Field-electron emission microscopy

D. Evidence from Field Electron Emission Microscopy and from Low-... 

Figure 4. Field-electron emission microscopy of small, organic adsorbates. (A) 110-oriented tungsten tip. (B) With copper-phthalocyanine adsorption.

Figure 5. Field-electron emission microscopy of protein adsorbates. (A) Active tip prior to deposition. (B) Deposition in buffer without ferritin-IgG. (C) Deposition in buffer with ferritin-IgG. (D) Control tip prior to air exposure. (E) After exposure to laboratory ambient. (F) After...

Bozdech, Ernst, Melmed, Field Ion] Bozdech, Georg/Norbert Emst/Allan J. Melmed Combined Field Ion and Field Electron Emission Microscopy and Energy-resolved Atom-probe Spectroscopy of Y, Ba2, CU3, Oy.x, Journal de Physique, Colloques 49 (1988), p. 453-458. 

The field emission microscope (FEM) and the field ion microscope (FIM) are in many respects complementary instruments. While the FIM can depict surface structure in atomic detail, study of field electron emission from the same specimen can yield information about the electronic structure of the surface layer. Field ion microscopy has been the subject of an earlier review and in this article more recent developments in field emission microscopy and its application to surface studies are reviewed. Earlier developments have been the subject of several reviews. 

While field ion microscopy has provided an effective means to visualize surface atoms and adsorbates, field emission is the preferred technique for measurement of the energetic properties of the surface. The effect of an applied field on the rate of electron emission was described by Fowler and Nordheim [65] and is shown schematically in Fig. Vlll 5. In the absence of a field, a barrier corresponding to the thermionic work function, prevents electrons from escaping from the Fermi level. An applied field, reduces this barrier to 4> - F, where the potential V decreases linearly with distance according to V = xF. Quantum-mechanical tunneling is now possible through this finite barrier, and the solufion for an electron in a finite potential box gives... 

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]. 

FEM Field emission microscopy [62, 101, 102] Electrons are emitted from a tip in a high field Surface structure... 

The source requited for aes is an electron gun similar to that described above for electron microscopy. The most common electron source is thermionic in nature with a W filament which is heated to cause electrons to overcome its work function. The electron flux in these sources is generally proportional to the square of the temperature. Thermionic electron guns are routinely used, because they ate robust and tehable. An alternative choice of electron gun is the field emission source which uses a large electric field to overcome the work function barrier. Field emission sources ate typically of higher brightness than the thermionic sources, because the electron emission is concentrated to the small area of the field emission tip. Focusing in both of these sources is done by electrostatic lenses. Today s thermionic sources typically produce spot sizes on the order of 0.2—0.5 p.m with beam currents of 10 A at 10 keV. If field emission sources ate used, spot sizes down to ca 10—50 nm can be achieved. 

Several ways exist to image these regions of different work function. We have already discussed scanning electron and field emission microscopy in this chapter. Scanning photoemission microscopy (SPM) is carried out by scanning a focussed UV beam (beam diameter of 0.5 pm) over the surface and recording the photoemis-... 

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... 

Electron microprobe analysis (EMA, EPMA) Energy dispersive X-ray analysis (EDX, EDAX) Field emission microscopy (FEM)... 

Less generally applicable than electron or scanning probe microscopy - but still capable of revealing great detail - are field emission microscopy (FEM) and field ion microscopy (FIM). These techniques are limited to the investigation of sharp metallic tips, however, with the attractive feature that the facets of such tips exhibit a variety of crystallographically different surface orientations, which can be studied simultaneously, for example in gas adsorption and reaction studies. 

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