Field ion image formation

There are basically two kinds of experiments which can be used for studying the mechanisms of the field ionization process near a field ion emitter surface and also the field ion image formation process. They are the measurement of field ion current as functions of tip voltage, tip temperature, and other experimental parameters, and the measurement of the ion energy distribution. 

The basic assumption used for the interpretation of the field ion images, viz, the overlapping mechanism, was independently confirmed by the recent atom-probe experiments which have supplied evidence for the formation of a quasichemical complex between an inert gas atom and a metal... 

Fig. 4.14 (a) A few atomically resolved images of surfaces of WSi2 where Si atoms arc invisible. In tungsten silicide, only the W sublatticc is seen in the field ion image. Sec Section 4.4.2 for discussions on silicide formation. (b) Atomic structure of WSi2 crystal. 

Fig. 4. 54 Helium field ion images showing formation of iridium silicide layers of two different atomic structures on the Ir (001) surface. The first micrograph in (a) show s the Ir surface before formation of an IrSi layer.

Fig. 13.8. Atomic metallic ion emission and nanotip formation, (a) At high temperature, the atoms on a W tip becomes mobile. The tip surface is macroscopically flat but microscopically rough, (b) By applying a high field (1.2-1.8 V/A,), the W atoms move to the protrusions, (c) The apex atom has the highest probability to be ionized and leave the tip. The W ions form an image of the tip on the fluorescence screen, (d) A well-defined pyramidal protrusion, often ended with a single atom, is formed. By cooling down the tip and reversing the bias, a field-emission image is observed on the fluorescence screen. The patterns are almost identical. (Reproduced from Binh and Garcia, 1992, with permission.)...

Field ion microscopy is one of the older techniques to involve ions and yet it remains the only means available for viewing individual atoms on a surface directly. Since an extensive literature exists, the subject will not be considered here except to mention some recent papers on surface diffusion and atomic interactions, cluster formation, the imaging atom-probe microscope, and reviews in the fields of metallurgy and surface chemistry. ... 

However, the probability that a helium atom accelerating toward a tip will be ionized is small, even at fields of 4 volts/A and at a distance of only 5A. Such an atom traveling toward the surface has a chance of only one in 100 to be ionized in traversing a distance of one angstrom. In order to understand the formation of the ion image, it is therefore necessary to examine the motion of the helium atom subsequent to colliding with the surface. 

The effect of He+ close to the surface is difficult to establish directly. However, it has been possible to demonstrate that hydrogen is also stripped when Ne and Ar are used for image formation. With Ar, complete ionization can be brought about some distance from the surface in a field of 4.5 volts/A. Under these conditions the ion should not... 

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