cleaved, tobacco mosaic virus particles on the apex of a tungsten field-emitter tip (imaged at 200kV). TMV sample kindly supplied by P. J. Butler, the MRC, Cambridge, England.

Table 2 summarizes some important characteristics of three electron sources currently used in SEMs. Clearly the tungsten field emitter is most attractive, due to its high brightness and low energy spread. There are, however, a number of drawbacks associated with this type of source. In the first place, a gun vacuum of 10 torr is required, which can result in problems when examining a specimen which outgasses. The emission current is unstable. The maximum... 

Fig. 47. Work function of the (110) facet of a tungsten field emitter at 300 K as a function of silver exposure. The authors interpret this data as follows Below an exposure treshold of about one ML, Ag adatoms migrate to the high-index facets that surround the W(llO) plane, and the emission properties are indistinguishable from those of the bare substrate. At 1 ML the adatoms invade the (110) plane, modifying the surface density of electron states and bringing about an abrupt increase in electron emission. Above 2 ML the additionally evaporated Ag atoms migrate again to the surrounding facets. From [95D].

In principle, energy-analyzer systems can be designed such that their electron-optical properties do not limit the energy resolution attainable, i. e. their intrinsic energy resolution is much better than the energy width of the primary electron beam, which is of the order of approximately 1.5-2.5 eV for a tungsten hairpin cathode, approximately 1 eV for a LaBg cathode, approximately 0.7 eV for a Schottky field emitter, and 0.3-0.5 eV for a pure cold-field emitter. 

Alternatives to activated tungsten wire emitters are also known, but less widespread in use. Cobalt and nickel [44,47] as well as silver [48] can be electrochemi-cally deposited on wires to produce activated FD emitters. Mechanically strong and efficient emitters can be made by growing fine silicon whiskers from silane gas on gold-coated tungsten or tantalum wires of 60 pm diameter. [45] Finally, on the fracture-surface of graphite rods fine microcrystallites are exposed, the sharpness of which provides field strengths sufficient for ionization. [49]... 

Figure 14.28 Field emitter of single crystal tungsten is shown (left) on its supporting hairpin and (right) in a close up view of the tip. (From Stinger.)...

LaBe and CeBe are well known for being excellent field emitters, and have actually been commercialized as thermionic cathode materials. With low work functions around 2.6 eV, they can provide greater brightness and lower operation temperatures (longer service life) than tungsten cathodes, for example. A simple method to grow high-quality physical vapour deposition (PVD) films of CeBe was recently reported. ... 

Fig. 2.29 Recovery of field ion current after the tungsten emitter surface is pulse field evaporated to deplete all the field adsorbed image gas atoms at the surface. Zero time refers to the time when the surface is completely depleted of field...

Fig. 4.1 Helium field ion image of a (110) oriented bcc tungsten tip of radius 140 A where atoms in the 001 and 111 are resolved. The image covers a 100° extension angle of the emitter surface.

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