Field Emission and Ion Microscopy

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, 

This entirely quantum mechanical phenomenon is called field emission. Expression (7-1) indicates that the current increases with increasing electric field and decreasing work function, in agreement with the potential energy diagram of Fig. 7.9. 

A spherical tip exposes many facets with different crystallographic orientations. 

In field ion microscopy, ions of a gas such as hydrogen or one of the rare gases image the tip. The principle is shown in Fig. 7.10 for helium. A high positive potential 

Atom probe microscopy is a variation of field ion microscopy in which either the field ionized atoms or evaporated atoms from the tip are detected with a mass spectrometer placed behind an aperture in the imaging screen. This allows one to identify the desorbing ions. If the tip is mounted on a manipulator, one can zoom in on a desired surface plane. The technique has, for example, been used to study the composition of alloy surfaces we refer to Tsong [32,33] for reviews. 

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Less generally applicable than electron or scanning probe microscopy, but capable of revealing great detail, are field emission and field ion microscopy (FEM and F1M). 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. 

R. Gomer, Field Emission and Field Ion Microscopy, Harvard University Press, 1961. 

The tungsten (110) surface is one of the best studied of all surfaces, especially in field emission and field ion microscopy for many reasons. It is a very stable surface without surface reconstruction or phase transformation. It is also inert to contaminations. For the study of adatom-adatom interactions, it is a very smooth plane with the largest density of adsorption sites available of any W surface. Lesser restrictions are imposed on the adatom-adatom separation. As the surface is structurally very smooth, wave mechanical interference effects are least affected by the surface atomic structure. 

This is a specialised technique which has been applied in field emission and field ion microscopy (see Section 2.1.5c). It is achieved by giving the tip a positive potential. Tungsten can then be removed at liquid helium temperatures with an applied field of 5.7 x 10 V.cm Perfectly regular surface structures are exposed containing many different lattice planes. Clean surfaces have been produced on tungsten, nickel, iron, platinum, copper, silicon and germanium. It is potentially applicable to a wide range of materials, but the area of clean surface exposed is only about 10 ° cm . 

A proper judgment of the validity of these findings, as well as any extension of such work, must rest upon a detailed appreciation of the experiments involved. It is the aim of this article to review the experimental methods upon which these advances have been based—the flash filament technique, flash desorption, field emission and field ion microscopy, and the use of ultrahigh vacuum procedures. 

Akabori, K., Yamamoto, Y, Kawahara, S., Jin-nai, H., and Nishioka, H. (2009). Field emission scanning electron microscopy combined with focused ion beam for rubbery material with nanomatrix structure. Journal of Physics Conference Series 184, 012-027. 

A sensor for nitric oxide was constructed utilizing multiwaUed carbon nanotubes that were previously modified with ethylenediamine and sonicated in a solution containing Co -TSPP. Drop cast electrodes were prepared and characterized by field-emission transmission electron microscopy, X-ray photoelectron spectroscopy and by electrochemical techniques [220]. Cyclic voltammetry, electrochemical impedance, and chronoamperometry were utilized to evaluate the electrocatalytic activity of the hybrid sensor that exhibited linear response in the 6.6 x 10 to 1.3 X 10 mol L range, with a detection limit of 6.6 x 10 mol L. Electrodes composed of porphyrins, platinum nanowires, and Nafion were proposed for photocatalytic reduction of water [221]. After an extensive search for optimization of each component and under the best conditions (—350 mV and visible light irradiation) a detectable amount of hydrogen was produced, but Nafion can diminish the diffusion of ions to the porphyrin active sites. 

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]. 

Mobility of this second kind is illustrated in Fig. XVIII-14, which shows NO molecules diffusing around on terraces with intervals of being trapped at steps. Surface diffusion can be seen in field emission microscopy (FEM) and can be measured by observing the growth rate of patches or fluctuations in emission from a small area [136,138] (see Section V111-2C), field ion microscopy [138], Auger and work function measurements, and laser-induced desorption... 

T.T. Tsong, Atom-Probe and Field Ion Microscopy Field-Ion Emission and Surfaces and Interfaces at Atomic Resolution, Cambridge University Press, Cambridge, 1990. 

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