Field-ion microscopy

Despite its amazing resolution, the utility of the ion microscope in probing adsorption is quite uncertain inasmuch as (1) the conditions under which adsorbed gas atoms and molecules become visible have have not been established (2) in observing atomic details with helium ions the adsorbed gas may be removed from the surface (3) even if the 

In this section we will explore the use of the ion microscope for studying adsorption phenomena, with special emphasis on the ability to detect single gas atoms. So far the microscope has been applied principally to the examination of the surface structure of metals and of imperfections in solids. These, as well as other bulk phenomena, will not be touched upon.  

The energy relations that must be obeyed to make field ionization possible are indicated schematically in Fig. 52a. In free space, the potential well of an atom placed in a uniform field is distorted symmetrically. At high fields (with helium, F 4.5 volts/A) the barrier behind which the electrons are trapped (shaded in the illustration) is sufficiently thinned and electrons can tunnel through. The rate of tunneling has been evaluated explicitly for hydrogen atoms (70, 71) and hydrogen molecule ions, H (72) for the former, the rate constant for field ionization can be written as 

The exponential represents the probability of an electron escaping into the field on colliding with the core potential. It has the form familiar from the Fowler-Nordheim relation [Eq. (30)], with the ionization potential replacing the work function as a measure of barrier height. The frequency of collisions with the core potential is given by the preexponential factor. At a field of 2 volts/A, this amounts to 0.51 x 1016 sec-1, compared with 0.66 x 1018 sec-1 for the Bohr frequency, yielding a rate constant kp = 2.12 x 107sec-1. 

For other atoms the form of the transmission coefficient should remain unchanged, the tunneling probability varying exponentially with the ratio 73/2/jF in the absence of detailed calculations it is convenient to retain the preexponential unchanged as well. 

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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]. 

FIM Field ion microscopy [63, 62, 103] He ions are formed in a high field at a metal tip Surface structure... 

E. W. Miiller and T. T. Tsong, Field Ion Microscopy, American Elsevier, New York, 1969. 

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... 

The last twenty years have seen a rapid development of surface physics. In particular, the properties of clean perfect surfaces (with two-dimensional periodicity) are henceforth well known and understood. In recent years, the focus has been put onto surfaces with defects (adatoms, steps, vacancies, impurities...) which can now be investigated experimentally due either to the progress of old techniques (field ion microscopy or He diffraction, for instance) or to the rapid development of new methods (STM, AFM, SEXAFS...). 

Field ion microscopy, scanning tunnelling microscopy (morphology analysis, etc.) L.E.E.D. (structure)... 

Field-ion Microscopy , Defects In Crystalline Solids Series, 2, North Hoiland Pub Co. (1970) 43) J.A. Swift, Electron Microscopes , Barnes Noble Publ (1970) 44) W.E. Voreck,... 

Many years have passed since the early days of AFM, when adhesion was seen as a hindrance, and it is now regarded as a useful parameter for identification of material as well as a key to understanding many important processes in biological function. In this area, the ability of AFM to map spatial variations of adhesion has not yet been fully exploited but in future could prove to be particularly useful. At present, the chemical nature and interaction area of the AFM probe are still rarely characterized to a desirable level. This may be improved dramatically by the use of nanotubes, carbon or otherwise, with functionalized end groups. However, reliance on other measurement techniques, such as transmission electron microscopy and field ion microscopy, will probably be essential in order to fully evaluate the tip-sample systems under investigation. 

FIM Field ion microscopy GFAAS Graphite furnace atomic absorption... 

Miller, M.K., Cerezo, A., Fletherington, M.G. Smith, G.D.W. (1996) Atom Probe Field Ion Microscopy, Oxford University Press, Oxford, UK. 

Fast Fourier Transform Flow Injection Analysis Field Ion Atom Probe Flame-Ionization Detector Field Ion Microscopy... 

Reflection contrast Reflection-imaging microscopy Field ion microscopy Quantification in gap between light and em microscopies Useful for imaging highly reflective particles such as silver grains in autoradiographs Atomic structure of crystals Immunoelectron Localization of cellular antigens... 

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. 

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... 

Figure 7.11 Field ion microscopy image of a platinum tip. As Pt has the fee structure, the fourfold symmetry of the picture implies that the center corresponds to a (100) facet. Dark areas near the four comers are (111) facets (from Muller [26]). 

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